broken hash use in fs/cifs/dfs_cache.c

Al Viro
2021-03-22T17:49:09Z

Found while trying to untangle some... unidiomatic string handling
in cifs:

static struct cache_entry *__lookup_cache_entry(const char *path)
{
struct cache_entry *ce;
unsigned int h;
bool found = false;

h = cache_entry_hash(path, strlen(path));

hlist_for_each_entry(ce, &cache_htable[h], hlist) {
if (!strcasecmp(path, ce->path)) {
found = true;
dump_ce(ce);
break;
}
}

if (!found)
ce = ERR_PTR(-ENOENT);
return ce;
}

combined with

static inline unsigned int cache_entry_hash(const void *data, int size)
{
unsigned int h;

h = jhash(data, size, 0);
return h & (CACHE_HTABLE_SIZE - 1);
}

That can't possibly work. The fundamental requirement for hashes is that
lookups for all keys matching an entry *MUST* lead to searches in the same
hash chain. Here the test is strcasecmp(), so "foo" and "Foo" are expected
to match the same entry. But jhash() yields different values on those -
it's a general-purpose hash, and it doesn't give a damn about upper and
lower case letters. Moreover, even though we look at the value modulo
32, I don't believe that it's going to be case-insensitive.

Either the key comparison or the hash function is wrong here. *IF* something
external guarantees the full match, we don't need strcasecmp() - strcmp()
would work. Otherwise, the hash function needs to be changed.