# Re: \[PATCH 1/4\] arm64: expose orig\_x0 in the user\_pt\_regs structure

- From: Andrei Vagin \<hidden\>
- Date: 2021-03-27 00:40:39 
- List: linux-arm-kernel
- Page: https://ratatoskr.run/linux-arm-kernel/2021/03/6317364
- Thread: https://ratatoskr.run/linux-arm-kernel/2021/03/6311396/t.md

```text
On Fri, Mar 26, 2021 at 11:28 AM Catalin Marinas
[ref] wrote:
>
> On Mon, Mar 22, 2021 at 03:50:50PM -0700, Andrei Vagin wrote:
> > diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
> > index 758ae984ff97..3c118c5b0893 100644
> > --- a/arch/arm64/include/uapi/asm/ptrace.h
> > +++ b/arch/arm64/include/uapi/asm/ptrace.h
> > @@ -90,6 +90,7 @@ struct user_pt_regs {
> >       __u64           sp;
> >       __u64           pc;
> >       __u64           pstate;
> > +     __u64           orig_x0;
> >  };
>
> That's a UAPI change, likely to go wrong. For example, a
> ptrace(PTRACE_GETREGSET, pid, REGSET_GPR, data) would write past the end
> of an old struct user_pt_regs in the debugger.

ptrace(PTRACE_GETREGSET, ...) receives iovec:
ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov)

iov contains a pointer to a buffer and its size and the kernel fills
only the part that fits the buffer.
I think this interface was invented to allow extending structures
without breaking backward compatibility.

>
> --
> Catalin

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