linux-hardening - Re: [PATCH][next] x86/mm/pgtable: Fix Wstringop-overflow warnings

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Date:   Fri, 1 Apr 2022 15:12:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] x86/mm/pgtable: Fix Wstringop-overflow warnings

On Thu, Mar 31, 2022 at 07:58:34PM -0500, Gustavo A. R. Silva wrote:
> Fix the following -Wstringop-overflow warnings when building with GCC-11:
> 
> .arch/x86/mm/pgtable.c: In function ‘pgd_alloc’:
> arch/x86/mm/pgtable.c:437:13: warning: ‘preallocate_pmds.constprop’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
>   437 |         if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
>       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:437:13: note: referencing argument 2 of type ‘pmd_t **’
> arch/x86/mm/pgtable.c:225:12: note: in a call to function ‘preallocate_pmds.constprop’
>   225 | static int preallocate_pmds(struct mm_struct *mm, pmd_t *pmds[], int count)
>       |            ^~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:440:13: warning: ‘preallocate_pmds.constprop’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
>   440 |         if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
>       |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:440:13: note: referencing argument 2 of type ‘pmd_t **’
> arch/x86/mm/pgtable.c:225:12: note: in a call to function ‘preallocate_pmds.constprop’
>   225 | static int preallocate_pmds(struct mm_struct *mm, pmd_t *pmds[], int count)
>       |            ^~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:462:9: warning: ‘free_pmds.constprop’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
>   462 |         free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:462:9: note: referencing argument 2 of type ‘pmd_t **’
> arch/x86/mm/pgtable.c:213:13: note: in a call to function ‘free_pmds.constprop’
>   213 | static void free_pmds(struct mm_struct *mm, pmd_t *pmds[], int count)
>       |             ^~~~~~~~~
> arch/x86/mm/pgtable.c:455:9: warning: ‘pgd_prepopulate_user_pmd’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
>   455 |         pgd_prepopulate_user_pmd(mm, pgd, u_pmds);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:455:9: note: referencing argument 3 of type ‘pmd_t **’
> arch/x86/mm/pgtable.c:320:13: note: in a call to function ‘pgd_prepopulate_user_pmd’
>   320 | static void pgd_prepopulate_user_pmd(struct mm_struct *mm,
>       |             ^~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:464:9: warning: ‘free_pmds.constprop’ accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
>   464 |         free_pmds(mm, pmds, PREALLOCATED_PMDS);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:464:9: note: referencing argument 2 of type ‘pmd_t **’
> arch/x86/mm/pgtable.c:213:13: note: in a call to function ‘free_pmds.constprop’
>   213 | static void free_pmds(struct mm_struct *mm, pmd_t *pmds[], int count)
>       |             ^~~~~~~~~
> 
> by using pointer notation instead of array notation as a workaround for
> the above GCC warnings.

'Workaround' implies the warning is on crack...

> This helps with the ongoing efforts to globally enable
> -Wstringop-overflow.

which puts to question this endeavour, why are you wanting to have this
if its crap?