On Mon, Feb 01, 2021 at 01:23:52PM +0100, Thomas Bogendoerfer wrote:
> On Fri, Jan 29, 2021 at 12:35:07PM +0800, Huang Pei wrote:
> > MIPS always save kernel stack pointer in regs[29]
> >
> > Signed-off-by: Huang Pei <huangpei@xxxxxxxxxxx>
> > ---
> > arch/mips/include/asm/ptrace.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/mips/include/asm/ptrace.h b/arch/mips/include/asm/ptrace.h
> > index 1e76774b36dd..daf3cf244ea9 100644
> > --- a/arch/mips/include/asm/ptrace.h
> > +++ b/arch/mips/include/asm/ptrace.h
> > @@ -53,7 +53,7 @@ struct pt_regs {
> >
> > static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
> > {
> > - return regs->regs[31];
> > + return regs->regs[29];
>
> hmm, I'm still wondering where the trick is... looks like this is used
> for uprobes, so nobody has ever used uprobes or I'm missing something.
>
> How did you find that ?
>
> Thomas.
>
> --
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea. [ RFC1925, 2.3 ]
Long story for short,
+. I think I had fix this bug in 2018, when I backported Uprobe from my
4.4 branch to CentOS 7 3.10. I just knwo it is *not* following MIPS
ABI, but I do not know how it destroy the cool function of
Kprobe/Uprobe, since the failure in porting eBPF from upstream to 3.10
just leave the fix in 3.10, totally forgotten.
+. In 2020, I was told to validate the effect of GNU XHash, and it came
to me that using Uprobe to count the number of "strcmp" called in ld.so,
so I found this fix again.
+. With more work on Kprobe/Kprobe_event/Uprobe, I found it hit only when
accessing arguments of Kprobe/Uprobe, so simple counting numbers of probe
fired would not trigger it
