linux-kernel - Re: [PATCH] oprofile, x86: Fix nmi-unsafe callgraph support in, 2.6.27 and later

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Date:	Thu, 9 Jun 2011 14:57:42 +0200
From:	Robert Richter <robert.richter@....com>
To:	John Lumby <johnlumby@...mail.com>
CC:	Maynard Johnson <maynardj@...ibm.com>,
	oprofile list <oprofile-list@...ts.sourceforge.net>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] oprofile, x86: Fix nmi-unsafe callgraph support in,
 2.6.27 and later

On 08.06.11 13:43:56, John Lumby wrote:
> I am also a bit confused by the kernel's statement that it's a NULL 
> pointer deref.     I had previously found that the pointer contains 
> 0x0000000a,  not NULL.   Does the kernel call it NULL for any invalid 
> value?   Or am I misunderstanding it.    Anyway ...

0x0000000a is also considered a NULL pointer access, typically this
happens if a member of a struct which points to NULL is accessed.

> 
> Have you or anyone tried this on an intel x86?     I'm just curious 
> whether it's everyone or only me.

>From your other mail:

"EIP is at print_context_stack=0x45/0xb0

and from a machine-code listing, I found that that offset corresponds 
to the line

       addr = *stack;

in arch/x86/kernel/dumpstack.c"

Actually this should not happen, because of checking the stack pointer
in valid_stack_ptr(). So could you apply the change below and test if
this throws a bug message?

Thanks,

-Robert


diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index e2a3f06..37693f5 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -73,6 +73,8 @@ static inline int valid_stack_ptr(struct thread_info *tinfo,
                        void *p, unsigned int size, void *end)
 {
        void *t = tinfo;
+
+       BUG_ON(p < (void *)THREAD_SIZE);
        if (end) {
                if (p < end && p >= (end-THREAD_SIZE))
                        return 1;


-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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