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Subject[PATCH 5.18 08/88] fs: sendfile handles O_NONBLOCK of out_fd
Date
From: Andrei Vagin <avagin@gmail.com>

commit bdeb77bc2c405fa9f954c20269db175a0bd2793f upstream.

sendfile has to return EAGAIN if out_fd is nonblocking and the write into
it would block.

Here is a small reproducer for the problem:

#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/sendfile.h>


#define FILE_SIZE (1UL << 30)
int main(int argc, char **argv) {
int p[2], fd;

if (pipe2(p, O_NONBLOCK))
return 1;

fd = open(argv[1], O_RDWR | O_TMPFILE, 0666);
if (fd < 0)
return 1;
ftruncate(fd, FILE_SIZE);

if (sendfile(p[1], fd, 0, FILE_SIZE) == -1) {
fprintf(stderr, "FAIL\n");
}
if (sendfile(p[1], fd, 0, FILE_SIZE) != -1 || errno != EAGAIN) {
fprintf(stderr, "FAIL\n");
}
return 0;
}

It worked before b964bf53e540, it is stuck after b964bf53e540, and it
works again with this fix.

This regression occurred because do_splice_direct() calls pipe_write
that handles O_NONBLOCK. Here is a trace log from the reproducer:

1) | __x64_sys_sendfile64() {
1) | do_sendfile() {
1) | __fdget()
1) | rw_verify_area()
1) | __fdget()
1) | rw_verify_area()
1) | do_splice_direct() {
1) | rw_verify_area()
1) | splice_direct_to_actor() {
1) | do_splice_to() {
1) | rw_verify_area()
1) | generic_file_splice_read()
1) + 74.153 us | }
1) | direct_splice_actor() {
1) | iter_file_splice_write() {
1) | __kmalloc()
1) 0.148 us | pipe_lock();
1) 0.153 us | splice_from_pipe_next.part.0();
1) 0.162 us | page_cache_pipe_buf_confirm();
... 16 times
1) 0.159 us | page_cache_pipe_buf_confirm();
1) | vfs_iter_write() {
1) | do_iter_write() {
1) | rw_verify_area()
1) | do_iter_readv_writev() {
1) | pipe_write() {
1) | mutex_lock()
1) 0.153 us | mutex_unlock();
1) 1.368 us | }
1) 1.686 us | }
1) 5.798 us | }
1) 6.084 us | }
1) 0.174 us | kfree();
1) 0.152 us | pipe_unlock();
1) + 14.461 us | }
1) + 14.783 us | }
1) 0.164 us | page_cache_pipe_buf_release();
... 16 times
1) 0.161 us | page_cache_pipe_buf_release();
1) | touch_atime()
1) + 95.854 us | }
1) + 99.784 us | }
1) ! 107.393 us | }
1) ! 107.699 us | }

Link: https://lkml.kernel.org/r/20220415005015.525191-1-avagin@gmail.com
Fixes: b964bf53e540 ("teach sendfile(2) to handle send-to-pipe directly")
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/read_write.c | 3 +++
1 file changed, 3 insertions(+)

--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1247,6 +1247,9 @@ static ssize_t do_sendfile(int out_fd, i
count, fl);
file_end_write(out.file);
} else {
+ if (out.file->f_flags & O_NONBLOCK)
+ fl |= SPLICE_F_NONBLOCK;
+
retval = splice_file_to_pipe(in.file, opipe, &pos, count, fl);
}


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