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 <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 <peff@peff.net>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/read-cache.c b/read-cache.c
index b09128b188431857450336481898f27f8894a241..32024029274828c12654e3c256de3d8d08f37734 100644 (file)
--- 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;