Out of bounds in `traceback.c` when `alloca()` is used over VLA

[fuhsnn]

Bug report

Bug description:

cpython/Python/traceback.c

Lines 42 to 48 in ebb150e

	#if defined(__STDC_NO_VLA__) && (__STDC_NO_VLA__ == 1)
	/* Use alloca() for VLAs. */
	# define VLA(type, name, size) type *name = alloca(size)
	#elif !defined(__STDC_NO_VLA__) || (__STDC_NO_VLA__ == 0)
	/* Use actual C VLAs.*/
	# define VLA(type, name, size) type name[size]
	#elif defined(CAN_C_BACKTRACE)

For the same size, VLA type[size] will allocate (sizeof(type) * size) but alloca(size) will only allocate (1 * size), which is significantly smaller than intended and will cause subsequent accesses of the allocation to be out of bounds, potentially corrupting the stack.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-145792: Fix incorrect alloca allocation size in traceback.c #145814

[sergey-miryanov]

@fuhsnn Would you like open a PR?