new_sync_write(): discard ->ki_pos unless the return value is positive

That allows ->write_iter() instances much more convenient life wrt
iocb->ki_pos (and fixes several filesystems with borderline POSIX
violations when zero-length write succeeds and changes the current
position).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/read_write.c b/fs/read_write.c
index 45d583c33879..819ef3faf1bb 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -477,7 +477,8 @@ static ssize_t new_sync_write(struct file *filp, const char __user *buf, size_t
 
         ret = filp->f_op->write_iter(&kiocb, &iter);
         BUG_ON(ret == -EIOCBQUEUED);
-        *ppos = kiocb.ki_pos;
+        if (ret > 0)
+                *ppos = kiocb.ki_pos;
         return ret;
 }