linux-kernel - Re: [GIT PULL] EFI fix

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Date:	Tue, 17 May 2016 12:20:49 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alex Thorlton <athorlton@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Borislav Petkov <bp@...en8.de>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [GIT PULL] EFI fix


* Matt Fleming <matt@...eblueprint.co.uk> wrote:

> On Tue, 17 May, at 10:04:34AM, Matt Fleming wrote:
> > 
> > Now I'm wondering whether other users of FRAME_BEGIN/FRAME_END make
> > this same mistake. Coccinelle might be able to detect it perhaps.
> 
> A quick bit of sed turned up the code in arch/x86/entry/entry_64.S,
> which looks to suffer from the same bug,

That would be arch/x86/entry/thunk_64.S, but yeah, good find!

>         /* rdi: arg1 ... normal C conventions. rax is saved/restored. */
>         .macro THUNK name, func, put_ret_addr_in_rdi=0
>         .globl \name
>         .type \name, @function
> \name:
>         FRAME_BEGIN
> 
>         /* this one pushes 9 elems, the next one would be %rIP */
>         pushq %rdi
>         pushq %rsi
>         pushq %rdx
>         pushq %rcx
>         pushq %rax
>         pushq %r8
>         pushq %r9
>         pushq %r10
>         pushq %r11
> 
>         .if \put_ret_addr_in_rdi
>         /* 9*8(%rsp) is return addr on stack */
>         movq 9*8(%rsp), %rdi
>         .endif
> 
> With CONFIG_FRAME_POINTER=y 9*8(%rsp) is actually the value of %rbp on
> entry, not the return address.

This seems to affect:

#ifdef CONFIG_TRACE_IRQFLAGS
        THUNK trace_hardirqs_on_thunk,trace_hardirqs_on_caller,1
        THUNK trace_hardirqs_off_thunk,trace_hardirqs_off_caller,1
#endif

and with TRACE_IRQFLAGS we already in most times force frame pointers, such as 
when LOCKDEP is enabled:

  config LOCKDEP
        bool
        depends on DEBUG_KERNEL && TRACE_IRQFLAGS_SUPPORT && STACKTRACE_SUPPORT && LOCKDEP_SUPPORT
  ...
        select FRAME_POINTER if !MIPS && !PPC && !ARM_UNWIND && !S390 && !MICROBLAZE && !ARC && !SCORE

Also, unlike the efi_call() case, this thunk_64.S bug affects the quality of 
debug/tracing information, not runtime correctness per se.

still all that is by accident, not by design - this bug should be fixed too.

Thanks,

	Ingo