Subject: Re: linux-next: build failure after merge of the kspp tree

From: Kees Cook <[email-masked]>
Date: Wed, 9 Mar 2022 08:52:06 -0800

On Wed, Mar 09, 2022 at 09:10:29PM +1100, Stephen Rothwell wrote:
> Hi all,
> 
> After merging the kspp tree, today's linux-next build (x86_64
> allmodconfig) failed like this:
> 
> In file included from include/linux/bitops.h:33,
>                  from include/linux/kernel.h:22,
>                  from drivers/platform/x86/thinkpad_acpi.c:37:
> drivers/platform/x86/thinkpad_acpi.c: In function 'tpacpi_dytc_profile_init':
> arch/x86/include/asm/bitops.h:207:22: error: array subscript 'long unsigned int[0]' is partly outside array bounds of 'int[1]' [-Werror=array-bounds]
>   207 |                 (addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
>       |                 ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/platform/x86/thinkpad_acpi.c:10385:18: note: while referencing 'output'
> 10385 |         int err, output;
>       |                  ^~~~~~
> 
> Actually the test_bit() in line 10409.
> 
> Caused by commit
> 
>   e1c21608e3cf ("platform/x86: thinkpad_acpi: Add PSC mode support")
> 
> from the drivers-x86 tree interacting with the enablement of
> -Werror=array-bounds.
> 
> I have reverted that commit for today.

Looking at https://lore.kernel.org/all/[email-masked]/

The problem is with the cast of "int output" to "unsigned long". Past
fixes have either made the variable unsigned long, or adding a new
variable and removal of casts. e.g. this pseudo-patch:

+	unsigned long bits;

	...
+	bits = output;
-	if (test_bit(DYTC_FC_MMC, (void *)&output)) { /* MMC MODE */
+	if (test_bit(DYTC_FC_MMC, &bits)) { /* MMC MODE */

...

-	} else if (test_bit(DYTC_FC_PSC, (void *)&output)) { /*PSC MODE */
+	} else if (test_bit(DYTC_FC_PSC, &bits)) { /*PSC MODE */

-- 
Kees Cook