[PATCH] fs: nfsd: Fix bug in compare_blob

From: Rasmus Villemoes
Date: Thu Dec 04 2014 - 10:21:19 EST

Bugs similar to the one in acbbe6fbb240 (kcmp: fix standard comparison
bug) are in rich supply.

In this variant, the problem is that struct xdr_netobj::len has type
unsigned int, so the expression o1->len - o2->len _also_ has type
unsigned int; it has completely well-defined semantics, and the result
is some non-negative integer, which is always representable in a long
long. But this means that if the conditional triggers, we are
guaranteed to return a positive value from compare_blob.

In this case it could be fixed by

- res = o1->len - o2->len;
+ res = (long long)o1->len - (long long)o2->len;

but I'd rather eliminate the usually broken 'return a - b;' idiom.

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>

How this could ever have worked is beyond me - compare_blob seems to
be used to maintain an rbtree, and I wouldn't expect rbtrees to behave
well if the comparison function doesn't satisfy the basic invariant
sign(cmp(a, b)) == -sign(cmp(b, a)).

fs/nfsd/nfs4state.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index e9c3afe4b5d3..d504cd6927f8 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1711,15 +1711,14 @@ static int copy_cred(struct svc_cred *target, struct svc_cred *source)
return 0;

-static long long
+static int
compare_blob(const struct xdr_netobj *o1, const struct xdr_netobj *o2)
- long long res;
- res = o1->len - o2->len;
- if (res)
- return res;
- return (long long)memcmp(o1->data, o2->data, o1->len);
+ if (o1->len < o2->len)
+ return -1;
+ if (o1->len > o2->len)
+ return 1;
+ return memcmp(o1->data, o2->data, o1->len);

static int same_name(const char *n1, const char *n2)
@@ -1907,7 +1906,7 @@ add_clp_to_name_tree(struct nfs4_client *new_clp, struct rb_root *root)
static struct nfs4_client *
find_clp_in_name_tree(struct xdr_netobj *name, struct rb_root *root)
- long long cmp;
+ int cmp;
struct rb_node *node = root->rb_node;
struct nfs4_client *clp;


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