96 lines
3.5 KiB
Markdown
96 lines
3.5 KiB
Markdown
# UNDF: UNDF-2026-000000591
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# libvirt-0001: virCPUx86UpdateLive() addedFeatures g_strv_contains O(F×A) per VM start/migration
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## Classification
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- **Severity**: MEDIUM
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- **CWE**: CWE-407 (Algorithmic Complexity — Inefficient Algorithmic Complexity)
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- **Component**: `src/cpu/cpu_x86.c`
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## Location
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`src/cpu/cpu_x86.c`, function `virCPUx86UpdateLive()`, lines 3189–3222
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```c
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for (i = 0; i < map->nfeatures; i++) { /* outer: ALL x86 features (~500) */
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virCPUx86Feature *feature = map->features[i];
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...
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if (!explicit &&
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model->addedFeatures &&
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g_strv_contains((const char **) model->addedFeatures, feature->name)) /* O(A) */
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ignore = true;
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...
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}
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```
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## Pattern
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`virCPUx86UpdateLive()` is called from `qemuProcessUpdateLiveGuestCPU()` on
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every QEMU VM start and live migration. It iterates all x86 CPU features in
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the global CPU map (`map->nfeatures` ≈ 500–800 on a modern host) and for each
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feature calls `g_strv_contains()` to test membership in
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`model->addedFeatures` — a `NULL`-terminated string array.
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`g_strv_contains()` is a GLib O(A) linear string scan. `A` = number of
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features added to the CPU model definition (e.g. Icelake-Server has ~20–50
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`addedFeatures`). Total: O(F × A) ≈ 500 × 50 = 25,000 string comparisons
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per VM start, per VM — with the virtio/KVM call chain holding libvirt
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driver-level locks.
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A secondary O(F×A) pattern exists in `qemuDomainDropAddedCPUFeatures()`
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called via `virCPUDefFilterFeatures()` during migration XML serialization
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(file `src/qemu/qemu_domain.c`, lines 5367–5371).
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## Call Path (semi-hot: every VM start + every live migration)
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```
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qemuProcessStart()
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qemuProcessFetchGuestCPU()
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qemuProcessUpdateLiveGuestCPU()
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virCPUUpdateLive()
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virCPUx86UpdateLive() # O(F×A) here
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for (i < map->nfeatures)
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g_strv_contains(model->addedFeatures, ...) # O(A)
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```
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## Speedup
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At F=500, A=50: 25,000 comparisons → 500 hash lookups (50× reduction).
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At F=800, A=100: 80,000 comparisons → 800 hash lookups (100× reduction).
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## Patch
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Convert `model->addedFeatures` from a `GStrv` (NULL-terminated `char**`) to
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a `GHashTable*` keyed by feature name for O(1) membership tests.
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```diff
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--- a/src/cpu/cpu_x86.c
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+++ b/src/cpu/cpu_x86.c
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@@ -187,7 +187,7 @@ struct _virCPUx86Model {
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char *name;
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virCPUx86Vendor *vendor;
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- GStrv addedFeatures;
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+ GHashTable *addedFeaturesSet; /* feature name → TRUE, for O(1) lookup */
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+ GStrv addedFeatures; /* kept for serialisation/API compat */
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virCPUx86Data data;
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char **blockers;
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};
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@@ -1745,6 +1745,8 @@ x86ModelParseCPUID(...)
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model->addedFeatures[nadded++] = g_strdup(ftname);
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+ if (!model->addedFeaturesSet)
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+ model->addedFeaturesSet = g_hash_table_new(g_str_hash, g_str_equal);
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+ g_hash_table_add(model->addedFeaturesSet, model->addedFeatures[nadded-1]);
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}
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@@ -1331,6 +1331,7 @@ x86ModelFree(virCPUx86Model *model)
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+ g_clear_pointer(&model->addedFeaturesSet, g_hash_table_unref);
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g_strfreev(model->addedFeatures);
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@@ -3219,7 +3219,7 @@ virCPUx86UpdateLive(...)
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if (!explicit &&
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- model->addedFeatures &&
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- g_strv_contains((const char **) model->addedFeatures, feature->name))
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+ model->addedFeaturesSet &&
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+ g_hash_table_contains(model->addedFeaturesSet, feature->name))
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ignore = true;
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```
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## Complexity
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- Before: O(F × A) — F = map->nfeatures (~500–800), A = addedFeatures count (~20–100)
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- After: O(F) — g_hash_table_contains is O(1) average
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