Thread

Re: PoC: VALGRIND_MAKE_MEM_NOACCESS for dynamic shared memory

Andres Freund <andres@anarazel.de> — 2026-05-04T13:56:28Z

Hi,

On 2026-05-04 10:18:31 +0200, Tomas Vondra wrote:
> A couple of days ago we got a report regarding an incorrect shmem size
> calculation in btree [1], leading to a buffer overflow / memory
> corruption. Which became a much less common issue in our code, thanks to
> valgrind and similar tools. But it took me a while to realize valgrind
> won't catch this because we only instrument private memory (palloc et
> al), while shmem is left alone.
> 
> I was wondering if it's feasible to improve this. Attached is a trivial
> patch that adjusts shm_toc.c to add a couple NOACCESS bytes after each
> entry in the segment. It seems to do the trick - with this we get a
> reasonable report (for the reproducer provided in the bug report, before
> it got fixed by 748d871b7c) from valgrind, with invalid accesses. See
> the attached .log for an example. It's much better than the confusing
> crashes due to corrupted state.
> 
> There's an issue, though. It seems the valgrind memory markings are not
> shared between processes. The leader sets the shm_toc up, marks the
> ranges as NOACCESS, and then checks it while accessing the memory. But
> the parallel workers don't seem to see this, and so will produce no
> reports. I'm assuming this is the case, because all the reports come
> from the leader, never from the workers. Maybe there's a different
> explanation (e.g. maybe it's just the leader touching the memory?).

I assume the issue is just that the workers don't have the NOACCESS markers? I
think you'd need to do them in every process using the shm_toc.  Either by
doing it in shm_toc_attach() or in shm_toc().



> An alternative would be to do mprotect(), but unfortunately it requires
> page-aligned ranges, which makes it somewhat useless for small overflows
> of a couple bytes (like here). It should work cross-process, I think,

It doesn't work across processes. Every process has their own mprotect "view".


> FWIW the PoC patch adds a 32-byte chunk, not just a single byte. This is
> intentional, because if the state is an array, it's quite possible the
> invalid access steps over a fair number of bytes. I'm actually thinking
> it should be even larger.
> 
> This modifies just the dynamic shmem, but maybe we should do this even
> for the "regular" shmem allocated at start. Issues in that would likely
> cause crashes pretty quick (unlike the btree issue, which requires a
> somewhat special reproducer), but a nice valgrind report helps.

I can tell you from experience that no, it's not necessarily quickly caught.
So yes, I think we should definitely do that.



>  /*
>   * Allocate shared memory from a segment managed by a table of contents.
>   *
> @@ -119,9 +127,19 @@ shm_toc_allocate(shm_toc *toc, Size nbytes)
>  	}
>  	vtoc->toc_allocated_bytes += nbytes;
>  
> +#ifdef USE_VALGRIND
> +	vtoc->toc_allocated_bytes += NUM_NOACCESS_BYTES;
> +#endif
> +
>  	SpinLockRelease(&toc->toc_mutex);
>  
> -	return ((char *) toc) + (total_bytes - allocated_bytes - nbytes);
> +#ifdef USE_VALGRIND
> +	/* make the bytes at the end no-access */
> +	VALGRIND_MAKE_MEM_NOACCESS(((char *) toc) + (total_bytes - allocated_bytes - NUM_NOACCESS_BYTES),
> +							   NUM_NOACCESS_BYTES);
> +#endif
> +
> +	return ((char *) toc) + (total_bytes - allocated_bytes - nbytes - NUM_NOACCESS_BYTES);
>  }

The size is already rounded up by that point:

	/*
	 * Make sure request is well-aligned.  XXX: MAXALIGN is not enough,
	 * because atomic ops might need a wider alignment.  We don't have a
	 * proper definition for the minimum to make atomic ops safe, but
	 * BUFFERALIGN ought to be enough.
	 */
	nbytes = BUFFERALIGN(nbytes);

Which means that you'll often have a up to 32byte pad at the end of the
(ALIGNOF_BUFFER=32) allocation already. I don't care about the waste, but the
ALIGNOF_BUFFER padding will often prevent detecting smaller out-of-bounds
accesses.

Greetings,

Andres Freund