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

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Message-ID: <CAHk-=wgqAcRU99Dp20+ZAux7Mdgbnw5deOguwOjdCJY0eNnSkA@mail.gmail.com>
Date:   Fri, 30 Aug 2019 08:48:49 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 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".

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.

               Linus