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

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Message-ID: <20160830152955.17633511@gandalf.local.home>
Date:   Tue, 30 Aug 2016 15:29:55 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...capital.net>
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, 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.

Then it can use the number to system call mapping.

Yes, this code needs some loving.

-- Steve