linux-kernel - Re: [PATCH 1/4] arm64: expose orig_x0 in the user_pt

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Date:   Sat, 27 Mar 2021 13:01:01 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Will Deacon <will@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Martin <Dave.Martin@....com>,
        Keno Fischer <keno@...iacomputing.com>
Subject: Re: [PATCH 1/4] arm64: expose orig_x0 in the user_pt_regs structure

On Fri, Mar 26, 2021 at 05:35:19PM -0700, Andrei Vagin wrote:
> On Fri, Mar 26, 2021 at 11:28 AM Catalin Marinas
> <catalin.marinas@....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.

You are right here, it doesn't write past the end of the iov buffer.
However, it's still an ABI change. An unaware program using a newer
user_pt_regs but running on an older kernel may be surprised that the
updated iov.len is smaller than sizeof (struct user_pt_regs).

Changing this structure also changes the core dump format, see ELF_NGREG
and ELF_CORE_COPY_REGS. Maybe this doesn't matter much either since the
ELF note would have size information but I'd prefer if we didn't modify
this structure.

-- 
Catalin