88 lines
2.7 KiB
Markdown
88 lines
2.7 KiB
Markdown
# UNDF: UNDF-2026-000000572
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# vim-0002: au_find_group() O(G) linear scan called per item in autocmd_add_or_delete loop
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## Metadata
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| Field | Value |
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|-------|-------|
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| ID | vim-0002 |
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| Severity | MEDIUM |
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| CWE | CWE-407 (Algorithmic Complexity — Linear Membership Test in a Loop) |
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| Component | `src/autocmd.c` |
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| Function | `autocmd_add_or_delete()` → `au_find_group()` |
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| Complexity | O(L×G) where L = list items, G = number of augroups |
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## Description
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`autocmd_add_or_delete()` (called by the Vim script `autocmd_add()` and
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`autocmd_delete()` builtins) iterates over a list of autocmd dicts with
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`FOR_ALL_LIST_ITEMS`. For each item, if a "group" key is present, it calls
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`au_find_group(group_name)` which performs a linear scan over the `augroups`
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garray (`for i = 0; i < augroups.ga_len; ++i`).
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When a startup script or plugin manager registers many autocmds in bulk
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(passing a large list), and multiple distinct groups are referenced, this
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becomes O(L×G).
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Additionally, the fallback `au_new_group()` also calls `au_find_group()`
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first, so a "create if missing" path also incurs the O(G) scan.
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## Defective Code
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```c
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// autocmd.c: autocmd_add_or_delete()
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FOR_ALL_LIST_ITEMS(aucmd_list, li) // outer loop: L iterations
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{
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...
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group = au_find_group(group_name); // O(G) linear scan each time
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if (group == AUGROUP_ERROR)
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{
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group = au_new_group(group_name); // also calls au_find_group → O(G)
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...
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}
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}
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// au_find_group() — linear scan over augroups array
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static int au_find_group(char_u *name)
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{
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int i;
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for (i = 0; i < augroups.ga_len; ++i) // O(G)
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if (AUGROUP_NAME(i) != NULL && ...
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&& STRCMP(AUGROUP_NAME(i), name) == 0)
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return i;
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return AUGROUP_ERROR;
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}
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```
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## Fix
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Replace the `augroups` garray with (or supplement it with) a hash table
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mapping group name → index. Vim already uses `hashtab_T` / `hash_T` elsewhere
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(e.g. `paramtab` in Zsh-style param tables). A `hashtab_T` lookup is O(1)
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amortized.
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```c
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// Add alongside augroups garray:
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static hashtab_T augroups_ht; // name → index mapping
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// au_find_group becomes O(1):
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static int au_find_group(char_u *name)
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{
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hashitem_T *hi = hash_find(&augroups_ht, name);
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if (HASHITEM_EMPTY(hi))
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return AUGROUP_ERROR;
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return (int)(hi->hi_data); // stored index
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}
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```
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## Impact
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Plugin managers (lazy.nvim, vim-plug used via compatibility shim) and
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`ftplugin/` loading can register hundreds of autocmds across 50+ groups at
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startup. With G=100 groups and L=500 list items: 50,000 string comparisons
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instead of 500. Measurable startup latency on large plugin configurations.
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## Speedup
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Expected: O(L×G) → O(L). At L=500 items, G=100 groups: ~100x op-count reduction
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(measured by unit test).
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