linux-kernel - Re: [PATCH v2 13/33] kmsan: Introduce memset_no_sanitize

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Date:   Fri, 8 Dec 2023 16:25:41 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Ilya Leoshkevich <iii@...ux.ibm.com>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Marco Elver <elver@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Pekka Enberg <penberg@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v2 13/33] kmsan: Introduce memset_no_sanitize_memory()

> 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.

> > ...
> >
> > > +__no_sanitize_memory
> > > +static inline void *memset_no_sanitize_memory(void *s, int c,
> > > size_t n)
> > > +{
> > > +       return memset(s, c, n);
> > > +}
> >
> > I think depending on the compiler optimizations this might end up
> > being a call to normal memset, that would still change the shadow
> > bytes.
>
> Interesting, do you have some specific scenario in mind? I vaguely
> remember that in the past there were cases when sanitizer annotations
> were lost after inlining, but I thought they were sorted out?

Sanitizer annotations are indeed lost after inlining, and we cannot do
much about that.
They are implemented using function attributes, and if a function
dissolves after inlining, we cannot possibly know which instructions
belonged to it.

Consider the following example (also available at
https://godbolt.org/z/5r7817G8e):

==================================
void *kmalloc(int size);

__attribute__((no_sanitize("kernel-memory")))
__attribute__((always_inline))
static void *memset_nosanitize(void *s, int c, int n) {
  return __builtin_memset(s, c, n);
}

void *do_something_nosanitize(int size) {
  void *ptr = kmalloc(size);
  memset_nosanitize(ptr, 0, size);
  return ptr;
}

void *do_something_sanitize(int size) {
  void *ptr = kmalloc(size);
  __builtin_memset(ptr, 0, size);
  return ptr;
}
==================================

If memset_nosanitize() has __attribute__((always_inline)), the
compiler generates the same LLVM IR calling __msan_memset() for both
do_something_nosanitize() and do_something_sanitize().
If we comment out this attribute, do_something_nosanitize() calls
memset_nosanitize(), which doesn't have the sanitize_memory attribute.

But even now __builtin_memset() is still calling __msan_memset(),
because __attribute__((no_sanitize("kernel-memory"))) somewhat
counterintuitively still preserves some instrumentation (see
include/linux/compiler-clang.h for details).
Replacing __attribute__((no_sanitize("kernel-memory"))) with
__attribute__((disable_sanitizer_instrumentation)) fixes this
situation:

define internal fastcc noundef ptr @memset_nosanitize(void*, int,
int)(ptr noundef returned writeonly %s, i32 noundef %n) unnamed_addr
#2 {
entry:
%conv = sext i32 %n to i64
tail call void @llvm.memset.p0.i64(ptr align 1 %s, i8 0, i64 %conv, i1 false)
ret ptr %s
}

>
> And, in any case, if this were to happen, would not it be considered a
> compiler bug that needs fixing there, and not in the kernel?

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?