Re: [PATCH v2 05/17] vdso: Avoid call to memset() by getrandom

Ard Biesheuvel
2024-08-28T17:13:08Z

On Wed, 28 Aug 2024 at 18:24, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> Hi!
>
> On Wed, Aug 28, 2024 at 05:40:23PM +0200, Ard Biesheuvel wrote:
> > On Wed, 28 Aug 2024 at 14:57, Segher Boessenkool
> > <segher@kernel.crashing.org> wrote:
> > >
> > > On Wed, Aug 28, 2024 at 12:24:12PM +0000, Arnd Bergmann wrote:
> > > > On Wed, Aug 28, 2024, at 11:18, Jason A. Donenfeld wrote:
> > > > > On Tue, Aug 27, 2024 at 05:53:30PM -0500, Segher Boessenkool wrote:
> > > > >> On Tue, Aug 27, 2024 at 11:08:19AM -0700, Eric Biggers wrote:
> > > > >> >
> > > > >> > Is there a compiler flag that could be used to disable the generation of calls
> > > > >> > to memset?
> > > > >>
> > > > >> -fno-tree-loop-distribute-patterns . But, as always, read up on it, see
> > > > >> what it actually does (and how it avoids your problem, and mostly: learn
> > > > >> what the actual problem *was*!)
> > > > >
> > > > > This might help with various loops, but it doesn't help with the matter
> > > > > that this patch fixes, which is struct initialization. I just tried it
> > > > > with the arm64 patch to no avail.
> > > >
> > > > Maybe -ffreestanding can help here? That should cause the vdso to be built
> > > > with the assumption that there is no libc, so it would neither add nor
> > > > remove standard library calls. Not sure if that causes other problems,
> > > > e.g. if the calling conventions are different.
> > >
> > > "GCC requires the freestanding
> > > environment provide 'memcpy', 'memmove', 'memset' and 'memcmp'."
> > >
> > > This is precisely to implement things like struct initialisation. Maybe
> > > we should have a "-ffreeerstanding" or "-ffreefloating" or think of
> > > something funnier still environment as well, this problem has been there
> > > since the -ffreestanding flag has existed, but the problem is as old as
> > > the night.
> > >
> > > -fno-builtin might help a bit more, but just attack the problem at
> > > its root, like I suggested?
> > >
> >
> > In my experience, this is likely to do the opposite: it causes the
> > compiler to 'forget' the semantics of memcpy() and memset(), so that
> > explicit trivial calls will no longer be elided and replaced with
> > plain loads and stores (as it can no longer guarantee the equivalence)
>
> No, the compiler will never forget those semantics. But if you tell it
> your function named memset() is not the actual standard memset -- via
> -fno-builtin-memset for example -- the compiler won't optimise things
> involving it quite as much. You told it so eh?
>

That is exactly the point I am making. So how would this help in this case?

> You can also tell it not to have a __builtin_memset function, but in
> this particular case that won;t quite work, since the compiler does need
> to have that builtin available to do struct and array initialisations
> and the like.
>
> > > (This isn't a new problem, originally it showed up as "GCC replaces
> > > (part of) my memcpy() implementation by a (recursive) call to memcpy()"
> > > and, well, that doesn't quite work!)
> > >
> >
> > This needs to be fixed for Clang as well, so throwing GCC specific
> > flags at it will at best be a partial solution.
>
> clang says it is a 100% plug-in replacement for GCC, so they will have
> to accept all GCC flags. And in many cases they do. Cases where they
> don't are bugs.
>

Even if this were true, we support Clang versions today that do not
support -fno-tree-loop-distribute-patterns so my point stands.

> > It is not a complete solution, unfortunately, and I guess there may be
> > other situations (compiler/arch combinations) where this might pop up
> > again.
>
> Why do mem* not work in VDSOs? Fix that, and all these problems
> disappear, and you do not need workrarounds :-)
>

Good point. We should just import memcpy and memset in the VDSO ELF metadata.

Not sure about static binaries, though: do those even use the VDSO?