linux-kernel - Re: [PATCH 1/2] tracing/syscalls: allow multiple syscall numbers per syscall

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Date:   Tue, 30 Aug 2016 12:53:53 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Marcin Nowakowski <marcin.nowakowski@...tec.com>,
        Linux API <linux-api@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        open list <linux-kernel@...r.kernel.org>,
        Linux MIPS Mailing List <linux-mips@...ux-mips.org>
Subject: Re: [PATCH 1/2] tracing/syscalls: allow multiple syscall numbers per syscall

On Tue, Aug 30, 2016 at 12:29 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 30 Aug 2016 11:52:39 -0700
> Andy Lutomirski <luto@...capital.net> wrote:
>
>
>> Okay, I think I see what's going on.  init_ftrace_syscalls() does:
>>
>>         meta = find_syscall_meta(addr);
>>
>> Unless I'm missing some reason why this is a sensible thing to do,
>> this seems overcomplicated and incorrect.  There is exactly one caller
>> of find_syscall_meta() and that caller knows the syscall number.  Why
>> doesn't it just look up the metadata by *number* instead of by syscall
>> implementation address?  There are plenty of architectures for which
>> multiple logically different syscalls can share an implementation
>> (e.g. pretty much everything that calls in_compat_syscall()).
>
> The problem is that the meta data is created at the syscalls
> themselves. Look at all the macro magic in include/linux/syscalls.h,
> and search for __syscall_metadata. The meta data is created via linker
> magic, and the find_syscall_meta() is what finds a specific system call
> and the meta data associated with it.

Egads!  OK, I see why this is a mess.

I guess we should be creating the metadata from the syscall tables
instead of from the syscall definitions, but I guess that's currently
a nasty per-arch mess.

Could we at least have an array of (arch, nr) instead of just an array
of nrs in the metadata?

--Andy