linux-kernel - [ARCH question] Do syscall_get_nr and syscall_get

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Date:	Tue, 4 Feb 2014 15:50:46 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>,
	linux-arch <linux-arch@...r.kernel.org>
Cc:	linux-audit@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Steve Grubb <sgrubb@...hat.com>, Eric Paris <eparis@...hat.com>
Subject: [ARCH question] Do syscall_get_nr and syscall_get_arguments always work?

On Tue, Feb 4, 2014 at 11:32 AM, Andy Lutomirski <luto@...capital.net> wrote:
> Now we get rid of __audit_syscall_entry.  (This speeds up even the
> auditing-is-on case.)  Instead we have __audit_start_record, which
> does more or less the same thing, except that (a) it doesn't BUG if
> in_syscall and (b) it *sets* TIF_SYSCALL_AUDIT.  This relies on the
> fact that syscall_get_nr and syscall_get_arguments are reliable on
> x86_64.  I suspect that they're reliable everywhere else, too.  The
> idea is that there's nothing wrong with calling __audit_start_record
> more than once.  (Maybe it should be called
> __audit_record_this_syscall.)

I'd like to make a change that can result in syscall_get_nr and
syscall_get_arguments being called (on current and
task_pt_regs(current)) from any system call (as opposed to being
called only from the audit/trace slowpaths).  Is this safe?

Here's my somewhat clueless analysis:

On x86_64, I've tested it, and it works.  The entry code saves all of
the argument registers, even in the fast path.

i386 and ia32_compat look okay, too.

If "stmia    sp, {r0 - r12}            @ Calling r0 - r12" does what I
think it does, then arm should be okay.

I'm totally guessing here, but e10_sync on aarch64 seems to save
enough registers.  I admit to being a little bit surprised, though --
aarch64 is new, and if I were designing an ABI, I specify that
syscalls *don't* save registers.

ia64 has a comment in ivt.S that streamlined syscalls save nr in r15.
The rest come from unwind info (!).  I assume this has something to do
with the magic ia64 register rotation thing.  I have no idea what
happens if there's a NaT in an argument register.

I can't even find the system call entry point on mips.

Is there a semi-official answer here?

--Andy
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