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Date:	Mon, 07 Feb 2005 11:28:02 -0800
From:	David Daney <ddaney@avtrex.com>
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To:	andreev <andreev@niisi.msk.ru>, Ralf Baechle <ralf@linux-mips.org>
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Subject: Re: Strace doesn't work on linux-2.4.28 and later
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andreev wrote:
> Hi, list.
> 
> We are using the latest kernel from mips-linux CVS and there is a 
> problem with ptrace.
> 
> When syscall with 5 or more arguments are traced, the fifth argument of 
> the syscall is overwritten
> by tracing code. This error causes problems with strace. For example, 
> you can't trace dynamically linked
> applications, because ld.so calls mmap which has 6 arguments.
> 

This patch broke it:

http://www.linux-mips.org/archives/linux-cvs/2004-11/msg00116.html

RCS file: /home/cvs/linux/arch/mips/kernel/Attic/scall_o32.S,v
retrieving revision 1.18.2.13
retrieving revision 1.18.2.14
diff -u -p -r1.18.2.13 -r1.18.2.14
--- linux/arch/mips/kernel/Attic/scall_o32.S	2004/04/26 15:06:02	1.18.2.13
+++ linux/arch/mips/kernel/Attic/scall_o32.S	2004/11/25 09:43:59	1.18.2.14
@@ -121,9 +121,9 @@ reschedule:

  trace_a_syscall:
  	SAVE_STATIC
-	sw	t2, PT_R1(sp)
+	sw	t2, PT_SCRATCH0(sp)
  	jal	syscall_trace
-	lw	t2, PT_R1(sp)
+	lw	t2, PT_SCRATCH0(sp)

  	lw	a0, PT_R4(sp)		# Restore argument registers
  	lw	a1, PT_R5(sp)

PT_SCRATCH0(sp) = 16(sp) which is where arg5 is stored.  This overwrites it.

In arch/mips/tools/offset.c we have:

	offset("#define PT_SCRATCH0 ", struct pt_regs, pad0[4]);
	offset("#define PT_SCRATCH1 ", struct pt_regs, pad0[5]);

I am thinking of testing a patch where I change them to:

	offset("#define PT_SCRATCH0 ", struct pt_regs, pad0[0]);
	offset("#define PT_SCRATCH1 ", struct pt_regs, pad0[1]);

Any needed argument registers are already saved in and restored from the 
regs array so overwriting the stack area reserved for them should be OK.

David Daney