From 8bcda98da5db9003145674d8a7cbe7b81cab0d24 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Ren=C3=A9=20Scharfe?= Date: Sat, 14 Aug 2021 22:00:38 +0200 Subject: [PATCH] oidtree: avoid unaligned access to crit-bit tree MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The flexible array member "k" of struct cb_node is used to store the key of the crit-bit tree node. It offers no alignment guarantees -- in fact the current struct layout puts it one byte after a 4-byte aligned address, i.e. guaranteed to be misaligned. oidtree uses a struct object_id as cb_node key. Since cf0983213c (hash: add an algo member to struct object_id, 2021-04-26) it requires 4-byte alignment. The mismatch is reported by UndefinedBehaviorSanitizer at runtime like this: hash.h:277:2: runtime error: member access within misaligned address 0x00015000802d for type 'struct object_id', which requires 4 byte alignment 0x00015000802d: note: pointer points here 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ^ SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior hash.h:277:2 in We can fix that by: 1. eliminating the alignment requirement of struct object_id, 2. providing the alignment in struct cb_node, or 3. avoiding the issue by only using memcpy to access "k". Currently we only store one of two values in "algo" in struct object_id. We could use a uint8_t for that instead and widen it only once we add support for our twohundredth algorithm or so. That would not only avoid alignment issues, but also reduce the memory requirements for each instance of struct object_id by ca. 9%. Supporting keys with alignment requirements might be useful to spread the use of crit-bit trees. It can be achieved by using a wider type for "k" (e.g. uintmax_t), using different types for the members "byte" and "otherbits" (e.g. uint16_t or uint32_t for each), or by avoiding the use of flexible arrays like khash.h does. This patch implements the third option, though, because it has the least potential for causing side-effects and we're close to the next release. If one of the other options is implemented later as well to get their additional benefits we can get rid of the extra copies introduced here. Reported-by: Andrzej Hunt Signed-off-by: René Scharfe Signed-off-by: Junio C Hamano --- cbtree.h | 2 +- hash.h | 2 +- oidtree.c | 20 +++++++++++++++----- 3 files changed, 17 insertions(+), 7 deletions(-) diff --git a/cbtree.h b/cbtree.h index fe4587087e..a04a312c3f 100644 --- a/cbtree.h +++ b/cbtree.h @@ -25,7 +25,7 @@ struct cb_node { */ uint32_t byte; uint8_t otherbits; - uint8_t k[FLEX_ARRAY]; /* arbitrary data */ + uint8_t k[FLEX_ARRAY]; /* arbitrary data, unaligned */ }; struct cb_tree { diff --git a/hash.h b/hash.h index 27a180248f..9e25c40e9a 100644 --- a/hash.h +++ b/hash.h @@ -115,7 +115,7 @@ static inline void git_SHA256_Clone(git_SHA256_CTX *dst, const git_SHA256_CTX *s struct object_id { unsigned char hash[GIT_MAX_RAWSZ]; - int algo; + int algo; /* XXX requires 4-byte alignment */ }; /* A suitably aligned type for stack allocations of hash contexts. */ diff --git a/oidtree.c b/oidtree.c index 580cab8ae2..0d39389bee 100644 --- a/oidtree.c +++ b/oidtree.c @@ -31,12 +31,19 @@ void oidtree_clear(struct oidtree *ot) void oidtree_insert(struct oidtree *ot, const struct object_id *oid) { struct cb_node *on; + struct object_id k; if (!oid->algo) BUG("oidtree_insert requires oid->algo"); on = mem_pool_alloc(&ot->mem_pool, sizeof(*on) + sizeof(*oid)); - oidcpy_with_padding((struct object_id *)on->k, oid); + + /* + * Clear the padding and copy the result in separate steps to + * respect the 4-byte alignment needed by struct object_id. + */ + oidcpy_with_padding(&k, oid); + memcpy(on->k, &k, sizeof(k)); /* * n.b. Current callers won't get us duplicates, here. If a @@ -68,17 +75,20 @@ int oidtree_contains(struct oidtree *ot, const struct object_id *oid) static enum cb_next iter(struct cb_node *n, void *arg) { struct oidtree_iter_data *x = arg; - const struct object_id *oid = (const struct object_id *)n->k; + struct object_id k; - if (x->algo != GIT_HASH_UNKNOWN && x->algo != oid->algo) + /* Copy to provide 4-byte alignment needed by struct object_id. */ + memcpy(&k, n->k, sizeof(k)); + + if (x->algo != GIT_HASH_UNKNOWN && x->algo != k.algo) return CB_CONTINUE; if (x->last_nibble_at) { - if ((oid->hash[*x->last_nibble_at] ^ x->last_byte) & 0xf0) + if ((k.hash[*x->last_nibble_at] ^ x->last_byte) & 0xf0) return CB_CONTINUE; } - return x->fn(oid, x->arg); + return x->fn(&k, x->arg); } void oidtree_each(struct oidtree *ot, const struct object_id *oid,