From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Herbert Xu <herbert@gondor.apana.org.au>,
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 22:27:58 -0600 [thread overview]
Message-ID: <20170113042758.whof5fk6eu7myctq@treble> (raw)
In-Reply-To: <CALCETrUFCFn-rKnr+NG3SU7J78ree9siJC=Kz8f_Bk6eG2HyPA@mail.gmail.com>
On Thu, Jan 12, 2017 at 07:23:18PM -0800, Andy Lutomirski wrote:
> On Thu, Jan 12, 2017 at 7:11 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > On Thu, Jan 12, 2017 at 05:46:55PM -0800, Andy Lutomirski wrote:
> >> On Thu, Jan 12, 2017 at 12:15 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >> > On Thu, Jan 12, 2017 at 12:08:07PM -0800, Andy Lutomirski wrote:
> >> >> On Thu, Jan 12, 2017 at 11:51 AM, Linus Torvalds
> >> >> <torvalds@linux-foundation.org> wrote:
> >> >> > On Thu, Jan 12, 2017 at 6:02 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >> >> >>
> >> >> >> 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?
> >> >> >
> >> >> > I think the argument is that we *could* try to align things, if we
> >> >> > just had some tool that actually then verified that we aren't missing
> >> >> > anything.
> >> >> >
> >> >> > I'm not entirely happy with checking the generated code, though,
> >> >> > because as Ingo says, you have a 50:50 chance of just getting it right
> >> >> > by mistake. So I'd much rather have some static tool that checks
> >> >> > things at a code level (ie coccinelle or sparse).
> >> >>
> >> >> What I meant was checking the entry code to see if it aligns stack
> >> >> frames, and good luck getting sparse to do that. Hmm, getting 16-byte
> >> >> alignment for real may actually be entirely a lost cause. After all,
> >> >> I think we have some inline functions that do asm volatile ("call
> >> >> ..."), and I don't see any credible way of forcing alignment short of
> >> >> generating an entirely new stack frame and aligning that.
> >> >
> >> > Actually we already found all such cases and fixed them by forcing a new
> >> > stack frame, thanks to objtool. For example, see 55a76b59b5fe.
> >>
> >> What I mean is: what guarantees that the stack is properly aligned for
> >> the subroutine call? gcc promises to set up a stack frame, but does
> >> it promise that rsp will be properly aligned to call a C function?
> >
> > Yes, I did an experiment and you're right. I had naively assumed that
> > all stack frames would be aligned.
>
> Just to check: did you do your experiment with -mpreferred-stack-boundary=4?
Yes, but it's too late for me to be doing hard stuff and I think my
first experiment was bogus. I didn't use all the other kernel-specific
gcc options.
I tried again with all the kernel gcc options, except with
-mpreferred-stack-boundary=4 instead of 3, and actually came up with the
opposite conclusion.
I used the following code:
void otherfunc(void);
static inline void bar(long *f)
{
asm volatile("call otherfunc" : : "m" (f) : );
}
void foo(void)
{
long buf[3] = {0, 0, 0};
bar(buf);
}
The stack frame was always 16-byte aligned regardless of whether the
buf array size was even or odd.
So my half-asleep brain is telling me that my original assumption was
right.
--
Josh
next prev parent reply other threads:[~2017-01-13 4:28 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
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 [this message]
[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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170113042758.whof5fk6eu7myctq@treble \
--to=jpoimboe@redhat.com \
--cc=ard.biesheuvel@linaro.org \
--cc=herbert@gondor.apana.org.au \
--cc=linux-crypto@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@amacapital.net \
--cc=luto@kernel.org \
--cc=mingo@kernel.org \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).