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

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Date:   Mon, 6 Mar 2023 17:07:42 -0800
From:   Tom Rix <trix@...hat.com>
To:     Kees Cook <kees@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>
Cc:     mhiramat@...nel.org, akpm@...ux-foundation.org,
        masahiroy@...nel.org, paulmck@...nel.org, hannes@...xchg.org,
        ojeda@...nel.org, thunder.leizhen@...wei.com,
        christophe.leroy@...roup.eu, vbabka@...e.cz,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] init/Kconfig: extend -Wno-array-bounds to gcc 13


On 3/6/23 3:02 PM, Kees Cook wrote:
> On March 6, 2023 2:20:50 PM PST, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>> + Kees
>> https://lore.kernel.org/lkml/20230306220947.1982272-1-trix@redhat.com/
>>
>> On Mon, Mar 6, 2023 at 2:10 PM Tom Rix <trix@...hat.com> 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.

The build with fail generally with gcc 13.

The warnings could be cleaned without having an error, but I looked at 
multiple errors, none of them were real.

imo this is a broken compiler option.

Tom

>
> -Kees
>
>
>