[llvmlinux] [arch/x86/kernel/head64.c] In function `early_make_pgtable': undefined reference to `mcount'

Sedat Dilek sedat.dilek at gmail.com
Tue Jul 16 09:15:44 UTC 2013

On Mon, Jul 15, 2013 at 11:25 AM, Sedat Dilek <sedat.dilek at gmail.com> wrote:
> On Mon, Jul 15, 2013 at 9:54 AM, PaX Team <pageexec at gmail.com> wrote:
>> On 13 Jul 2013 at 14:57, Sedat Dilek wrote:
>>> >> # CONFIG_FTRACE is not set
>>> >>
>>> [ CCing pageexec ]
>> [btw, i've been subscribed since the beginning ;)]
>>> Unfortunately, it is the opposite:
>>> With CONFIG_FTRACE=n (disabled) I am able to build a kernel (with the
>>> kernel-config I normally use for my Linux-next builds).
>>> But disabling it means dispense with a lot of tracing-support, this is no good.
>> i haven't tried building with any of the tracers in a long time but what
>> i do know is that at runtime the kernel will fail because the stack walker
>> code has various assumptions about the function call depth and will
>> dereference bad memory when you enable this under clang (that's what the
>> CALLER_ADDR* defines try to work around).
>> another issue i remember is that clang applies the instrumentation needed
>> by ftrace/etc in a different way, in particular functions get instrumented
>> before inlining, so you'll get multiple calls to mcount from functions that
>> contain inlined bodies and that's certainly not the behaviour the rest of
>> the kernel infrastructure expects (i had a hackish workaround for this some
>> years ago but i think it didn't get accepted).
>> then there's the whole issue of clang/llvm not supporting the 'asm goto' gcc
>> extension which is used for efficient instrumentation.
>> all in all, forget instrumentation based tracing support for now, it will
>> require quite some work.
> Cool, that's really good explanations.
> So, CONFIG_FTRACE should be disabled at all or is
> The latter makes the "mcount" issue go away.
> Speaking of the changes to x86-ftrace.h in 0033 (see [1]).
> So, these workarounds can be dropped and CONFIG_FTRACE=n
> from config_x86_64 in [2]?

Hi pageexec,

I try to rework my "checkpoint 1" patch for x86_64.
Can you confirm which of the kernel-config "tracing" options should be disabled?
That would help me a lot!

- Sedat -
> And all this should be $somewhere documented.
> I am willing to do that. In general there should be a file like
> KNOWN-ISSUE or WORKAROUNDS in llvmlinux-topdir.
> - Sedat -
> [1] http://git.linuxfoundation.org/?p=llvmlinux.git;a=blob;f=arch/x86_64/patches/0033-Various-fixes-in-arch-x86-include-asm.patch
> [2] git.linuxfoundation.org/?p=llvmlinux.git;a=tree;f=targets/x86_64/config_x86_6

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