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1. Added "$(objpfx)tst-cmsghdr: $(libdl)" to socket/Makefile since we still
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   need $(libdl) in RHEL8.
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2. Included stddef.h in socket/tst-cmsghdr-skeleton.c because it uses NULL.
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commit 9c443ac4559a47ed99859bd80d14dc4b6dd220a1
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Author: Arjun Shankar <arjun@redhat.com>
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Date:   Tue Aug 2 11:10:25 2022 +0200
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    socket: Check lengths before advancing pointer in CMSG_NXTHDR
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    The inline and library functions that the CMSG_NXTHDR macro may expand
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    to increment the pointer to the header before checking the stride of
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    the increment against available space.  Since C only allows incrementing
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    pointers to one past the end of an array, the increment must be done
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    after a length check.  This commit fixes that and includes a regression
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    test for CMSG_FIRSTHDR and CMSG_NXTHDR.
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    The Linux, Hurd, and generic headers are all changed.
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    Tested on Linux on armv7hl, i686, x86_64, aarch64, ppc64le, and s390x.
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    [BZ #28846]
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    Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
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Conflicts:
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	socket/Makefile
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	  (usual test backport differences)
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diff --git a/bits/socket.h b/bits/socket.h
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index 725798882e4b803b..0474613a9c003eeb 100644
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--- a/bits/socket.h
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+++ b/bits/socket.h
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@@ -245,6 +245,12 @@ struct cmsghdr
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 			 + CMSG_ALIGN (sizeof (struct cmsghdr)))
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 #define CMSG_LEN(len)   (CMSG_ALIGN (sizeof (struct cmsghdr)) + (len))
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+/* Given a length, return the additional padding necessary such that
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+   len + __CMSG_PADDING(len) == CMSG_ALIGN (len).  */
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+#define __CMSG_PADDING(len) ((sizeof (size_t) \
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+                              - ((len) & (sizeof (size_t) - 1))) \
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+                             & (sizeof (size_t) - 1))
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+
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 extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
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 				      struct cmsghdr *__cmsg) __THROW;
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 #ifdef __USE_EXTERN_INLINES
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@@ -254,18 +260,38 @@ extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
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 _EXTERN_INLINE struct cmsghdr *
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 __NTH (__cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg))
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 {
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+  /* We may safely assume that __cmsg lies between __mhdr->msg_control and
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+     __mhdr->msg_controllen because the user is required to obtain the first
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+     cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
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+     via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
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+     trust the value of __cmsg->cmsg_len and therefore do not use it in any
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+     pointer arithmetic until we check its value.  */
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+
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+  unsigned char * __msg_control_ptr = (unsigned char *) __mhdr->msg_control;
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+  unsigned char * __cmsg_ptr = (unsigned char *) __cmsg;
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+
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+  size_t __size_needed = sizeof (struct cmsghdr)
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+                         + __CMSG_PADDING (__cmsg->cmsg_len);
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+
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+  /* The current header is malformed, too small to be a full header.  */
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   if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr))
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-    /* The kernel header does this so there may be a reason.  */
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     return (struct cmsghdr *) 0;
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+  /* There isn't enough space between __cmsg and the end of the buffer to
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+  hold the current cmsg *and* the next one.  */
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+  if (((size_t)
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+         (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr)
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+       < __size_needed)
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+      || ((size_t)
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+            (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr
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+             - __size_needed)
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+          < __cmsg->cmsg_len))
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+
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+    return (struct cmsghdr *) 0;
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+
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+  /* Now, we trust cmsg_len and can use it to find the next header.  */
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   __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg
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 			       + CMSG_ALIGN (__cmsg->cmsg_len));
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-  if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control
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-					+ __mhdr->msg_controllen)
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-      || ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len)
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-	  > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen)))
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-    /* No more entries.  */
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-    return (struct cmsghdr *) 0;
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   return __cmsg;
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 }
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 #endif	/* Use `extern inline'.  */
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diff --git a/socket/Makefile b/socket/Makefile
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index 8975a65c2aabbfbc..a445383f8739351e 100644
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--- a/socket/Makefile
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+++ b/socket/Makefile
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@@ -31,7 +31,12 @@ routines := accept bind connect getpeername getsockname getsockopt	\
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 	    setsockopt shutdown socket socketpair isfdtype opensock	\
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 	    sockatmark accept4 recvmmsg sendmmsg sockaddr_un_set
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-tests := tst-accept4
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+tests := \
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+  tst-accept4 \
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+  tst-cmsghdr \
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+  # tests
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+
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+$(objpfx)tst-cmsghdr: $(libdl)
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 tests-internal := \
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   tst-sockaddr_un_set \
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diff --git a/socket/tst-cmsghdr-skeleton.c b/socket/tst-cmsghdr-skeleton.c
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new file mode 100644
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index 0000000000000000..7accfa6e54708e2a
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--- /dev/null
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+++ b/socket/tst-cmsghdr-skeleton.c
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@@ -0,0 +1,93 @@
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+/* Test ancillary data header creation.
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+   Copyright (C) 2022 Free Software Foundation, Inc.
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+   This file is part of the GNU C Library.
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+
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+   The GNU C Library is free software; you can redistribute it and/or
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+   modify it under the terms of the GNU Lesser General Public
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+   License as published by the Free Software Foundation; either
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+   version 2.1 of the License, or (at your option) any later version.
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+
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+   The GNU C Library is distributed in the hope that it will be useful,
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+   but WITHOUT ANY WARRANTY; without even the implied warranty of
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+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
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+   Lesser General Public License for more details.
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+
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+   You should have received a copy of the GNU Lesser General Public
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+   License along with the GNU C Library; if not, see
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+   <https://www.gnu.org/licenses/>.  */
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+
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+/* We use the preprocessor to generate the function/macro tests instead of
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+   using indirection because having all the macro expansions alongside
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+   each other lets the compiler warn us about suspicious pointer
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+   arithmetic across subsequent CMSG_{FIRST,NXT}HDR expansions.  */
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+
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+#include <stdint.h>
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+#include <stddef.h>
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+
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+#define RUN_TEST_CONCAT(suffix) run_test_##suffix
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+#define RUN_TEST_FUNCNAME(suffix) RUN_TEST_CONCAT (suffix)
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+
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+static void
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+RUN_TEST_FUNCNAME (CMSG_NXTHDR_IMPL) (void)
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+{
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+  struct msghdr m = {0};
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+  struct cmsghdr *cmsg;
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+  char cmsgbuf[3 * CMSG_SPACE (sizeof (PAYLOAD))] = {0};
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+
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+  m.msg_control = cmsgbuf;
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+  m.msg_controllen = sizeof (cmsgbuf);
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+
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+  /* First header should point to the start of the buffer.  */
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+  cmsg = CMSG_FIRSTHDR (&m);
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+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
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+
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+  /* If the first header length consumes the entire buffer, there is no
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+     space remaining for additional headers.  */
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+  cmsg->cmsg_len = sizeof (cmsgbuf);
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+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
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+  TEST_VERIFY_EXIT (cmsg == NULL);
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+
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+  /* The first header length is so big, using it would cause an overflow.  */
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+  cmsg = CMSG_FIRSTHDR (&m);
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+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
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+  cmsg->cmsg_len = SIZE_MAX;
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+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
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+  TEST_VERIFY_EXIT (cmsg == NULL);
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+
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+  /* The first header leaves just enough space to hold another header.  */
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+  cmsg = CMSG_FIRSTHDR (&m);
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+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
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+  cmsg->cmsg_len = sizeof (cmsgbuf) - sizeof (struct cmsghdr);
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+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
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+  TEST_VERIFY_EXIT (cmsg != NULL);
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+
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+  /* The first header leaves space but not enough for another header.  */
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+  cmsg = CMSG_FIRSTHDR (&m);
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+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
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+  cmsg->cmsg_len ++;
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+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
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+  TEST_VERIFY_EXIT (cmsg == NULL);
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+
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+  /* The second header leaves just enough space to hold another header.  */
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+  cmsg = CMSG_FIRSTHDR (&m);
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+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
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+  cmsg->cmsg_len = CMSG_LEN (sizeof (PAYLOAD));
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+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
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+  TEST_VERIFY_EXIT (cmsg != NULL);
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+  cmsg->cmsg_len = sizeof (cmsgbuf)
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+                   - CMSG_SPACE (sizeof (PAYLOAD)) /* First header.  */
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+                   - sizeof (struct cmsghdr);
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+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
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+  TEST_VERIFY_EXIT (cmsg != NULL);
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+
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+  /* The second header leaves space but not enough for another header.  */
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+  cmsg = CMSG_FIRSTHDR (&m);
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+  TEST_VERIFY_EXIT ((char *) cmsg == cmsgbuf);
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+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
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+  TEST_VERIFY_EXIT (cmsg != NULL);
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+  cmsg->cmsg_len ++;
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+  cmsg = CMSG_NXTHDR_IMPL (&m, cmsg);
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+  TEST_VERIFY_EXIT (cmsg == NULL);
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+
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+  return;
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+}
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diff --git a/socket/tst-cmsghdr.c b/socket/tst-cmsghdr.c
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new file mode 100644
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index 0000000000000000..68c96d3c9dd2bce8
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--- /dev/null
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+++ b/socket/tst-cmsghdr.c
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@@ -0,0 +1,56 @@
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+/* Test ancillary data header creation.
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+   Copyright (C) 2022 Free Software Foundation, Inc.
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+   This file is part of the GNU C Library.
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+
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+   The GNU C Library is free software; you can redistribute it and/or
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+   modify it under the terms of the GNU Lesser General Public
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+   License as published by the Free Software Foundation; either
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+   version 2.1 of the License, or (at your option) any later version.
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+
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+   The GNU C Library is distributed in the hope that it will be useful,
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+   but WITHOUT ANY WARRANTY; without even the implied warranty of
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+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
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+   Lesser General Public License for more details.
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+
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+   You should have received a copy of the GNU Lesser General Public
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+   License along with the GNU C Library; if not, see
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+   <https://www.gnu.org/licenses/>.  */
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+
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+#include <sys/socket.h>
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+#include <gnu/lib-names.h>
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+#include <support/xdlfcn.h>
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+#include <support/check.h>
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+
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+#define PAYLOAD "Hello, World!"
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+
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+/* CMSG_NXTHDR is a macro that calls an inline function defined in
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+   bits/socket.h.  In case the function cannot be inlined, libc.so carries
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+   a copy.  Both versions need to be tested.  */
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+
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+#define CMSG_NXTHDR_IMPL CMSG_NXTHDR
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+#include "tst-cmsghdr-skeleton.c"
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+#undef CMSG_NXTHDR_IMPL
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+
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+static struct cmsghdr * (* cmsg_nxthdr) (struct msghdr *, struct cmsghdr *);
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+
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+#define CMSG_NXTHDR_IMPL cmsg_nxthdr
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+#include "tst-cmsghdr-skeleton.c"
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+#undef CMSG_NXTHDR_IMPL
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+
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+static int
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+do_test (void)
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+{
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+  static void *handle;
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+
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+  run_test_CMSG_NXTHDR ();
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+
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+  handle = xdlopen (LIBC_SO, RTLD_LAZY);
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+  cmsg_nxthdr = (struct cmsghdr * (*) (struct msghdr *, struct cmsghdr *))
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+                  xdlsym (handle, "__cmsg_nxthdr");
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+
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+  run_test_cmsg_nxthdr ();
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+
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+  return 0;
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+}
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+
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+#include <support/test-driver.c>
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diff --git a/sysdeps/mach/hurd/bits/socket.h b/sysdeps/mach/hurd/bits/socket.h
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index 18959139dc7d325b..cc66684061e3e179 100644
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--- a/sysdeps/mach/hurd/bits/socket.h
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+++ b/sysdeps/mach/hurd/bits/socket.h
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@@ -249,6 +249,12 @@ struct cmsghdr
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 			 + CMSG_ALIGN (sizeof (struct cmsghdr)))
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 #define CMSG_LEN(len)   (CMSG_ALIGN (sizeof (struct cmsghdr)) + (len))
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+/* Given a length, return the additional padding necessary such that
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+   len + __CMSG_PADDING(len) == CMSG_ALIGN (len).  */
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+#define __CMSG_PADDING(len) ((sizeof (size_t) \
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+                              - ((len) & (sizeof (size_t) - 1))) \
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+                             & (sizeof (size_t) - 1))
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+
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 extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
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 				      struct cmsghdr *__cmsg) __THROW;
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 #ifdef __USE_EXTERN_INLINES
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@@ -258,18 +264,38 @@ extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
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 _EXTERN_INLINE struct cmsghdr *
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 __NTH (__cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg))
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 {
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+  /* We may safely assume that __cmsg lies between __mhdr->msg_control and
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+     __mhdr->msg_controllen because the user is required to obtain the first
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+     cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
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+     via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
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+     trust the value of __cmsg->cmsg_len and therefore do not use it in any
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+     pointer arithmetic until we check its value.  */
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+
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+  unsigned char * __msg_control_ptr = (unsigned char *) __mhdr->msg_control;
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+  unsigned char * __cmsg_ptr = (unsigned char *) __cmsg;
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+
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+  size_t __size_needed = sizeof (struct cmsghdr)
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+                         + __CMSG_PADDING (__cmsg->cmsg_len);
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+
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+  /* The current header is malformed, too small to be a full header.  */
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   if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr))
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-    /* The kernel header does this so there may be a reason.  */
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     return (struct cmsghdr *) 0;
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+  /* There isn't enough space between __cmsg and the end of the buffer to
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+  hold the current cmsg *and* the next one.  */
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+  if (((size_t)
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+         (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr)
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+       < __size_needed)
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+      || ((size_t)
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+            (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr
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+             - __size_needed)
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+          < __cmsg->cmsg_len))
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+
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+    return (struct cmsghdr *) 0;
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+
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+  /* Now, we trust cmsg_len and can use it to find the next header.  */
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   __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg
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 			       + CMSG_ALIGN (__cmsg->cmsg_len));
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-  if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control
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-					+ __mhdr->msg_controllen)
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-      || ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len)
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-	  > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen)))
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-    /* No more entries.  */
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-    return (struct cmsghdr *) 0;
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   return __cmsg;
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 }
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 #endif	/* Use `extern inline'.  */
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diff --git a/sysdeps/unix/sysv/linux/bits/socket.h b/sysdeps/unix/sysv/linux/bits/socket.h
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index c3fbb2110296273c..6b895b89831d2cb5 100644
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--- a/sysdeps/unix/sysv/linux/bits/socket.h
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+++ b/sysdeps/unix/sysv/linux/bits/socket.h
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@@ -302,6 +302,12 @@ struct cmsghdr
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 			 + CMSG_ALIGN (sizeof (struct cmsghdr)))
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 #define CMSG_LEN(len)   (CMSG_ALIGN (sizeof (struct cmsghdr)) + (len))
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+/* Given a length, return the additional padding necessary such that
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+   len + __CMSG_PADDING(len) == CMSG_ALIGN (len).  */
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+#define __CMSG_PADDING(len) ((sizeof (size_t) \
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+                              - ((len) & (sizeof (size_t) - 1))) \
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+                             & (sizeof (size_t) - 1))
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+
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 extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
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 				      struct cmsghdr *__cmsg) __THROW;
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 #ifdef __USE_EXTERN_INLINES
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@@ -311,18 +317,38 @@ extern struct cmsghdr *__cmsg_nxthdr (struct msghdr *__mhdr,
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 _EXTERN_INLINE struct cmsghdr *
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 __NTH (__cmsg_nxthdr (struct msghdr *__mhdr, struct cmsghdr *__cmsg))
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 {
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+  /* We may safely assume that __cmsg lies between __mhdr->msg_control and
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+     __mhdr->msg_controllen because the user is required to obtain the first
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+     cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
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+     via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
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+     trust the value of __cmsg->cmsg_len and therefore do not use it in any
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+     pointer arithmetic until we check its value.  */
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+
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+  unsigned char * __msg_control_ptr = (unsigned char *) __mhdr->msg_control;
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+  unsigned char * __cmsg_ptr = (unsigned char *) __cmsg;
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+
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+  size_t __size_needed = sizeof (struct cmsghdr)
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+                         + __CMSG_PADDING (__cmsg->cmsg_len);
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+
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+  /* The current header is malformed, too small to be a full header.  */
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   if ((size_t) __cmsg->cmsg_len < sizeof (struct cmsghdr))
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-    /* The kernel header does this so there may be a reason.  */
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     return (struct cmsghdr *) 0;
e38cb5
 
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+  /* There isn't enough space between __cmsg and the end of the buffer to
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+  hold the current cmsg *and* the next one.  */
e38cb5
+  if (((size_t)
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+         (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr)
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+       < __size_needed)
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+      || ((size_t)
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+            (__msg_control_ptr + __mhdr->msg_controllen - __cmsg_ptr
e38cb5
+             - __size_needed)
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+          < __cmsg->cmsg_len))
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+
e38cb5
+    return (struct cmsghdr *) 0;
e38cb5
+
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+  /* Now, we trust cmsg_len and can use it to find the next header.  */
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   __cmsg = (struct cmsghdr *) ((unsigned char *) __cmsg
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 			       + CMSG_ALIGN (__cmsg->cmsg_len));
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-  if ((unsigned char *) (__cmsg + 1) > ((unsigned char *) __mhdr->msg_control
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-					+ __mhdr->msg_controllen)
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-      || ((unsigned char *) __cmsg + CMSG_ALIGN (__cmsg->cmsg_len)
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-	  > ((unsigned char *) __mhdr->msg_control + __mhdr->msg_controllen)))
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-    /* No more entries.  */
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-    return (struct cmsghdr *) 0;
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   return __cmsg;
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 }
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 #endif	/* Use `extern inline'.  */
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diff --git a/sysdeps/unix/sysv/linux/cmsg_nxthdr.c b/sysdeps/unix/sysv/linux/cmsg_nxthdr.c
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index bab0be6884d9da1c..16594622211c1c8b 100644
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--- a/sysdeps/unix/sysv/linux/cmsg_nxthdr.c
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+++ b/sysdeps/unix/sysv/linux/cmsg_nxthdr.c
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@@ -23,18 +23,38 @@
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 struct cmsghdr *
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 __cmsg_nxthdr (struct msghdr *mhdr, struct cmsghdr *cmsg)
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 {
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+  /* We may safely assume that cmsg lies between mhdr->msg_control and
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+     mhdr->msg_controllen because the user is required to obtain the first
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+     cmsg via CMSG_FIRSTHDR, set its length, then obtain subsequent cmsgs
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+     via CMSG_NXTHDR, setting lengths along the way.  However, we don't yet
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+     trust the value of cmsg->cmsg_len and therefore do not use it in any
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+     pointer arithmetic until we check its value.  */
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+
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+  unsigned char * msg_control_ptr = (unsigned char *) mhdr->msg_control;
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+  unsigned char * cmsg_ptr = (unsigned char *) cmsg;
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+
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+  size_t size_needed = sizeof (struct cmsghdr)
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+                       + __CMSG_PADDING (cmsg->cmsg_len);
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+
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+  /* The current header is malformed, too small to be a full header.  */
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   if ((size_t) cmsg->cmsg_len < sizeof (struct cmsghdr))
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-    /* The kernel header does this so there may be a reason.  */
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-    return NULL;
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+    return (struct cmsghdr *) 0;
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+
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+  /* There isn't enough space between cmsg and the end of the buffer to
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+  hold the current cmsg *and* the next one.  */
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+  if (((size_t)
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+         (msg_control_ptr + mhdr->msg_controllen - cmsg_ptr)
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+       < size_needed)
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+      || ((size_t)
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+            (msg_control_ptr + mhdr->msg_controllen - cmsg_ptr
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+             - size_needed)
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+          < cmsg->cmsg_len))
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+
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+    return (struct cmsghdr *) 0;
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+  /* Now, we trust cmsg_len and can use it to find the next header.  */
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   cmsg = (struct cmsghdr *) ((unsigned char *) cmsg
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 			     + CMSG_ALIGN (cmsg->cmsg_len));
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-  if ((unsigned char *) (cmsg + 1) > ((unsigned char *) mhdr->msg_control
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-				      + mhdr->msg_controllen)
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-      || ((unsigned char *) cmsg + CMSG_ALIGN (cmsg->cmsg_len)
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-	  > ((unsigned char *) mhdr->msg_control + mhdr->msg_controllen)))
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-    /* No more entries.  */
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-    return NULL;
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   return cmsg;
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 }
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 libc_hidden_def (__cmsg_nxthdr)