From 4a6ed30f96c0313dc55fc052707b33d4d3518912 Mon Sep 17 00:00:00 2001 From: Victoria Dye Date: Wed, 28 Sep 2022 17:19:00 +0000 Subject: [PATCH] read-cache: avoid misaligned reads in index v4 The process for reading the index into memory from disk is to first read its contents into a single memory-mapped file buffer (type 'char *'), then sequentially convert each on-disk index entry into a corresponding incore 'cache_entry'. To access the contents of the on-disk entry for processing, a moving pointer within the memory-mapped file is cast to type 'struct ondisk_cache_entry *'. In index v4, the entries in the on-disk index file are written *without* aligning their first byte to a 4-byte boundary; entries are a variable length (depending on the entry name and whether or not extended flags are used). As a result, casting the 'char *' buffer pointer to 'struct ondisk_cache_entry *' then accessing its contents in a 'SANITIZE=undefined' build can trigger the following error: read-cache.c:1886:46: runtime error: member access within misaligned address
for type 'struct ondisk_cache_entry', which requires 4 byte alignment Avoid this error by reading fields directly from the 'char *' buffer, using the 'offsetof' individual fields in 'struct ondisk_cache_entry'. Additionally, add documentation describing why the new approach avoids the misaligned address error, as well as advice on how to improve the implementation in the future. Reported-by: Jeff King Signed-off-by: Victoria Dye Signed-off-by: Junio C Hamano --- read-cache.c | 61 ++++++++++++++++++++++++++++++++++++---------------- 1 file changed, 42 insertions(+), 19 deletions(-) diff --git a/read-cache.c b/read-cache.c index b09128b188..3202402927 100644 --- a/read-cache.c +++ b/read-cache.c @@ -1873,9 +1873,20 @@ static int read_index_extension(struct index_state *istate, return 0; } +/* + * Parses the contents of the cache entry contained within the 'ondisk' buffer + * into a new incore 'cache_entry'. + * + * Note that 'char *ondisk' may not be aligned to a 4-byte address interval in + * index v4, so we cannot cast it to 'struct ondisk_cache_entry *' and access + * its members. Instead, we use the byte offsets of members within the struct to + * identify where 'get_be16()', 'get_be32()', and 'oidread()' (which can all + * read from an unaligned memory buffer) should read from the 'ondisk' buffer + * into the corresponding incore 'cache_entry' members. + */ static struct cache_entry *create_from_disk(struct mem_pool *ce_mem_pool, unsigned int version, - struct ondisk_cache_entry *ondisk, + const char *ondisk, unsigned long *ent_size, const struct cache_entry *previous_ce) { @@ -1883,7 +1894,7 @@ static struct cache_entry *create_from_disk(struct mem_pool *ce_mem_pool, size_t len; const char *name; const unsigned hashsz = the_hash_algo->rawsz; - const uint16_t *flagsp = (const uint16_t *)(ondisk->data + hashsz); + const char *flagsp = ondisk + offsetof(struct ondisk_cache_entry, data) + hashsz; unsigned int flags; size_t copy_len = 0; /* @@ -1901,15 +1912,15 @@ static struct cache_entry *create_from_disk(struct mem_pool *ce_mem_pool, if (flags & CE_EXTENDED) { int extended_flags; - extended_flags = get_be16(flagsp + 1) << 16; + extended_flags = get_be16(flagsp + sizeof(uint16_t)) << 16; /* We do not yet understand any bit out of CE_EXTENDED_FLAGS */ if (extended_flags & ~CE_EXTENDED_FLAGS) die(_("unknown index entry format 0x%08x"), extended_flags); flags |= extended_flags; - name = (const char *)(flagsp + 2); + name = (const char *)(flagsp + 2 * sizeof(uint16_t)); } else - name = (const char *)(flagsp + 1); + name = (const char *)(flagsp + sizeof(uint16_t)); if (expand_name_field) { const unsigned char *cp = (const unsigned char *)name; @@ -1935,20 +1946,32 @@ static struct cache_entry *create_from_disk(struct mem_pool *ce_mem_pool, ce = mem_pool__ce_alloc(ce_mem_pool, len); - ce->ce_stat_data.sd_ctime.sec = get_be32(&ondisk->ctime.sec); - ce->ce_stat_data.sd_mtime.sec = get_be32(&ondisk->mtime.sec); - ce->ce_stat_data.sd_ctime.nsec = get_be32(&ondisk->ctime.nsec); - ce->ce_stat_data.sd_mtime.nsec = get_be32(&ondisk->mtime.nsec); - ce->ce_stat_data.sd_dev = get_be32(&ondisk->dev); - ce->ce_stat_data.sd_ino = get_be32(&ondisk->ino); - ce->ce_mode = get_be32(&ondisk->mode); - ce->ce_stat_data.sd_uid = get_be32(&ondisk->uid); - ce->ce_stat_data.sd_gid = get_be32(&ondisk->gid); - ce->ce_stat_data.sd_size = get_be32(&ondisk->size); + /* + * NEEDSWORK: using 'offsetof()' is cumbersome and should be replaced + * with something more akin to 'load_bitmap_entries_v1()'s use of + * 'read_be16'/'read_be32'. For consistency with the corresponding + * ondisk entry write function ('copy_cache_entry_to_ondisk()'), this + * should be done at the same time as removing references to + * 'ondisk_cache_entry' there. + */ + ce->ce_stat_data.sd_ctime.sec = get_be32(ondisk + offsetof(struct ondisk_cache_entry, ctime) + + offsetof(struct cache_time, sec)); + ce->ce_stat_data.sd_mtime.sec = get_be32(ondisk + offsetof(struct ondisk_cache_entry, mtime) + + offsetof(struct cache_time, sec)); + ce->ce_stat_data.sd_ctime.nsec = get_be32(ondisk + offsetof(struct ondisk_cache_entry, ctime) + + offsetof(struct cache_time, nsec)); + ce->ce_stat_data.sd_mtime.nsec = get_be32(ondisk + offsetof(struct ondisk_cache_entry, mtime) + + offsetof(struct cache_time, nsec)); + ce->ce_stat_data.sd_dev = get_be32(ondisk + offsetof(struct ondisk_cache_entry, dev)); + ce->ce_stat_data.sd_ino = get_be32(ondisk + offsetof(struct ondisk_cache_entry, ino)); + ce->ce_mode = get_be32(ondisk + offsetof(struct ondisk_cache_entry, mode)); + ce->ce_stat_data.sd_uid = get_be32(ondisk + offsetof(struct ondisk_cache_entry, uid)); + ce->ce_stat_data.sd_gid = get_be32(ondisk + offsetof(struct ondisk_cache_entry, gid)); + ce->ce_stat_data.sd_size = get_be32(ondisk + offsetof(struct ondisk_cache_entry, size)); ce->ce_flags = flags & ~CE_NAMEMASK; ce->ce_namelen = len; ce->index = 0; - oidread(&ce->oid, ondisk->data); + oidread(&ce->oid, (const unsigned char *)ondisk + offsetof(struct ondisk_cache_entry, data)); if (expand_name_field) { if (copy_len) @@ -2117,12 +2140,12 @@ static unsigned long load_cache_entry_block(struct index_state *istate, unsigned long src_offset = start_offset; for (i = offset; i < offset + nr; i++) { - struct ondisk_cache_entry *disk_ce; struct cache_entry *ce; unsigned long consumed; - disk_ce = (struct ondisk_cache_entry *)(mmap + src_offset); - ce = create_from_disk(ce_mem_pool, istate->version, disk_ce, &consumed, previous_ce); + ce = create_from_disk(ce_mem_pool, istate->version, + mmap + src_offset, + &consumed, previous_ce); set_index_entry(istate, i, ce); src_offset += consumed;