linux-kernel - Re: ACPI: kmemcheck: Caught 16-bit read from freed memory (f7c12ec6)

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Date:	Thu, 8 May 2008 08:05:27 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Lin Ming" <ming.m.lin@...el.com>
Cc:	"Pekka Enberg" <penberg@...helsinki.fi>,
	"Bob Moore" <robert.moore@...el.com>,
	"Alexey Starikovskiy" <astarikovskiy@...e.de>,
	"Len Brown" <len.brown@...el.com>, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: ACPI: kmemcheck: Caught 16-bit read from freed memory (f7c12ec6)

Hello,

On Thu, May 8, 2008 at 7:35 AM, Lin Ming <ming.m.lin@...el.com> wrote:
> Here comes a simple patch that fixes the warning in my machine.
>
>  Vegard, would you please help to test it in your machine?
>

Thanks for the try, but unfortunately this does not solve the problem.

Please note that kmemcheck is an patch to the kernel; without it you
will never see the warning. You can pull it from
git://git.kernel.org/pub/scm/linux/kernel/git/vegard/kmemcheck.git current
though it is unlikely that it will help you more than looking at the
code (or the report) will do.

>  Thanks,
>  Lin Ming
>
>  Signed-off-by: Lin Ming <ming.m.lin@...el.com>
>  ---
>  diff --git a/drivers/acpi/parser/psargs.c b/drivers/acpi/parser/psargs.c
>  index f1e8bf6..ef55d24 100644
>  --- a/drivers/acpi/parser/psargs.c
>  +++ b/drivers/acpi/parser/psargs.c
>  @@ -268,7 +268,7 @@ acpi_ps_get_next_namepath(struct acpi_walk_state
>  *walk_state,

>          */
>         if (ACPI_SUCCESS(status) &&
>             possible_method_call && (node->type == ACPI_TYPE_METHOD)) {
>  -               if (walk_state->op->common.aml_opcode == AML_UNLOAD_OP) {
>  +               if (walk_state->op && walk_state->op->common.aml_opcode ==
>  AML_UNLOAD_OP) {
>                         /*
>                          * acpi_ps_get_next_namestring has increased the AML pointer,
>                          * so we need to restore the saved AML pointer for method call.

Also, noticing your change, I can see why it makes no difference:
Pekka already found that it is walk_state->op that has the value of
0xf7c12ec6 (e.g. the pointer being dereferenced), so the test will
still succeed.

On the other hand, I have discovered what seems to be a deficiency in
kmemcheck (i.e. it might be my fault entirely), so it is possible that
the warning is bogus. Will send an update shortly.


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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