Re: [PATCH] ARM: UNWINDER_FRAME_POINTER implementation for Clang

From: Nick Desaulniers <ndesaulniers@google.com>
To: Nathan Huckleberry <nhuck@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>
Cc: "Russell King" <linux@armlinux.org.uk>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	"Miles Chen (陳民樺)" <miles.chen@mediatek.com>,
	"Tri Vo" <trong@google.com>, "Arnd Bergmann" <arnd@arndb.de>,
	"Ard Biesheuvel" <ard.biesheuvel@linaro.org>
Subject: Re: [PATCH] ARM: UNWINDER_FRAME_POINTER implementation for Clang
Date: Wed, 21 Aug 2019 19:33:02 -0700	[thread overview]
Message-ID: <CAKwvOd=Uvzw5_azQuSjUovSEDTNAaB=pTht1-zMiA8mqfmJ0zw@mail.gmail.com> (raw)
In-Reply-To: <CAJkfWY4cHz+i8kYg2i1Krs-32nh7-WQU+psT=DRGYnTje6yj4Q@mail.gmail.com>

On Wed, Aug 21, 2019 at 10:43 AM Nathan Huckleberry <nhuck@google.com> wrote:
>
> On Tue, Aug 20, 2019 at 2:39 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Tue, Aug 20, 2019 at 12:44 PM Nathan Huckleberry <nhuck@google.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