Re: [RFC][ATCH 1/3] ptrace: Remove maxargs from task_current_syscall()

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Nov 8, 2016 at 8:16 AM, Linus Torvalds
<torvalds@linux-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@goodmis.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