commit 73c7f5a87971de2797f261e1a447f68dce09284b
Author: Florian Weimer <fweimer@redhat.com>
Date:   Mon Sep 20 14:56:08 2021 +0200

    nptl: pthread_kill needs to return ESRCH for old programs (bug 19193)
    
    The fix for bug 19193 breaks some old applications which appear
    to use pthread_kill to probe if a thread is still running, something
    that is not supported by POSIX.
    
    (cherry picked from commit 95dba35bf05e4a5d69dfae5e9c9d4df3646a7f93)

diff --git a/nptl/pthread_kill.c b/nptl/pthread_kill.c
index fb7862eff787a94f..a44dc8f2d9baa925 100644
--- a/nptl/pthread_kill.c
+++ b/nptl/pthread_kill.c
@@ -21,8 +21,11 @@
 #include <pthreadP.h>
 #include <shlib-compat.h>
 
-int
-__pthread_kill_internal (pthread_t threadid, int signo)
+/* Sends SIGNO to THREADID.  If the thread is about to exit or has
+   already exited on the kernel side, return NO_TID.  Otherwise return
+   0 or an error code. */
+static int
+__pthread_kill_implementation (pthread_t threadid, int signo, int no_tid)
 {
   struct pthread *pd = (struct pthread *) threadid;
   if (pd == THREAD_SELF)
@@ -52,11 +55,8 @@ __pthread_kill_internal (pthread_t threadid, int signo)
        signal is either not observable (the target thread has already
        blocked signals at this point), or it will fail, or it might be
        delivered to a new, unrelated thread that has reused the TID.
-       So do not actually send the signal.  Do not report an error
-       because the threadid argument is still valid (the thread ID
-       lifetime has not ended), and ESRCH (for example) would be
-       misleading.  */
-    ret = 0;
+       So do not actually send the signal.  */
+    ret = no_tid;
   else
     {
       /* Using tgkill is a safety measure.  pd->exit_lock ensures that
@@ -71,6 +71,15 @@ __pthread_kill_internal (pthread_t threadid, int signo)
   return ret;
 }
 
+int
+__pthread_kill_internal (pthread_t threadid, int signo)
+{
+  /* Do not report an error in the no-tid case because the threadid
+     argument is still valid (the thread ID lifetime has not ended),
+     and ESRCH (for example) would be misleading.  */
+  return __pthread_kill_implementation (threadid, signo, 0);
+}
+
 int
 __pthread_kill (pthread_t threadid, int signo)
 {
@@ -81,6 +90,7 @@ __pthread_kill (pthread_t threadid, int signo)
 
   return __pthread_kill_internal (threadid, signo);
 }
+
 /* Some architectures (for instance arm) might pull raise through libgcc, so
    avoid the symbol version if it ends up being used on ld.so.  */
 #if !IS_IN(rtld)
@@ -88,6 +98,17 @@ libc_hidden_def (__pthread_kill)
 versioned_symbol (libc, __pthread_kill, pthread_kill, GLIBC_2_34);
 
 # if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_34)
-compat_symbol (libc, __pthread_kill, pthread_kill, GLIBC_2_0);
+/* Variant which returns ESRCH in the no-TID case, for backwards
+   compatibility.  */
+int
+attribute_compat_text_section
+__pthread_kill_esrch (pthread_t threadid, int signo)
+{
+  if (__is_internal_signal (signo))
+    return EINVAL;
+
+  return __pthread_kill_implementation (threadid, signo, ESRCH);
+}
+compat_symbol (libc, __pthread_kill_esrch, pthread_kill, GLIBC_2_0);
 # endif
 #endif
diff --git a/sysdeps/pthread/tst-pthread_kill-exited.c b/sysdeps/pthread/tst-pthread_kill-exited.c
index 7575fb6d58cae99c..a2fddad526666c8c 100644
--- a/sysdeps/pthread/tst-pthread_kill-exited.c
+++ b/sysdeps/pthread/tst-pthread_kill-exited.c
@@ -16,11 +16,15 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-/* This test verifies that pthread_kill returns 0 (and not ESRCH) for
-   a thread that has exited on the kernel side.  */
+/* This test verifies that the default pthread_kill returns 0 (and not
+   ESRCH) for a thread that has exited on the kernel side.  */
 
+#include <errno.h>
+#include <pthread.h>
+#include <shlib-compat.h>
 #include <signal.h>
 #include <stddef.h>
+#include <support/check.h>
 #include <support/support.h>
 #include <support/xthread.h>
 
@@ -30,6 +34,12 @@ noop_thread (void *closure)
   return NULL;
 }
 
+#if TEST_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_34) && PTHREAD_IN_LIBC
+extern __typeof (pthread_kill) compat_pthread_kill;
+compat_symbol_reference (libpthread, compat_pthread_kill, pthread_kill,
+                         GLIBC_2_0);
+#endif
+
 static int
 do_test (void)
 {
@@ -37,7 +47,14 @@ do_test (void)
 
   support_wait_for_thread_exit ();
 
+  /* NB: Always uses the default symbol due to separate compilation.  */
   xpthread_kill (thr, SIGUSR1);
+
+#if TEST_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_34) && PTHREAD_IN_LIBC
+  /* Old binaries need the non-conforming ESRCH error code.  */
+  TEST_COMPARE (compat_pthread_kill (thr, SIGUSR1), ESRCH);
+#endif
+
   xpthread_join (thr);
 
   return 0;