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