linux-kernel - Re: [PATCHSET x86/master] add stack protector support for x86

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Message-ID: <20090209135557.GB14785@elte.hu>
Date:	Mon, 9 Feb 2009 14:55:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	hpa@...or.com, jeremy@...p.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, x86@...nel.org, rusty@...tcorp.com.au
Subject: Re: [PATCHSET x86/master] add stack protector support for x86_32


* Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> This patchset adds stack protector support for x86_32.  The basics are
> the same with x86_64 but there are some noticeable differences.
> 
> * x86_32 uses %fs for percpu base.  %gs is unused by the kernel and
>   managed lazily.  %gs is used for userland TLS and loading %gs with
>   different value on kernel entry is known to cost quite a bit on some
>   chips.
> 
>   Lazy %gs handling is made optional and disabled if stack protector
>   is enabled.  To do this, entry for %gs is added to pt_regs.  This
>   adds one "pushl $0" to SAVE_ALL in entry_32.S when lazy %gs is on.
>   However, no overhead is added to common exit path and error_code
>   entry path shed a few instructions.  I don't think there will be
>   noticeable overhead but then again it does add an instruction to a
>   very hot path.  Would this be okay?

Yeah, looks good.

> * x86_32 doesn't support direct access to shadow part of %gs and
>   there's no swapgs, so GDT entry should be built for stack canary.
> 
>   GDT entry 28 is used for this.  The boot cpu one is setup from
>   head_32.S.  Others while setting up percpu areas.

Yeah.

> * math_emu register access was completely broken.  Fixed.

=> i'll queue this up for mainline too as this breakage is independent
   of stackprotector.

> * x86_32 exception handlers take register frame verbatim as struct
>   pt_regs.  With -fstack-protector, gcc copies pt_regs into the
>   callee's stack frame to put it after the stack canary.  Of course it
>   doesn't copy back (as the callee owns the argument) and any change
>   made to pt_regs is lost on return.  This is currently worked around
>   by adding -fno-stack-protector to any file containing such
>   functions.  We really need to teach gcc about the calling
>   convention.

Or we could just push in struct pt_regs * ? Even if it's one more
instruction that will avoid trouble not just with the canary but also
with over-eager tail-call optimizations, etc.

> This patchset contains the following eleven patches.

Note, i sorted out the dependencies (it depended on x86/uaccess) and have
put the commits into tip:core/percpu. I might not get around testing it
today and pushing it out into tip:master, but i pushed out the core/percpu
bits, should you queue up further changes.

	Ingo
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