Doc/c-api/memory.rst: extend --without-pymalloc doc with ASan information

[disconnect3d]

This commit extends the documentation for disabling pymalloc with the --without-pymalloc flag regarding why it is worth to use it when enabling AddressSanitizer for Python build (which is done, e.g., in CPython's CI builds).

I have tested the CPython latest main build with both ASan and pymalloc enabled and it seems to work just fine. I did run the python -m test suite which didn't uncover any ASan crashes (though, it detected some memory leaks, which I believe are irrelevant here).

I have discussed ASan and this flag with @encukou on the CPython Core sprint on EuroPython 2025. We initially thought that the --without-pymalloc flag is needed for ASan builds due to the fact pymalloc must hit the begining of page when determining if the memory to be freed comes from pymalloc or was allocated by the system malloc. In other words, we thought, that ASan would crash CPython during free of big objects (allocated by system malloc). It may be that this was the case in the past, but it is not the case anymore as the address_in_range function used by pymalloc is annotated to be skipped from the ASan instrumentation.

This code can be seen here:

cpython/Objects/obmalloc.c

Lines 2096 to 2110 in acefb97

	static bool _Py_NO_SANITIZE_ADDRESS
	_Py_NO_SANITIZE_THREAD
	_Py_NO_SANITIZE_MEMORY
	address_in_range(OMState *state, void *p, poolp pool)
	{
	// Since address_in_range may be reading from memory which was not allocated
	// by Python, it is important that pool->arenaindex is read only once, as
	// another thread may be concurrently modifying the value without holding
	// the GIL. The following dance forces the compiler to read pool->arenaindex
	// only once.
	uint arenaindex = *((volatile uint *)&pool->arenaindex);
	return arenaindex < maxarenas &&
	(uintptr_t)p - allarenas[arenaindex].address < ARENA_SIZE &&
	allarenas[arenaindex].address != 0;
	}

While the annotation macro is defined here:

cpython/Include/pyport.h

Lines 582 to 598 in acefb97

	# if __has_feature(address_sanitizer)
	# if !defined(_Py_ADDRESS_SANITIZER)
	# define _Py_ADDRESS_SANITIZER
	# define _Py_NO_SANITIZE_ADDRESS __attribute__((no_sanitize_address))
	# endif
	# endif
	# if __has_feature(thread_sanitizer)
	# if !defined(_Py_THREAD_SANITIZER)
	# define _Py_THREAD_SANITIZER
	# define _Py_NO_SANITIZE_THREAD __attribute__((no_sanitize_thread))
	# endif
	# endif
	#elif defined(__GNUC__)
	# if defined(__SANITIZE_ADDRESS__)
	# define _Py_ADDRESS_SANITIZER
	# define _Py_NO_SANITIZE_ADDRESS __attribute__((no_sanitize_address))
	# endif

And the corresponding attribute is documented in:

• for gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-no_005fsanitize_005faddress-function-attribute
• for clang: https://clang.llvm.org/docs/AttributeReference.html#no-sanitize-address-no-address-safety-analysis

---

📚 Documentation preview 📚: https://cpython-previews--136790.org.readthedocs.build/

[encukou]

Thank you the PR, and for taking the time to make it this short!

[miss-islington-app]

Thanks @disconnect3d for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[miss-islington-app]

Thanks @disconnect3d for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-136798 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-136799 is a backport of this pull request to the 3.13 branch.