linux-kernel - Re: [PATCH v3 14/51] x86/asm/head: put real return address on idle task stack

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Date:	Wed, 17 Aug 2016 15:30:55 -0500
From:	Nilay Vaish <nilayvaish@...il.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, x86 <x86@...nel.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH v3 14/51] x86/asm/head: put real return address on idle
 task stack

On 12 August 2016 at 09:28, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> The frame at the end of each idle task stack has a zeroed return
> address.  This is inconsistent with real task stacks, which have a real
> return address at that spot.  This inconsistency can be confusing for
> stack unwinders.
>
> Make it a real address by using the side effect of a call instruction to
> push the instruction pointer on the stack.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  arch/x86/kernel/head_64.S | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 3621ad2..c90f481 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -298,8 +298,9 @@ ENTRY(start_cpu)
>          *      REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
>          *              address given in m16:64.
>          */
> -       movq    initial_code(%rip),%rax
> -       pushq   $0              # fake return address to stop unwinder
> +       call    1f              # put return address on stack for unwinder
> +1:     xorq    %rbp, %rbp      # clear frame pointer
> +       movq    initial_code(%rip), %rax
>         pushq   $__KERNEL_CS    # set correct cs
>         pushq   %rax            # target address in negative space
>         lretq


Josh,  I have a couple of questions.

It seems to me that this patch and the patch 16/51 are both aiming at
the same thing, but are for two different architectures: 32-bit and
64-bit versions of x86.  But you have taken slightly different
approaches in the two patches (for 64-bit, we first jump and then make
a function call, for 32-bit we directly call the function).  Is there
any particular reason for this?  May be I missed out on something.

Second, this is for the whole patch series.  If I wanted to test this
series, how should I go about doing so?

Thanks
Nilay