Re: v4.10: kernel stack frame pointer .. has bad value (null)

From: Josh Poimboeuf
Date: Wed Mar 08 2017 - 16:37:24 EST


[adding Steven Rostedt to CC as an FYI]

On Wed, Mar 08, 2017 at 10:25:01AM -0800, Linus Torvalds wrote:
> On Wed, Mar 8, 2017 at 9:37 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > - CONFIG_FUNCTION_GRAPH_TRACER sets it on x86-32 because of a gcc bug
> > where the stack gets aligned before the mcount call. This issue
> > should be mostly obsolete as most modern compilers now have -mfentry.
> > We could make it dependent on CC_USING_FENTRY.
>
> Yeah. At some point we might even upgrade the compiler requirements to
> no longer accept the mcount model.

The plot slightly thickens...

So I was mistaken about this problem not existing with newer versions of
gcc, because the x86-32 ftrace code doesn't use -mfentry. It still
relies on mcount. So CONFIG_FUNCTION_GRAPH_TRACER will still need
-maccumulate-outgoing-args for *all* versions of gcc on x86-32.

(Of course, that situation would improve if ftrace on x86-32 were ported
to use -mfentry.)

Also, since -Os tells gcc to ignore -maccumulate-outgoing-args, this
means that CONFIG_FUNCTION_GRAPH_TRACER with mcount needs a dependency
on CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE.

I suspect these issues also affect x86-64 with gcc 4.4.x and 4.5.x,
which corresponds to the window after the funky DRAP prologue was
introduced but before -mfentry was introduced.

In summary, here are the changes I'm looking at:

- set -maccumulate-outgoing-args if CONFIG_FUNCTION_GRAPH_TRACER &&
!CC_USING_ENTRY
(for both 32- and 64-bit)

- somehow make CONFIG_FUNCTION_GRAPH_TRACER depend on either
CC_USING_FENTRY or CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
(for both 32- and 64-bit)

(not sure how to do that -- maybe just fail the build in the
graph tracer + mcount + '-Os' case)

- set -maccumulate-outgoing-args if CONFIG_JUMP_LABEL && gcc < 4.5.2
(for both 32-bit and 64-bit)

--
Josh