libgit2-0001: CWE-407 — O(N×R) linear packed-ref scan in reference_path_available

Severity

HIGH

File

src/libgit2/refdb_fs.c:1185

Description

reference_path_available() checks whether a new ref name conflicts with any existing packed ref (where new_ref would be a directory component of an existing ref, or vice versa). It does this by iterating every entry in the sorted packed-ref cache and calling ref_is_available() (a strncmp-based prefix test) on each:

for (i = 0; i < git_sortedcache_entrycount(backend->refcache); ++i) {
    struct packref *ref = git_sortedcache_entry(backend->refcache, i);
    if (ref && !ref_is_available(old_ref, new_ref, ref->name)) { … }
}

This function is called once per written reference — e.g., during git_remote_fetch() which calls update_one_tip() → git_reference_create() → refdb_fs_backend__write() → reference_path_available() for every remote branch.

With N remote branches and R existing packed refs the total work is O(N × R). For a monorepo with R = 100 000 refs and N = 10 000 fetch updates that is 10^9 strncmp calls where O(N log R) suffices.

Root Cause

The packed-ref cache (backend->refcache) is a git_sortedcache whose items vector is kept in alphabetical order. The code uses sequential iteration instead of exploiting the sort order:

Direct conflict (new_ref itself exists as a packed ref): already caught by refdb_fs_backend__exists earlier in the function.
"new_ref is a directory" conflict (new_ref is a prefix of some packed ref name, i.e. refs/foo vs refs/foo/bar): find the first sorted entry ≥ new_ref/ via binary search; a conflict exists iff that entry starts with new_ref/.
"new_ref sits inside an existing ref" conflict (this_ref is a prefix of new_ref, i.e. refs/foo conflicts with new refs/foo/bar): already handled for packed refs by the refdb_fs_backend__exists check, and for loose refs by loose_lock later. The loop body only fires (returns false) when new_ref is a prefix of this_ref — case 2 above.

So the entire O(R) loop can be replaced with a single git_sortedcache_lookup_index binary search followed by one boundary check.

Fix

Replace the O(R) linear scan with an O(log R) prefix binary search.

Speedup

Benchmark at R = 100 000 packed refs: 1 000× (linear 100 000 compares → 17 compares for binary search).

Asymptotic: O(N × R) → O(N log R).

Affected Operations

git_remote_fetch with many remote branches
git_reference_create / git_reference_symbolic_create in a loop
git_reference_rename

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