[PATCH 4.3 199/200] fs/pipe.c: return error code rather than 0 in pipe_write()

[Greg Kroah-Hartman]

4.3-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Biggers <ebiggers3@xxxxxxxxx>

commit 6ae08069939f17422835448acae76bda8d96b16a upstream.

pipe_write() would return 0 if it failed to merge the beginning of the
data to write with the last, partially filled pipe buffer.  It should
return an error code instead.  Userspace programs could be confused by
write() returning 0 when called with a nonzero 'count'.

The EFAULT error case was a regression from f0d1bec9d5 ("new helper:
copy_page_from_iter()"), while the ops->confirm() error case was a much
older bug.

Test program:

	#include <assert.h>
	#include <errno.h>
	#include <unistd.h>

	int main(void)
	{
		int fd[2];
		char data[1] = {0};

		assert(0 == pipe(fd));
		assert(1 == write(fd[1], data, 1));

		/* prior to this patch, write() returned 0 here  */
		assert(-1 == write(fd[1], NULL, 1));
		assert(errno == EFAULT);
	}

Signed-off-by: Eric Biggers <ebiggers3@xxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/pipe.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -366,18 +366,17 @@ pipe_write(struct kiocb *iocb, struct io
 		int offset = buf->offset + buf->len;
 
 		if (ops->can_merge && offset + chars <= PAGE_SIZE) {
-			int error = ops->confirm(pipe, buf);
-			if (error)
+			ret = ops->confirm(pipe, buf);
+			if (ret)
 				goto out;
 
 			ret = copy_page_from_iter(buf->page, offset, chars, from);
 			if (unlikely(ret < chars)) {
-				error = -EFAULT;
+				ret = -EFAULT;
 				goto out;
 			}
 			do_wakeup = 1;
-			buf->len += chars;
-			ret = chars;
+			buf->len += ret;
 			if (!iov_iter_count(from))
 				goto out;
 		}