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SubjectRe: test-case (Was: [PATCH 12/X] uprobes: x86: introduce abort_xol())
On 10/25, Srikar Dronamraju wrote:
> >
> > static inline void *uc_ip(struct ucontext *ctxt)
> > {
> > return (void*)ctxt->uc_mcontext.gregs[16];
> > }
> > ...
> >
> I have tested this on both x86_32 and x86_64 and can confirm that the
> behaviour is same with or without uprobes placed at that instruction.
> This is on the uprobes code with your changes.

Great, thanks.

> However on x86_32; the output is different from x86_64.
>
> On x86_32 (I have additionally printed the uc_ip and fault_insn.
>
> SIGSEGV! ip=0x10246 addr=0x12345678
> ERR!! wrong ip uc_ip(ctxt) = 10246 fault_insn = 804856c

Yep. uc_ip() is not correct on x86_32. Sorry, I forgot to mention this.

I was really surprised when I wrote this test. I simply can't understand
how can I play with ucontext in the user-space. I guess uc_ip() should use
REG_EIP instead of 16, but I wasn't able to compile it even if I added
__USE_GNU. It would be even better to use sigcontext instead of the ugly
mcontext_t, but this looks "impossible". The kernel is much simpler ;)


> I still trying to dig up what uc_ip is and why its different on x86_32.

See above. I guess it needs ctxt->uc_mcontext.gregs[14]. Or REG_EIP.

uc_ip() simply reads sigcontext->ip passed by setup_sigcontext().

> Also I was thinking on your suggestion of making abort_xol a weak
> function. In which case we could have architecture independent function
> in kernel/uprobes.c which is just a wrapper for set_instruction_pointer.
>
> void __weak abort_xol(struct pt_regs *regs, struct uprobe_task *utask)
> {
> set_instruction_pointer(regs, utask->vaddr);
> }
>
> where it would called from uprobe_notify_resume() as
>
> abort_xol(regs, utask);
>
> If other archs would want to do something else, they could override
> abort_xol definition.

I didn't suggest this ;) But looks reasonable to me. And afaics x86_32
can use this arch-independent function.

Oleg.



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