From: Victoria Dye <vdye@xxxxxxxxxx>
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 <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'.
Reported-by: Jeff King <peff@xxxxxxxx>
Signed-off-by: Victoria Dye <vdye@xxxxxxxxxx>
---
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+@xxxxxxxxxxxxxxxxxxxxxxx/
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
--
gitgitgadget
