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Message-ID: <CALCETrUBLL+2bU+A03sEns=ESjTXModxwuxiH1EHj__SDp2hgA@mail.gmail.com>
Date: Tue, 8 Nov 2016 08:20:48 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Roland McGrath <roland@...k.frob.com>,
Oleg Nesterov <oleg@...hat.com>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC][ATCH 1/3] ptrace: Remove maxargs from task_current_syscall()
On Tue, Nov 8, 2016 at 8:16 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> So I definitely approve of the change, but I wonder if we should go
> one step further:
>
> On Mon, Nov 7, 2016 at 1:26 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>
>> extern int task_current_syscall(struct task_struct *target, long *callno,
>> - unsigned long args[6], unsigned int maxargs,
>> - unsigned long *sp, unsigned long *pc);
>> + unsigned long args[6], unsigned long *sp,
>> + unsigned long *pc);
>
> The thing is, in C, having an array in a function declaration is
> pretty much exactly the same as just having a pointer, so from a type
> checking standpoint it doesn't really help all that much (but from a
> "human documentation" side the "args[6]" is much better than "*args").
>
> However, what would really help type checking is making it a
> structure. And maybe that structure could just contain "callno", "sp"
> and "pc" too? That would not only fix the type checking, it would make
> the calling convention even cleaner. Just have one single structure
> that contains all the relevant data.
I would propose calling this 'struct seccomp_data'.
>
> For example, kernel/seccomp.c does this instead:
>
> sd->nr = syscall_get_nr(task, regs);
> sd->arch = syscall_get_arch();
> syscall_get_arguments(task, regs, 0, 6, args);
> sd->args[0] = args[0];
> sd->args[1] = args[1];
> sd->args[2] = args[2];
> sd->args[3] = args[3];
> sd->args[4] = args[4];
> sd->args[5] = args[5];
> sd->instruction_pointer = KSTK_EIP(task);
It's a bit hard to tell from seccomp.c, but x86 carefully arranges for
that code to never get run -- instead the entry code supplies a struct
seccomp_data. Other arches could follow suit for a nice speedup.
--Andy
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