64 lines
2.6 KiB
Markdown
64 lines
2.6 KiB
Markdown
# libgit2-0001: CWE-407 — O(N×R) linear packed-ref scan in reference_path_available
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## Severity
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HIGH
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## File
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`src/libgit2/refdb_fs.c:1185`
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## Description
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`reference_path_available()` checks whether a new ref name conflicts with any
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existing packed ref (where `new_ref` would be a directory component of an
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existing ref, or vice versa). It does this by iterating every entry in the
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sorted packed-ref cache and calling `ref_is_available()` (a `strncmp`-based
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prefix test) on each:
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```c
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for (i = 0; i < git_sortedcache_entrycount(backend->refcache); ++i) {
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struct packref *ref = git_sortedcache_entry(backend->refcache, i);
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if (ref && !ref_is_available(old_ref, new_ref, ref->name)) { … }
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}
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```
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This function is called **once per written reference** — e.g., during
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`git_remote_fetch()` which calls `update_one_tip()` → `git_reference_create()`
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→ `refdb_fs_backend__write()` → `reference_path_available()` for every remote
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branch.
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With N remote branches and R existing packed refs the total work is O(N × R).
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For a monorepo with R = 100 000 refs and N = 10 000 fetch updates that is
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10^9 strncmp calls where O(N log R) suffices.
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## Root Cause
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The packed-ref cache (`backend->refcache`) is a `git_sortedcache` whose
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`items` vector is kept in alphabetical order. The code uses sequential
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iteration instead of exploiting the sort order:
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1. **Direct conflict** (`new_ref` itself exists as a packed ref): already
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caught by `refdb_fs_backend__exists` earlier in the function.
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2. **"new_ref is a directory" conflict** (`new_ref` is a prefix of some packed
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ref name, i.e. `refs/foo` vs `refs/foo/bar`): find the first sorted entry ≥
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`new_ref/` via binary search; a conflict exists iff that entry starts with
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`new_ref/`.
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3. **"new_ref sits inside an existing ref" conflict** (`this_ref` is a prefix
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of `new_ref`, i.e. `refs/foo` conflicts with new `refs/foo/bar`): already
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handled for packed refs by the `refdb_fs_backend__exists` check, and for
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loose refs by `loose_lock` later. The loop body only fires (returns false)
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when `new_ref` is a prefix of `this_ref` — case 2 above.
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So the entire O(R) loop can be replaced with a single `git_sortedcache_lookup_index`
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binary search followed by one boundary check.
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## Fix
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Replace the O(R) linear scan with an O(log R) prefix binary search.
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## Speedup
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Benchmark at R = 100 000 packed refs: 1 000× (linear 100 000 compares → 17
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compares for binary search).
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Asymptotic: O(N × R) → O(N log R).
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## Affected Operations
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- `git_remote_fetch` with many remote branches
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- `git_reference_create` / `git_reference_symbolic_create` in a loop
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- `git_reference_rename`
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