[PATCH] Fix range checks in kernfs_get_target_path

[Bernd Edlinger]

The strncpy causes a warning [-Wstringop-truncation] here,
which indicates that it never appends a NUL byte to the path.
The NUL byte is only there because the buffer is allocated
with kzalloc(PAGE_SIZE, GFP_KERNEL), but since the range-check
is also off-by-one, and PAGE_SIZE==PATH_MAX the returned string
will not be zero-terminated if it is exactly PATH_MAX characters.
Furthermore also the initial loop may theoretically exceed PATH_MAX
and cause a fault.

Signed-off-by: Bernd Edlinger <bernd.edlin...@hotmail.de>
---
  fs/kernfs/symlink.c | 10 +++++++---
  1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
index 08ccabd..c8b7d44a 100644
--- a/fs/kernfs/symlink.c
+++ b/fs/kernfs/symlink.c
@@ -63,7 +63,10 @@ static int kernfs_get_target_path(struct kernfs_node 
*parent,
                if (base == kn)
                        break;

-               strcpy(s, "../");
+               if ((s - path) + 3 >= PATH_MAX)
+                       return -ENAMETOOLONG;
+
+               memcpy(s, "../", 3);
                s += 3;
                base = base->parent;
        }
@@ -79,16 +82,17 @@ static int kernfs_get_target_path(struct kernfs_node 
*parent,
        if (len < 2)
                return -EINVAL;
        len--;
-       if ((s - path) + len > PATH_MAX)
+       if ((s - path) + len >= PATH_MAX)
                return -ENAMETOOLONG;

        /* reverse fillup of target string from target to base */
        kn = target;
+       s[len] = '\0';
        while (kn->parent && kn != base) {
                int slen = strlen(kn->name);

                len -= slen;
-               strncpy(s + len, kn->name, slen);
+               memcpy(s + len, kn->name, slen);
                if (len)
                        s[--len] = '/';

-- 
1.9.1