LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Jakub Jelinek <jakub@redhat.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: pageexec@freemail.hu, Sam Ravnborg <sam@ravnborg.org>,
	Arjan van de Ven <arjan@infradead.org>,
	linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [x86.git#mm] stack protector fixes, vmsplice exploit
Date: Thu, 14 Feb 2008 17:35:53 -0500	[thread overview]
Message-ID: <20080214223553.GW24887@devserv.devel.redhat.com> (raw)
In-Reply-To: <20080214202535.GA25316@elte.hu>

On Thu, Feb 14, 2008 at 09:25:35PM +0100, Ingo Molnar wrote:
> The per function call overhead from stackprotector is already pretty 
> serious IMO, but at least that's something that GCC _could_ be doing 
> (much) smarter (why doesnt it jne forward out to __check_stk_failure, 
> instead of generating 4 instructions, one of them a default-mispredicted 
> branch instruction??), so that overhead could in theory be something 
> like 4 fall-through instructions per function, instead of the current 6.

Where do you see a mispredicted branch?
int foo (void)
{
  char buf[64];
  bar (buf);
  return 6;
}

-O2 -fstack-protector -m64:
        subq    $88, %rsp
        movq    %fs:40, %rax
        movq    %rax, 72(%rsp)
        xorl    %eax, %eax
        movq    %rsp, %rdi
        call    bar
        movq    72(%rsp), %rdx
        xorq    %fs:40, %rdx
        movl    $6, %eax
        jne     .L5
        addq    $88, %rsp
        ret
.L5:
        .p2align 4,,6
        .p2align 3
        call    __stack_chk_fail
-O2 -fstack-protector -m32:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $88, %esp
        movl    %gs:20, %eax
        movl    %eax, -4(%ebp)
        xorl    %eax, %eax
        leal    -68(%ebp), %eax
        movl    %eax, (%esp)
        call    bar
        movl    $6, %eax
        movl    -4(%ebp), %edx
        xorl    %gs:20, %edx
        jne     .L5
        leave
        ret
.L5:
        .p2align 4,,7
        .p2align 3
        call    __stack_chk_fail
-O2 -fstack-protector -m64 -mcmodel=kernel:
        subq    $88, %rsp
        movq    %gs:40, %rax
        movq    %rax, 72(%rsp)
        xorl    %eax, %eax
        movq    %rsp, %rdi
        call    bar
        movq    72(%rsp), %rdx
        xorq    %gs:40, %rdx
        movl    $6, %eax
        jne     .L5
        addq    $88, %rsp
        ret
.L5:
        .p2align 4,,6
        .p2align 3
        call    __stack_chk_fail

both with gcc 4.1.x and 4.3.0.
BTW, you can use -fstack-protector --param=ssp-buffer-size=4
etc. to tweak the size of buffers to trigger stack protection, the
default is 8, but e.g. whole Fedora is compiled with 4.

	Jakub

  parent reply	other threads:[~2008-02-14 22:36 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-14 17:00 Ingo Molnar
2008-02-14 17:16 ` pageexec
2008-02-14 19:00   ` Ingo Molnar
2008-02-14 18:55     ` pageexec
2008-02-14 20:25       ` Ingo Molnar
2008-02-14 21:00         ` Arjan van de Ven
2008-02-14 21:12         ` pageexec
2008-02-14 22:35         ` Jakub Jelinek [this message]
2008-02-14 21:43           ` pageexec
2008-02-14 23:19             ` Ingo Molnar
2008-02-14 23:16           ` Ingo Molnar

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=20080214223553.GW24887@devserv.devel.redhat.com \
    --to=jakub@redhat.com \
    --cc=arjan@infradead.org \
    --cc=hpa@zytor.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@elte.hu \
    --cc=pageexec@freemail.hu \
    --cc=sam@ravnborg.org \
    --cc=tglx@linutronix.de \
    --cc=torvalds@linux-foundation.org \
    --subject='Re: [x86.git#mm] stack protector fixes, vmsplice exploit' \
    /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

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).