From 8df650c7c5adc2bb24a0077d8332f5ee342e7fd8 Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Fri, 26 Feb 2021 10:25:24 +0100
Subject: [PATCH] copy: handle copy_file_range() weirdness on procfs/sysfs
This addresses the issue described in https://lwn.net/Articles/846403/
and makes sure we will be able to stream bytes from procfs/sysfs via
copy_bytes() if people ask us to.
Based on: ee1aa61c4710ae567a2b844e0f0bb8cb0456ab8c
Related: #1970860
---
src/basic/copy.c | 75 +++++++++++++++++++++++++++++---------------
src/test/test-copy.c | 17 ++++++++++
2 files changed, 66 insertions(+), 26 deletions(-)
diff --git a/src/basic/copy.c b/src/basic/copy.c
index e06a503a29..a48c42c5c6 100644
--- a/src/basic/copy.c
+++ b/src/basic/copy.c
@@ -92,7 +92,7 @@ int copy_bytes_full(
void **ret_remains,
size_t *ret_remains_size) {
- bool try_cfr = true, try_sendfile = true, try_splice = true;
+ bool try_cfr = true, try_sendfile = true, try_splice = true, copied_something = false;
int r, nonblock_pipe = -1;
size_t m = SSIZE_MAX; /* that is the maximum that sendfile and c_f_r accept */
@@ -185,9 +185,20 @@ int copy_bytes_full(
try_cfr = false;
/* use fallback below */
- } else if (n == 0) /* EOF */
- break;
- else
+ } else if (n == 0) { /* likely EOF */
+
+ if (copied_something)
+ break;
+
+ /* So, we hit EOF immediately, without having copied a single byte. This
+ * could indicate two things: the file is actually empty, or we are on some
+ * virtual file system such as procfs/sysfs where the syscall actually
+ * doesn't work but doesn't return an error. Try to handle that, by falling
+ * back to simple read()s in case we encounter empty files.
+ *
+ * See: https://lwn.net/Articles/846403/ */
+ try_cfr = try_sendfile = try_splice = false;
+ } else
/* Success! */
goto next;
}
@@ -201,9 +212,14 @@ int copy_bytes_full(
try_sendfile = false;
/* use fallback below */
- } else if (n == 0) /* EOF */
+ } else if (n == 0) { /* likely EOF */
+
+ if (copied_something)
+ break;
+
+ try_sendfile = try_splice = false; /* same logic as above for copy_file_range() */
break;
- else
+ } else
/* Success! */
goto next;
}
@@ -213,14 +229,14 @@ int copy_bytes_full(
/* splice()'s asynchronous I/O support is a bit weird. When it encounters a pipe file
* descriptor, then it will ignore its O_NONBLOCK flag and instead only honour the
- * SPLICE_F_NONBLOCK flag specified in its flag parameter. Let's hide this behaviour here, and
- * check if either of the specified fds are a pipe, and if so, let's pass the flag
- * automatically, depending on O_NONBLOCK being set.
+ * SPLICE_F_NONBLOCK flag specified in its flag parameter. Let's hide this behaviour
+ * here, and check if either of the specified fds are a pipe, and if so, let's pass
+ * the flag automatically, depending on O_NONBLOCK being set.
*
- * Here's a twist though: when we use it to move data between two pipes of which one has
- * O_NONBLOCK set and the other has not, then we have no individual control over O_NONBLOCK
- * behaviour. Hence in that case we can't use splice() and still guarantee systematic
- * O_NONBLOCK behaviour, hence don't. */
+ * Here's a twist though: when we use it to move data between two pipes of which one
+ * has O_NONBLOCK set and the other has not, then we have no individual control over
+ * O_NONBLOCK behaviour. Hence in that case we can't use splice() and still guarantee
+ * systematic O_NONBLOCK behaviour, hence don't. */
if (nonblock_pipe < 0) {
int a, b;
@@ -238,12 +254,13 @@ int copy_bytes_full(
(a == FD_IS_BLOCKING_PIPE && b == FD_IS_NONBLOCKING_PIPE) ||
(a == FD_IS_NONBLOCKING_PIPE && b == FD_IS_BLOCKING_PIPE))
- /* splice() only works if one of the fds is a pipe. If neither is, let's skip
- * this step right-away. As mentioned above, if one of the two fds refers to a
- * blocking pipe and the other to a non-blocking pipe, we can't use splice()
- * either, hence don't try either. This hence means we can only use splice() if
- * either only one of the two fds is a pipe, or if both are pipes with the same
- * nonblocking flag setting. */
+ /* splice() only works if one of the fds is a pipe. If neither is,
+ * let's skip this step right-away. As mentioned above, if one of the
+ * two fds refers to a blocking pipe and the other to a non-blocking
+ * pipe, we can't use splice() either, hence don't try either. This
+ * hence means we can only use splice() if either only one of the two
+ * fds is a pipe, or if both are pipes with the same nonblocking flag
+ * setting. */
try_splice = false;
else
@@ -259,9 +276,13 @@ int copy_bytes_full(
try_splice = false;
/* use fallback below */
- } else if (n == 0) /* EOF */
- break;
- else
+ } else if (n == 0) { /* likely EOF */
+
+ if (copied_something)
+ break;
+
+ try_splice = false; /* same logic as above for copy_file_range() + sendfile() */
+ } else
/* Success! */
goto next;
}
@@ -312,11 +333,13 @@ int copy_bytes_full(
assert(max_bytes >= (uint64_t) n);
max_bytes -= n;
}
- /* sendfile accepts at most SSIZE_MAX-offset bytes to copy,
- * so reduce our maximum by the amount we already copied,
- * but don't go below our copy buffer size, unless we are
- * close the limit of bytes we are allowed to copy. */
+
+ /* sendfile accepts at most SSIZE_MAX-offset bytes to copy, so reduce our maximum by the
+ * amount we already copied, but don't go below our copy buffer size, unless we are close the
+ * limit of bytes we are allowed to copy. */
m = MAX(MIN(COPY_BUFFER_SIZE, max_bytes), m - n);
+
+ copied_something = true;
}
return 0; /* return 0 if we hit EOF earlier than the size limit */
diff --git a/src/test/test-copy.c b/src/test/test-copy.c
index 2e8d251ac1..29ac33e47a 100644
--- a/src/test/test-copy.c
+++ b/src/test/test-copy.c
@@ -253,6 +253,22 @@ static void test_copy_atomic(void) {
assert_se(copy_file_atomic("/etc/fstab", q, 0644, 0, COPY_REPLACE) >= 0);
}
+static void test_copy_proc(void) {
+ _cleanup_(rm_rf_physical_and_freep) char *p = NULL;
+ _cleanup_free_ char *f = NULL, *a = NULL, *b = NULL;
+
+ /* Check if copying data from /proc/ works correctly, i.e. let's see if https://lwn.net/Articles/846403/ is a problem for us */
+
+ assert_se(mkdtemp_malloc(NULL, &p) >= 0);
+ assert_se(f = path_join(NULL, p, "version"));
+ assert_se(copy_file("/proc/version", f, 0, (mode_t) -1, 0, 0) >= 0);
+
+ assert_se(read_one_line_file("/proc/version", &a) >= 0);
+ assert_se(read_one_line_file(f, &b) >= 0);
+ assert_se(streq(a, b));
+ assert_se(strlen(a) > 0);
+}
+
int main(int argc, char *argv[]) {
log_set_max_level(LOG_DEBUG);
@@ -267,6 +283,7 @@ int main(int argc, char *argv[]) {
test_copy_bytes_regular_file(argv[0], false, 32000); /* larger than copy buffer size */
test_copy_bytes_regular_file(argv[0], true, 32000);
test_copy_atomic();
+ test_copy_proc();
return 0;
}