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Bug in the syscall tracing code

To: linux-mips@linux-mips.org
Subject: Bug in the syscall tracing code
From: "Gleb O. Raiko" <raiko@niisi.msk.ru>
Date: Thu, 06 Oct 2005 21:21:49 +0400
Organization: NIISI RAN
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mozilla Thunderbird 1.0.7 (Windows/20050923)
Hello,

The story continues. The last fix of the syscall tracing code was wrong, unfortunately. (The bug was a user could invoke any function in the kernel. The fix was not to use t2 as pointer to a syscall, s0 was chosen for it.) The problem we discovered is a few syscalls do SAVE_STATIC (those declared as save_static_function), so s0 (which holds pointer to the syscall at the time the syscall is invoked) is saved on the stack overwriting a value saved from the process being traced. No wonder, s0 that restored on syscall exit differs from s0 saved on syscall enter.

See, arch/mips/kernel/scall32-o32.S, syscall_trace_entry, for example. The rest of ABIs are the same.

There are several ways to fix this:

1. Make syscall handling code to be close to other arches. I mean, check for the trace flag first, then parse arguments and invoke a syscall.

2. Remove save_static_functions and do SAVE_STATIC early for several syscalls (yes, one big switch or its asm equivalent).

3. Store t2 in pt_regs (it means we have to expand this structure).

4. I know there should be yet another way.

Any ideas ?

Regards,
Gleb.

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