Re: [PATCH] init/Kconfig: extend -Wno-array-bounds to gcc 13

[Kees Cook]

On March 6, 2023 2:20:50 PM PST, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>+ Kees
>https://lore.kernel.org/lkml/20230306220947.1982272-1-trix@xxxxxxxxxx/
>
>On Mon, Mar 6, 2023 at 2:10 PM Tom Rix <trix@xxxxxxxxxx> wrote:
>>
>> With gcc 13.0.1 on x86, there are several false positives like
>>
>> drivers/net/ethernet/microchip/sparx5/sparx5_psfp.c:167:31:
>>   error: array subscript 4 is above array bounds of ‘const struct sparx5_psfp_gce[4]’ [-Werror=array-bounds=]
>>   167 |                 gce = &sg->gce[i];
>>       |                        ~~~~~~~^~~
>> In file included from drivers/net/ethernet/microchip/sparx5/sparx5_psfp.c:8:
>> drivers/net/ethernet/microchip/sparx5/sparx5_main.h:506:32: note: while referencing ‘gce’
>>   506 |         struct sparx5_psfp_gce gce[SPX5_PSFP_GCE_CNT];
>>       |                                ^~~
>>
>> The code lines for the reported problem
>>         /* For each scheduling entry */
>>         for (i = 0; i < sg->num_entries; i++) {
>>                 gce = &sg->gce[i];
>>
>> i is bounded by num_entries, which is set in sparx5_tc_flower.c
>>         if (act->gate.num_entries >= SPX5_PSFP_GCE_CNT) {
>>                 NL_SET_ERR_MSG_MOD(extack, "Invalid number of gate entries");
>>                 return -EINVAL;
>>         }
>> ..
>>         sg->num_entries = act->gate.num_entries;
>>
>> So disable array-bounds as was done on gcc 11 and 12

GCC 13 isn't released yet, and we've been working to make Linux warning-free under -Wareay-bounds. (And we succeeded briefly with GCC 11.)

I'd much rather get GCC fixed. This is due to the shift sanitizer reducing the scope of num_entries (via macro args) to 0-31, which is still >4. This seems like a hinting bug in GCC: just because the variable was used in a shift doesn't mean the compiler can make any value assumptions.

-Kees



-- 
Kees Cook