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![Jeff King](/assets/img/avatar_default.png)
A pack and its matching .idx file are limited to 2^32 objects, because the pack format contains a 32-bit field to store the number of objects. Hence we use uint32_t in the code. But the byte count of even a .idx file can be much larger than that, because it stores at least a hash and an offset for each object. So using SHA-1, a v2 .idx file will cross the 4GB boundary at 153,391,650 objects. This confuses load_idx(), which computes the minimum size like this: unsigned long min_size = 8 + 4*256 + nr*(hashsz + 4 + 4) + hashsz + hashsz; Even though min_size will be big enough on most 64-bit platforms, the actual arithmetic is done as a uint32_t, resulting in a truncation. We actually exceed that min_size, but then we do: unsigned long max_size = min_size; if (nr) max_size += (nr - 1)*8; to account for the variable-sized table. That computation doesn't overflow quite so low, but with the truncation for min_size, we end up with a max_size that is much smaller than our actual size. So we complain that the idx is invalid, and can't find any of its objects. We can fix this case by casting "nr" to a size_t, which will do the multiplication in 64-bits (assuming you're on a 64-bit platform; this will never work on a 32-bit system since we couldn't map the whole .idx anyway). Likewise, we don't have to worry about further additions, because adding a smaller number to a size_t will convert the other side to a size_t. A few notes: - obviously we could just declare "nr" as a size_t in the first place (and likewise, packed_git.num_objects). But it's conceptually a uint32_t because of the on-disk format, and we correctly treat it that way in other contexts that don't need to compute byte offsets (e.g., iterating over the set of objects should and generally does use a uint32_t). Switching to size_t would make all of those other cases look wrong. - it could be argued that the proper type is off_t to represent the file offset. But in practice the .idx file must fit within memory, because we mmap the whole thing. And the rest of the code (including the idx_size variable we're comparing against) uses size_t. - we'll add the same cast to the max_size arithmetic line. Even though we're adding to a larger type, which will convert our result, the multiplication is still done as a 32-bit value and can itself overflow. I didn't check this with my test case, since it would need an even larger pack (~530M objects), but looking at compiler output shows that it works this way. The standard should agree, but I couldn't find anything explicit in 6.3.1.8 ("usual arithmetic conversions"). The case in load_idx() was the most immediate one that I was able to trigger. After fixing it, looking up actual objects (including the very last one in sha1 order) works in a test repo with 153,725,110 objects. That's because bsearch_hash() works with uint32_t entry indices, and the actual byte access: int cmp = hashcmp(table + mi * stride, sha1); is done with "stride" as a size_t, causing the uint32_t "mi" to be promoted to a size_t. This is the way most code will access the index data. However, I audited all of the other byte-wise accesses of packed_git.index_data, and many of the others are suspect (they are similar to the max_size one, where we are adding to a properly sized offset or directly to a pointer, but the multiplication in the sub-expression can overflow). I didn't trigger any of these in practice, but I believe they're potential problems, and certainly adding in the cast is not going to hurt anything here. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
206 lines
5.8 KiB
C
206 lines
5.8 KiB
C
#include "cache.h"
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#include "pack-revindex.h"
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#include "object-store.h"
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#include "packfile.h"
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/*
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* Pack index for existing packs give us easy access to the offsets into
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* corresponding pack file where each object's data starts, but the entries
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* do not store the size of the compressed representation (uncompressed
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* size is easily available by examining the pack entry header). It is
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* also rather expensive to find the sha1 for an object given its offset.
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*
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* The pack index file is sorted by object name mapping to offset;
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* this revindex array is a list of offset/index_nr pairs
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* ordered by offset, so if you know the offset of an object, next offset
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* is where its packed representation ends and the index_nr can be used to
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* get the object sha1 from the main index.
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*/
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/*
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* This is a least-significant-digit radix sort.
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*
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* It sorts each of the "n" items in "entries" by its offset field. The "max"
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* parameter must be at least as large as the largest offset in the array,
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* and lets us quit the sort early.
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*/
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static void sort_revindex(struct revindex_entry *entries, unsigned n, off_t max)
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{
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/*
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* We use a "digit" size of 16 bits. That keeps our memory
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* usage reasonable, and we can generally (for a 4G or smaller
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* packfile) quit after two rounds of radix-sorting.
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*/
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#define DIGIT_SIZE (16)
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#define BUCKETS (1 << DIGIT_SIZE)
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/*
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* We want to know the bucket that a[i] will go into when we are using
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* the digit that is N bits from the (least significant) end.
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*/
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#define BUCKET_FOR(a, i, bits) (((a)[(i)].offset >> (bits)) & (BUCKETS-1))
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/*
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* We need O(n) temporary storage. Rather than do an extra copy of the
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* partial results into "entries", we sort back and forth between the
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* real array and temporary storage. In each iteration of the loop, we
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* keep track of them with alias pointers, always sorting from "from"
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* to "to".
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*/
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struct revindex_entry *tmp, *from, *to;
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int bits;
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unsigned *pos;
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ALLOC_ARRAY(pos, BUCKETS);
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ALLOC_ARRAY(tmp, n);
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from = entries;
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to = tmp;
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/*
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* If (max >> bits) is zero, then we know that the radix digit we are
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* on (and any higher) will be zero for all entries, and our loop will
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* be a no-op, as everybody lands in the same zero-th bucket.
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*/
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for (bits = 0; max >> bits; bits += DIGIT_SIZE) {
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unsigned i;
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memset(pos, 0, BUCKETS * sizeof(*pos));
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/*
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* We want pos[i] to store the index of the last element that
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* will go in bucket "i" (actually one past the last element).
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* To do this, we first count the items that will go in each
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* bucket, which gives us a relative offset from the last
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* bucket. We can then cumulatively add the index from the
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* previous bucket to get the true index.
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*/
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for (i = 0; i < n; i++)
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pos[BUCKET_FOR(from, i, bits)]++;
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for (i = 1; i < BUCKETS; i++)
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pos[i] += pos[i-1];
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/*
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* Now we can drop the elements into their correct buckets (in
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* our temporary array). We iterate the pos counter backwards
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* to avoid using an extra index to count up. And since we are
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* going backwards there, we must also go backwards through the
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* array itself, to keep the sort stable.
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*
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* Note that we use an unsigned iterator to make sure we can
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* handle 2^32-1 objects, even on a 32-bit system. But this
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* means we cannot use the more obvious "i >= 0" loop condition
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* for counting backwards, and must instead check for
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* wrap-around with UINT_MAX.
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*/
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for (i = n - 1; i != UINT_MAX; i--)
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to[--pos[BUCKET_FOR(from, i, bits)]] = from[i];
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/*
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* Now "to" contains the most sorted list, so we swap "from" and
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* "to" for the next iteration.
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*/
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SWAP(from, to);
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}
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/*
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* If we ended with our data in the original array, great. If not,
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* we have to move it back from the temporary storage.
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*/
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if (from != entries)
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COPY_ARRAY(entries, tmp, n);
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free(tmp);
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free(pos);
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#undef BUCKET_FOR
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#undef BUCKETS
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#undef DIGIT_SIZE
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}
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/*
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* Ordered list of offsets of objects in the pack.
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*/
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static void create_pack_revindex(struct packed_git *p)
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{
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const unsigned num_ent = p->num_objects;
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unsigned i;
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const char *index = p->index_data;
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const unsigned hashsz = the_hash_algo->rawsz;
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ALLOC_ARRAY(p->revindex, num_ent + 1);
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index += 4 * 256;
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if (p->index_version > 1) {
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const uint32_t *off_32 =
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(uint32_t *)(index + 8 + (size_t)p->num_objects * (hashsz + 4));
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const uint32_t *off_64 = off_32 + p->num_objects;
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for (i = 0; i < num_ent; i++) {
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const uint32_t off = ntohl(*off_32++);
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if (!(off & 0x80000000)) {
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p->revindex[i].offset = off;
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} else {
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p->revindex[i].offset = get_be64(off_64);
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off_64 += 2;
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}
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p->revindex[i].nr = i;
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}
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} else {
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for (i = 0; i < num_ent; i++) {
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const uint32_t hl = *((uint32_t *)(index + (hashsz + 4) * i));
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p->revindex[i].offset = ntohl(hl);
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p->revindex[i].nr = i;
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}
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}
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/*
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* This knows the pack format -- the hash trailer
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* follows immediately after the last object data.
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*/
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p->revindex[num_ent].offset = p->pack_size - hashsz;
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p->revindex[num_ent].nr = -1;
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sort_revindex(p->revindex, num_ent, p->pack_size);
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}
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int load_pack_revindex(struct packed_git *p)
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{
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if (!p->revindex) {
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if (open_pack_index(p))
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return -1;
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create_pack_revindex(p);
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}
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return 0;
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}
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int find_revindex_position(struct packed_git *p, off_t ofs)
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{
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int lo = 0;
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int hi = p->num_objects + 1;
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const struct revindex_entry *revindex = p->revindex;
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do {
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const unsigned mi = lo + (hi - lo) / 2;
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if (revindex[mi].offset == ofs) {
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return mi;
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} else if (ofs < revindex[mi].offset)
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hi = mi;
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else
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lo = mi + 1;
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} while (lo < hi);
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error("bad offset for revindex");
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return -1;
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}
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struct revindex_entry *find_pack_revindex(struct packed_git *p, off_t ofs)
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{
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int pos;
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if (load_pack_revindex(p))
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return NULL;
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pos = find_revindex_position(p, ofs);
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if (pos < 0)
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return NULL;
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return p->revindex + pos;
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}
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