Re: [PATCH v2 13/33] kmsan: Introduce memset_no_sanitize_memory()

[Ilya Leoshkevich]

On Fri, 2023-12-08 at 16:25 +0100, Alexander Potapenko wrote:
> > A problem with __memset() is that, at least for me, it always ends
> > up being a call. There is a use case where we need to write only 1
> > byte, so I thought that introducing a call there (when compiling
> > without KMSAN) would be unacceptable.
> 
> Wonder what happens with that use case if we e.g. build with fortify-
> source.
> Calling memset() for a single byte might be indicating the code is
> not hot.

The original code has a simple assignment. Here is the relevant diff:

        if (s->flags & __OBJECT_POISON) {
-               memset(p, POISON_FREE, poison_size - 1);
-               p[poison_size - 1] = POISON_END;
+               memset_no_sanitize_memory(p, POISON_FREE, poison_size -
1);
+               memset_no_sanitize_memory(p + poison_size - 1,
POISON_END, 1);
        }

[...]


> As stated above, I don't think this is more or less working as
> intended.
> If we really want the ability to inline __memset(), we could
> transform
> it into memset() in non-sanitizer builds, but perhaps having a call
> is
> also acceptable?

Thanks for the detailed explanation and analysis. I will post
a version with a __memset() and let the slab maintainers decide if
the additional overhead is acceptable.