linux-kernel - Re: Oprofile bug ?

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Date:	Fri, 12 Nov 2010 16:45:14 -0500
From:	fche@...hat.com (Frank Ch. Eigler)
To:	"Sylvain GENEVES" <Sylvain.Geneves@...ialpes.fr>
Cc:	"oprofile-list@...ts.sf.net" <oprofile-list@...ts.sf.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Oprofile bug ?

"Sylvain GENEVES" <Sylvain.Geneves@...ialpes.fr> writes:

> [...]
> I'm encountering unexpected behaviour with OProfile when the profiled
> system is under heavy load : "BUG: unable to handle kernel paging request
> at 0000000000004cc3" (full console message is attached).
> [...]
> Anyone has any idea on what is happening ?

Just glancing at that oops & my local random kernel build, it appears
as though this part of arch/x86/kernel/time.c:profile_pc is failing:

unsigned long profile_pc(struct pt_regs *regs)
{
        unsigned long pc = instruction_pointer(regs);

        if (!user_mode_vm(regs) && in_lock_functions(pc)) {
#ifdef CONFIG_FRAME_POINTER
                return *(unsigned long *)(regs->bp + sizeof(long));
#else
                       ^^^^^^^^^^^^^^^^^^
[...]

regs->bp must have been 0x4cbb, which this code turns into an
unchecked dereferences at 0x4cbb+8 = 0x4cc3.  I don't have a theory
as to why regs->bp should have that value in it, but the kernel
should probably use probe_kernel_read() or somesuch to validate the
value before dereferencing it.

- FChE
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