From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>,
Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Andy Lutomirski <luto@kernel.org>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: Re: x86-64: Maintain 16-byte stack alignment
Date: Thu, 12 Jan 2017 08:02:15 -0600 [thread overview]
Message-ID: <20170112140215.rh247gwk55fjzmg7@treble> (raw)
In-Reply-To: <CALCETrVb-NyKJajGEsLOrQqQrQ7cRm4sqLRxNr_yVE2-mjiDrg@mail.gmail.com>
On Wed, Jan 11, 2017 at 10:21:07PM -0800, Andy Lutomirski wrote:
> On Tue, Jan 10, 2017 at 10:01 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Tue, Jan 10, 2017 at 8:35 PM, Herbert Xu <herbert@gondor.apana.org.au> wrote:
> >> On Tue, Jan 10, 2017 at 08:17:17PM -0800, Linus Torvalds wrote:
> >>>
> >>> That said, I do think that the "don't assume stack alignment, do it by
> >>> hand" may be the safer thing. Because who knows what the random rules
> >>> will be on other architectures.
> >>
> >> Sure we can ban the use of attribute aligned on stacks. But
> >> what about indirect uses through structures? For example, if
> >> someone does
> >>
> >> struct foo {
> >> } __attribute__ ((__aligned__(16)));
> >>
> >> int bar(...)
> >> {
> >> struct foo f;
> >>
> >> return baz(&f);
> >> }
> >>
> >> then baz will end up with an unaligned argument. The worst part
> >> is that it is not at all obvious to the person writing the function
> >> bar.
> >
> > Linus, I'm starting to lean toward agreeing with Herbert here, except
> > that we should consider making it conditional on having a silly GCC
> > version. After all, the silly GCC versions are wasting space and time
> > with alignment instructions no matter what we do, so this would just
> > mean tweaking the asm and adding some kind of check_stack_alignment()
> > helper to throw out a WARN_ONCE() if we miss one. The problem with
> > making it conditional is that making pt_regs effectively live at a
> > variable offset from %rsp is just nasty.
>
> So actually doing this is gross because we have calls from asm to C
> all over the place. But... maybe we can automate all the testing.
> Josh, how hard would it be to teach objtool to (if requested by an
> option) check that stack frames with statically known size preserve
> 16-byte stack alignment?
>
> I find it rather annoying that gcc before 4.8 malfunctions when it
> sees __aligned__(16) on x86_64 kernels. Sigh.
Just to clarify, I think you're asking if, for versions of gcc which
don't support -mpreferred-stack-boundary=3, objtool can analyze all C
functions to ensure their stacks are 16-byte aligned.
It's certainly possible, but I don't see how that solves the problem.
The stack will still be misaligned by entry code. Or am I missing
something?
--
Josh
next prev parent reply other threads:[~2017-01-12 14:02 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-10 14:33 x86-64: Maintain 16-byte stack alignment Herbert Xu
2017-01-10 14:39 ` Herbert Xu
2017-01-10 17:05 ` Linus Torvalds
2017-01-10 17:09 ` Andy Lutomirski
2017-01-11 3:11 ` Herbert Xu
2017-01-11 3:30 ` Linus Torvalds
2017-01-11 4:17 ` Linus Torvalds
2017-01-11 4:35 ` Herbert Xu
2017-01-11 6:01 ` Andy Lutomirski
2017-01-12 6:21 ` Andy Lutomirski
2017-01-12 7:40 ` Ingo Molnar
2017-01-12 14:02 ` Josh Poimboeuf [this message]
2017-01-12 19:51 ` Linus Torvalds
2017-01-12 20:08 ` Andy Lutomirski
2017-01-12 20:15 ` Josh Poimboeuf
2017-01-12 20:55 ` Josh Poimboeuf
2017-01-12 21:40 ` Linus Torvalds
2017-01-13 8:38 ` Herbert Xu
2017-01-13 1:46 ` Andy Lutomirski
2017-01-13 3:11 ` Josh Poimboeuf
2017-01-13 3:23 ` Andy Lutomirski
2017-01-13 4:27 ` Josh Poimboeuf
[not found] ` <CA+55aFzRrSwGxxfZk-RUEnsz=xhcSmOwE1CenfCPBWtsS9MwDw@mail.gmail.com>
2017-01-13 5:07 ` Josh Poimboeuf
2017-01-13 8:43 ` Herbert Xu
2017-01-13 8:42 ` Herbert Xu
2017-01-13 8:39 ` Herbert Xu
2017-01-13 8:36 ` Herbert Xu
2017-01-13 13:07 ` Josh Poimboeuf
[not found] ` <CA+55aFw+Z_ieo6DzTVB6_-TvQ0jj60s=T0mvXfqkBVFdKFPw_Q@mail.gmail.com>
2017-01-11 8:06 ` Ard Biesheuvel
2017-01-11 8:09 ` Herbert Xu
2017-01-11 18:20 ` Andy Lutomirski
2017-01-12 7:05 ` Herbert Xu
2017-01-12 7:46 ` Ingo Molnar
2017-01-12 14:49 ` Josh Poimboeuf
2017-01-12 7:51 ` Andy Lutomirski
2017-01-12 8:04 ` Herbert Xu
2017-01-12 8:18 ` Ingo Molnar
2017-01-12 15:03 ` Josh Poimboeuf
2017-01-12 15:06 ` Herbert Xu
2017-01-12 15:18 ` Josh Poimboeuf
2017-01-12 15:10 ` Josh Poimboeuf
2017-01-10 17:30 ` Ard Biesheuvel
2017-01-10 19:00 ` Andy Lutomirski
2017-01-10 19:16 ` Ard Biesheuvel
2017-01-10 19:22 ` Andy Lutomirski
2017-01-10 20:00 ` Ard Biesheuvel
2017-01-10 23:25 ` Andy Lutomirski
2017-01-11 3:26 ` Herbert Xu
2017-01-11 3:26 ` Herbert Xu
2017-01-11 3:16 ` Herbert Xu
2017-01-11 3:15 ` Herbert Xu
2017-01-12 6:12 ` Herbert Xu
2017-01-12 8:01 ` Ard Biesheuvel
2017-01-12 8:06 ` Herbert Xu
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