libfs: make simple_read_from_buffer conventional

Impact: have simple_read_from_buffer conform to standards

It was brought to my attention by Andrew Morton, Theodore Tso, and H.
Peter Anvin that a read from userspace should only return -EFAULT if
nothing was actually read.

Looking at the simple_read_from_buffer I noticed that this function does
not conform to that rule.  This patch fixes that function.

[akpm@linux-foundation.org: simplification suggested by hpa]
[hpa@zytor.com: fix count==0 handling]
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 6 insertions, 2 deletions

diff --git a/fs/libfs.c b/fs/libfs.c
index dcec3d3ea64f..662a28e4f667 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -527,14 +527,18 @@ ssize_t simple_read_from_buffer(void __user *to, size_t count, loff_t *ppos,
                                 const void *from, size_t available)
 {
         loff_t pos = *ppos;
+        size_t ret;
+
         if (pos < 0)
                 return -EINVAL;
-        if (pos >= available)
+        if (pos >= available || !count)
                 return 0;
         if (count > available - pos)
                 count = available - pos;
-        if (copy_to_user(to, from + pos, count))
+        ret = copy_to_user(to, from + pos, count);
+        if (ret == count)
                 return -EFAULT;
+        count -= ret;
         *ppos = pos + count;
         return count;
 }