From: Phillip Wood Date: Sun, 25 Sep 2022 08:25:08 GMT Subject: Re: [PATCH] read-cache: avoid misaligned reads in index v4 Message-ID: In-Reply-To: Hi Victoria On 23/09/2022 20:43, Victoria Dye via GitGitGadget wrote: > From: Victoria Dye > > 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'. I was confused as to why this was safe as it means we're still reading the individual fields from unaligned addresses. The reason it is safe is that we use get_bexx() to read the fields and those functions handle unaligned access. I wonder if it is worth adding a note to clarify that if you re-roll. Best Wishes Phillip > > Reported-by: Jeff King > Signed-off-by: Victoria Dye > --- > read-cache: avoid misaligned reads in index v4 > > This fixes the bug reported in [1], where unaligned index entries in the > memory-mapped index file triggered a 'SANITIZE=undefined' error due to > casting to & accessing unaligned data a 4 byte-aligned 'struct > ondisk_cache_entry *' type. > > In addition to the originally-reported 't9210-scalar.sh' now passing in > a 'SANITIZE=undefined' build, I did some light testing by first writing > a v4 index with a released version of Git (v2.37), then running some > index-modifying operations ('git status', 'git add') with this patch's > changes, then again running 'git status' with the stable version. I > didn't see anything out of the ordinary but, considering how critical > "reading the index" is, I'd very much appreciate some extra-thorough > reviews on this patch. :) > > Thanks! > > * Victoria > > [1] > https://lore.kernel.org/git/YywzNTzd72tox8Z+@coredump.intra.peff.net/ > > Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1366%2Fvdye%2Fbugfix%2Findex-v4-v1 > Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1366/vdye/bugfix/index-v4-v1 > Pull-Request: https://github.com/gitgitgadget/git/pull/1366 > > read-cache.c | 42 +++++++++++++++++++++++------------------- > 1 file changed, 23 insertions(+), 19 deletions(-) > > diff --git a/read-cache.c b/read-cache.c > index b09128b1884..d16eb979060 100644 > --- a/read-cache.c > +++ b/read-cache.c > @@ -1875,7 +1875,7 @@ static int read_index_extension(struct index_state *istate, > > 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 +1883,7 @@ 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 +1901,15 @@ 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 +1935,24 @@ 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); > + 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 +2121,12 @@ 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; > > base-commit: 1b3d6e17fe83eb6f79ffbac2f2c61bbf1eaef5f8