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 <20210326182839.GE5126@arm.com>
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From:   Andrei Vagin <avagin@gmail.com>
Date:   Fri, 26 Mar 2021 17:35:19 -0700
Message-ID: <CANaxB-xTrMkJ2D19wwjSbCfizdRDnNO52fibpaRXP5g3hOcQwQ@mail.gmail.com>
Subject: Re: [PATCH 1/4] arm64: expose orig_x0 in the user_pt_regs structure
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Will Deacon <will@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Keno Fischer <keno@juliacomputing.com>
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On Fri, Mar 26, 2021 at 11:28 AM Catalin Marinas
<catalin.marinas@arm.com> 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