linux-kernel - Re: [PATCH] ARM: UNWINDER_FRAME_POINTER implementation for Clang

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Date:   Wed, 21 Aug 2019 19:33:02 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Huckleberry <nhuck@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Miles Chen (陳民樺) 
        <miles.chen@...iatek.com>, Tri Vo <trong@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] ARM: UNWINDER_FRAME_POINTER implementation for Clang

On Wed, Aug 21, 2019 at 10:43 AM Nathan Huckleberry <nhuck@...gle.com> wrote:
>
> On Tue, Aug 20, 2019 at 2:39 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Tue, Aug 20, 2019 at 12:44 PM Nathan Huckleberry <nhuck@...gle.com> wrote:
...snip...
> > > +tst    r1, #0x10               @ 26 or 32-bit mode?
> > > +moveq  mask, #0xfc000003
> >
> > Should we be using different masks for ARM vs THUMB as per the
> > reference implementation?
> The change that introduces the arm/thumb code looked like a script
> that was run over all arm in the kernel. Neither this code nor the
> reference solution is compatible with arm, so there's no need for the
> change.

Looks like you're referring to commit 8b592783a2e8 ("Thumb-2:
Implement the unified arch/arm/lib functions").

Currently, arch/arm/Kconfig.debug has:
  57 config UNWINDER_FRAME_POINTER
  58   bool "Frame pointer unwinder"
  59   depends on !THUMB2_KERNEL && !CC_IS_CLANG

So it looks like UNWINDER_FRAME_POINTER and THUMB2_KERNEL are mutually
exclusive.  Probably could send a patch cleaning that up. (ie.
removing the different masks; essentially removing the hunk from
arch/arm/lib/backtrace.S from 8b592783a2e8).

> > > +for_each_frame:        tst     frame, mask             @ Check for address exceptions
> > > +               bne     no_frame
> > > +
> > > +/*
> > > + * sv_fp is the stack frame with the locals for the current considered
> > > + *     function.
> > > + * sv_pc is the saved lr frame the frame above. This is a pointer to a
> > > + *     code address within the current considered function, but
> > > + *     it is not the function start. This value gets updated to be
> > > + *     the function start later if it is possible.
> > > + */
> > > +1001:          ldr     sv_pc, [frame, #4]      @ get saved 'pc'
> > > +1002:          ldr     sv_fp, [frame, #0]      @ get saved fp
> >
> > The reference implementation applies the mask to sv_pc and sv_fp.  I
> > assume we want to, too?
> The mask is already applied to both. See for_each_frame:

ah, under the finished_setup label.
-- 
Thanks,
~Nick Desaulniers