On Tue, Feb 02, 2021 at 09:32:31AM +0800, Huang Pei wrote:
> 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
Thank you for the explanation, applied to mips-next.
Thomas.
--
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea. [ RFC1925, 2.3 ]
