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

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Message-ID: <CALCETrWjipqo3ZfbrrS4MkdFgcisrjvu=CcWe6T-h1HV7tzUrg@mail.gmail.com>
Date:   Tue, 30 Aug 2016 17:01:57 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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>,
        Marcin Nowakowski <marcin.nowakowski@...tec.com>
Subject: Re: [PATCH 1/2] tracing/syscalls: allow multiple syscall numbers per syscall

On Tue, Aug 30, 2016 at 4:28 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 30 Aug 2016 16:09:04 -0700
> Andy Lutomirski <luto@...capital.net> wrote:
>
>> But none of this should be a problem at all for MIPS, right?  AFAICT
>> the only problem for MIPS is that there *is* a mapping from metadata
>> to nr.  If that mapping got removed, MIPS should just work, right?
>
> Wait, where's the mapping of metadata to nr. I don't see that, nor do I
> see a need for that. The issue is that we have metadata that expresses
> how to record a syscall, and we map syscall nr to metadata, because
> when tracing is active, the only thing we have to find that metadata is
> the syscall nr.

It's in init_ftrace_syscalls():

        meta->syscall_nr = i;

and everything that uses that.  I think that this is the main problem
that the patch that started this thread changes, and I think that
deleting it would be cleaner than this patch.

>
> Now if a syscall nr has more than one way to record (a single nr for
> multiple syscalls), then we get into trouble. That's why we have
> trouble with compat syscalls. The same number maps to different
> syscalls, and we don't know how to differentiate that.

>
>
>>
>> For x86 compat, I think that adding arch should be sufficient.
>> Specifically, rather than having just one enter_syscall_files array,
>> have one per audit arch.  Then call syscall_get_arch() as well as
>> syscall_get_nr() and use both to lookup the metadata.  AFAIK this
>> should work on all architectures, although you might need some arch
>> helpers to enumerate all the arches and their respective syscall
>> tables (and max syscall nrs).
>
> OK, if the regs can get us to the arch, then this might work.
>
> That is, perhaps we can have multiple tables (not really sure how to
> make that happen in an arch agnostic way), and then have two functions:
>
> trace_get_syscall_nr(current, regs)
> trace_get_syscall_arch(current, regs)

Sadly, syscall_get_arch() doesn't take a regs parameter -- it looks at
current.  If it were made more general, it would need a task pointer,
not a regs pointer, but would just looking at current be okay for
tracing?

syscall_get_arch() does work on all archs that support seccomp filters, though.