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SubjectRe: [BUG] gcov causes vread_tsc to increment kernel memory

On Thu, 2 Jul 2009, Peter Oberparleiter wrote:
> You're right, I missed that file. This should be fixed with the patch
> below. As the problem didn't occur on my test machine, please retest
> with the patch applied. Thanks!
>
> Also seeing as function tracer and gcov work on a similar basis and
> require similar files to be excluded from profiling, it would be nice
> if we wouldn't need to mark those files separately. Instead it would
> be great if the Makefile could be used to specify that a certain
> object file has a certain property (e.g. PROPERTY_USERPACE_file.o := y)
> and the mechanism (e.g. function tracer) would only need to specify
> that the extra gcc options should not be applied when that property is
> set. What do you think?

I think a property for vsyscall would be nice. Then you can add your
restrictions there. But ftrace also has to deal with other issues as well,
not just vsyscall.

>
> =================
> Subject: [PATCH] gcov: exclude code operating in userspace from profiling
>
> From: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
>
> Fix for this issue on x86_64:
>
> rostedt@goodmis.org wrote:
> > On bootup of the latest kernel my init segfaults. Debugging it,
> > I found that vread_tsc (a vsyscall) increments some strange
> > kernel memory:
> >
> > 0000000000000000 <vread_tsc>:
> > 0: 55 push %rbp
> > 1: 48 ff 05 00 00 00 00 incq 0(%rip)
> > # 8 <vread_tsc+0x8>
> > 4: R_X86_64_PC32 .bss+0x3c
> > 8: 48 89 e5 mov %rsp,%rbp
> > b: 66 66 90 xchg %ax,%ax
> > e: 48 ff 05 00 00 00 00 incq 0(%rip)
> > # 15 <vread_tsc+0x15>
> > 11: R_X86_64_PC32 .bss+0x44
> > 15: 66 66 90 xchg %ax,%ax
> > 18: 48 ff 05 00 00 00 00 incq 0(%rip)
> > # 1f <vread_tsc+0x1f>
> > 1b: R_X86_64_PC32 .bss+0x4c
> > 1f: 0f 31 rdtsc
> >
> >
> > Those "incq" is very bad to happen in vsyscall memory, since
> > userspace can not modify it. You need to make something prevent
> > profiling of vsyscall memory (like I do with ftrace).
>

Reported-and-tested-by: Steven Rostedt <rostedt@goodmis.org>

Thanks!

-- Steve

> Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
> ---
> arch/x86/kernel/Makefile | 2 ++
> 1 file changed, 2 insertions(+)
>
> Index: linux-2.6.31-rc1/arch/x86/kernel/Makefile
> ===================================================================
> --- linux-2.6.31-rc1.orig/arch/x86/kernel/Makefile
> +++ linux-2.6.31-rc1/arch/x86/kernel/Makefile
> @@ -26,6 +26,8 @@ CFLAGS_tsc.o := $(nostackp)
> CFLAGS_paravirt.o := $(nostackp)
> GCOV_PROFILE_vsyscall_64.o := n
> GCOV_PROFILE_hpet.o := n
> +GCOV_PROFILE_tsc.o := n
> +GCOV_PROFILE_paravirt.o := n
>
> obj-y := process_$(BITS).o signal.o entry_$(BITS).o
> obj-y += traps.o irq.o irq_$(BITS).o dumpstack_$(BITS).o
>
>


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