[PATCH 2/2] fs/pipe.c: return error code rather than 0 in pipe_write()

[Eric Biggers <ebiggers3@gmail.com>]

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

diff --git a/fs/pipe.c b/fs/pipe.c
index 997de34..42cf8dd 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -366,18 +366,17 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 		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;
 		}
-- 
2.6.1