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allocate a GC root value for the Value cache pointer

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keeping it as a simple data member means it won't be scanned by the GC, so
eventually the GC will collect a cache that is still referenced (resulting in
use-after-free of cache elements).

fixes #5962
This commit is contained in:
pennae 2022-01-22 21:17:35 +01:00
parent 5f08db69d1
commit fd5aa6ee3e
2 changed files with 11 additions and 6 deletions
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15
src/libexpr/eval.cc
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@ -435,6 +435,11 @@ EvalState::EvalState(
, store(store)
, buildStore(buildStore ? buildStore : store)
, regexCache(makeRegexCache())
#if HAVE_BOEHMGC
, valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
#else
, valueAllocCache(std::make_shared<void *>(nullptr))
#endif
, baseEnv(allocEnv(128))
, staticBaseEnv(false, 0)
{
@ -852,15 +857,15 @@ Value * EvalState::allocValue()
GC_malloc_many returns a linked list of objects of the given size, where the first word
of each object is also the pointer to the next object in the list. This also means that we
have to explicitly clear the first word of every object we take. */
if (!valueAllocCache) {
valueAllocCache = GC_malloc_many(sizeof(Value));
if (!valueAllocCache) throw std::bad_alloc();
if (!*valueAllocCache) {
*valueAllocCache = GC_malloc_many(sizeof(Value));
if (!*valueAllocCache) throw std::bad_alloc();
}
/* GC_NEXT is a convenience macro for accessing the first word of an object.
Take the first list item, advance the list to the next item, and clear the next pointer. */
void * p = valueAllocCache;
GC_PTR_STORE_AND_DIRTY(&valueAllocCache, GC_NEXT(p));
void * p = *valueAllocCache;
GC_PTR_STORE_AND_DIRTY(&*valueAllocCache, GC_NEXT(p));
GC_NEXT(p) = nullptr;
nrValues++;

2
src/libexpr/eval.hh
View file

@ -133,7 +133,7 @@ private:
std::shared_ptr<RegexCache> regexCache;
/* Allocation cache for GC'd Value objects. */
void * valueAllocCache = nullptr;
std::shared_ptr<void *> valueAllocCache;
public: