linux-kernel - Re: objtool warning "uses BP as a scratch register" with clang-9

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Date:   Fri, 30 Aug 2019 11:49:27 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: objtool warning "uses BP as a scratch register" with clang-9

On Fri, Aug 30, 2019 at 08:48:49AM -0700, Linus Torvalds wrote:
> On Fri, Aug 30, 2019 at 8:02 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > For KASAN, the Clang threshold for inserting memset() is *2* consecutive
> > writes instead of 17.  Isn't that likely to cause tearing-related
> > surprises?
> 
> Tearing isn't likely to be a problem.
> 
> It's not like memcpy() does byte-by-byte copies. If you pass it a
> word-aligned pointer, it will do word-aligned accesses simply for
> performance reasons.
> 
> Even on x86, where we use "rep movsb", we (a) tend to disable it for
> small copies and (b) it turns out that microcode that does the
> optimized movsb (which is the only case we use it) probably ends up
> doing atomic things anyway. Note the "probably". I don't have
> microcode source code, but there are other indications like "we know
> it doesn't take interrupts on a byte-per-byte level, only on the
> cacheline level".

The microcode argument is not all that comforting :-)

Also what about unaligned accesses, e.g. if a struct member isn't on a
word boundary?  Arnd's godbolt link showed those can get combined too.

I don't see x86 memcpy() doing any destination alignment checks.

Have we audited other arches' memset/memcpy implementations?

> So it's probably not an issue from a tearing standpoint - but it
> worries me because of "this has to be a leaf function" kind of issues
> where we may be using individual stores on purpose. We do have things
> like that.

It sounds like everybody's in agreement that replacing accesses with
memset/memcpy is bad in a kernel context.  Should we push for a new
fine-grained compiler option to disable it?

-- 
Josh