Messages in this thread | | | Date | Wed, 8 Mar 2017 15:29:59 -0600 | From | Josh Poimboeuf <> | Subject | Re: v4.10: kernel stack frame pointer .. has bad value (null) |
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[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@redhat.com> 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
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