Re: [PATCH 03/19] x86/dumpstack: remove unnecessary stack pointer arguments
From: Brian Gerst
Date: Thu Jul 21 2016 - 23:08:48 EST
On Thu, Jul 21, 2016 at 9:41 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Thu, Jul 21, 2016 at 02:56:52PM -0700, Andy Lutomirski wrote:
>> On Thu, Jul 21, 2016 at 2:21 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> > When calling show_stack_log_lvl() or dump_trace() with a regs argument,
>> > providing a stack pointer or frame pointer is redundant.
>> > diff --git a/arch/x86/kernel/dumpstack_32.c b/arch/x86/kernel/dumpstack_32.c
>> > index 358fe1c..c533b8b 100644
>> > --- a/arch/x86/kernel/dumpstack_32.c
>> > +++ b/arch/x86/kernel/dumpstack_32.c
>> > @@ -122,7 +122,7 @@ void show_regs(struct pt_regs *regs)
>> > u8 *ip;
>> > pr_emerg("Stack:\n");
>> > - show_stack_log_lvl(NULL, regs, ®s->sp, 0, KERN_EMERG);
>> > + show_stack_log_lvl(NULL, regs, NULL, 0, KERN_EMERG);
>> This is weird -- note the &. You're at some risk of exposing a bug in
>> x86_32's kernel_stack_pointer() function, which is a mess. (I don't
>> see why it's written the way it is -- the actual return stack pointer
>> given a pt_regs is quite well defined -- if regs->cs & 3 != 0, then
>> it's regs->sp, else it's ®s->sp.)
>> That being said, this isn't a big deal, so:
>> Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> If you want to make this all a bit more reliably on x86_32, you could
>> fix kernel_stack_pointer().
> Ok. The whole '®s->sp' thing threw me for a loop. I have no idea
> what kernel_stack_pointer() is trying to do. I just assumed it was
> correct. I'll take a look at it and try to fix it in another patch.
On 32-bit, when an interrupt doesn't change CPL, SS:ESP is not pushed.
So, effectively, the old stack pointer is ®s->sp.