linux-kernel - Re: [PATCH v5 24/25] ptrace: add PTRACE_GET_SYSCALL

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Date:   Tue, 11 Dec 2018 16:29:54 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Dmitry V. Levin" <ldv@...linux.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Elvira Khabirova <lineprinter@...linux.org>,
        Eugene Syromyatnikov <esyr@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, linux-api@...r.kernel.org,
        strace-devel@...ts.strace.io, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 24/25] ptrace: add PTRACE_GET_SYSCALL_INFO request

On 12/10, Dmitry V. Levin wrote:
>
> On Mon, Dec 10, 2018 at 03:11:07PM +0100, Oleg Nesterov wrote:
> > On 12/10, Dmitry V. Levin wrote:
> > >
> > > +struct ptrace_syscall_info {
> > > +	__u8 op;	/* PTRACE_SYSCALL_INFO_* */
> > > +	__u8 __pad0[3];
> > > +	__u32 arch;
> > > +	__u64 instruction_pointer;
> > > +	__u64 stack_pointer;
> > > +	__u64 frame_pointer;
> > > +	union {
> > > +		struct {
> > > +			__u64 nr;
> > > +			__u64 args[6];
> > > +		} entry;
> > > +		struct {
> > > +			__s64 rval;
> > > +			__u8 is_error;
> > > +			__u8 __pad1[7];
> > > +		} exit;
> > > +		struct {
> > > +			__u64 nr;
> > > +			__u64 args[6];
> > > +			__u32 ret_data;
> > > +			__u8 __pad2[4];
> > > +		} seccomp;
> > > +	};
> > > +};
> >
> > Could you explain why ptrace_syscall_info needs __pad{0,1,2} ? I simply can't
> > understand why...
>
> I suppose the idea behind the use of these pads was to make the structure
> arch-independent.

Still can't understand... are you saying that without (say) __pad2[4]
sizeof(ptrace_syscall_info) or offsetofend(ptrace_syscall_info, seccomp)
will depend on arch? Or what? I am just curious.

> I don't think we really need to keep it exactly the same on all
> architectures - the only practical requirement is to avoid any compat
> issues, but I don't mind keeping the structure arch-independent.

OK, but may be you can add a short comment to explain these pads.

Oleg.