Subject: Re: [PATCH] Fix unwind_frame for clang-built kernels

From: Miles Chen <[email-masked]>
Date: Fri, 19 Jun 2020 09:55:04 +0800

On Thu, 2020-06-18 at 13:22 -0700, Nick Desaulniers wrote:
> On Tue, Jun 16, 2020 at 3:36 PM 'Nathan Huckleberry' via Clang Built
> Linux <[email-masked]> wrote:
> >
> > Since clang does not push pc and sp in function prologues, the current
> > implementation of unwind_frame does not work. By using the previous
> > frame's lr/fp instead of saved pc/sp we get valid unwinds on clang-built
> > kernels.
> >
> > The bounds check on next frame pointer must be changed as well since
> > there are 8 less bytes between frames.
> >
> > This fixes /proc/<pid>/stack.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/912
> > Signed-off-by: Nathan Huckleberry <[email-masked]>
> 
> Thanks for the patch, Nathan!  When I looked into this, I found the
> latest ARM AAPCS [0] specifically says (with `it` referring to `a
> platform`: "It may permit the frame pointer register to be used as a
> general-purpose callee-saved register, but provide a platform-specific
> mechanism for external agents to reliably locate the chain of frame
> records."   While it's good that's acknowledged in the documentation,
> the current wording is relaxed in order to not force current
> implementations to converge.  This has the unfortunate side effect of
> making finding the frame pointer toolchain dependendent, hence this
> patch and your previous commit 6dc5fd93b2f1 ("ARM: 8900/1:
> UNWINDER_FRAME_POINTER implementation for Clang").  Being more
> specific in the documentation would force at least one implementation
> to change, and I think that would also imply an ABI break.  So I can
> see it both ways, though I still would prefer that the language pin
> this down, even if we had to change LLVM.  Just providing additional
> context for folks on the thread.
> 
> This should also have a reported by tag from Miles, in v2.
> 
> Reported-by: Miles Chen <[email-masked]>
> 
> Miles mentioned to me that he tested it, but maybe Miles can confirm
> that publicly on-list via an explicit Tested-by: tag?

Thanks for the patch.

Tested-by: Miles Chen <[email-masked]>

> 
> This would be useful for us to have in stable; otherwise we'll have to
> carry out of tree in Android and CrOS, which I'd rather not do.  Via
> Documentation/process/stable-kernel-rules.rst, if you add this tag to
> V2, that will greatly simplify submitting this to stable:
> Cc: [email-masked]
> 
> Miles also showed me the behavior of this patch for different kernel
> versions, which varies anywhere from empty or single entry traces to
> panics, so this is pretty important that this works for Clang builds.
> 
> [0] https://static.docs.arm.com/ihi0042/i/aapcs32.pdf
> 
> > ---
> >  arch/arm/kernel/stacktrace.c | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c
> > index cc726afea023..76ea4178a55c 100644
> > --- a/arch/arm/kernel/stacktrace.c
> > +++ b/arch/arm/kernel/stacktrace.c
> > @@ -22,6 +22,19 @@
> >   * A simple function epilogue looks like this:
> >   *     ldm     sp, {fp, sp, pc}
> >   *
> > + * When compiled with clang, pc and sp are not pushed. A simple function
> > + * prologue looks like this when built with clang:
> > + *
> > + *     stmdb   {..., fp, lr}
> > + *     add     fp, sp, #x
> > + *     sub     sp, sp, #y
> > + *
> > + * A simple function epilogue looks like this when built with clang:
> > + *
> > + *     sub     sp, fp, #x
> > + *     ldm     {..., fp, pc}
> > + *
> > + *
> >   * Note that with framepointer enabled, even the leaf functions have the same
> >   * prologue and epilogue, therefore we can ignore the LR value in this case.
> >   */
> > @@ -34,6 +47,16 @@ int notrace unwind_frame(struct stackframe *frame)
> >         low = frame->sp;
> >         high = ALIGN(low, THREAD_SIZE);
> >
> > +#ifdef CONFIG_CC_IS_CLANG
> > +       /* check current frame pointer is within bounds */
> > +       if (fp < low + 4 || fp > high - 4)
> 
> The patch LGTM; maybe Russell or Catalin could triple check this
> bounds check?  Assuming there's no issue, you can include on a v2 my
> reviewed by:
> 
> Reviewed-by: Nick Desaulniers <[email-masked]>
> 
> I'd probably wait the remainder of a week before sending a v2 to
> collect additional feedback.  Thank you again.
> 
> > +               return -EINVAL;
> > +
> > +       frame->sp = frame->fp;
> > +       frame->fp = *(unsigned long *)(fp);
> > +       frame->pc = frame->lr;
> > +       frame->lr = *(unsigned long *)(fp + 4);
> > +#else
> >         /* check current frame pointer is within bounds */
> >         if (fp < low + 12 || fp > high - 4)
> >                 return -EINVAL;
> > @@ -42,6 +65,7 @@ int notrace unwind_frame(struct stackframe *frame)
> >         frame->fp = *(unsigned long *)(fp - 12);
> >         frame->sp = *(unsigned long *)(fp - 8);
> >         frame->pc = *(unsigned long *)(fp - 4);
> > +#endif
> >
> >         return 0;
> >  }
> > --
> > 2.27.0.290.gba653c62da-goog
> >
> > --
> 

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