fs: sendfile handles O_NONBLOCK of out_fd

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 b964bf53, it is stuck after b964bf53, 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: b964bf53 ("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>

fs/read_write.c

@@ -1263,6 +1263,9 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
 					  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);
 	}