linux-hardening - RE: [PATCH] memremap: Replace 0-length array with flexible array

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Message-ID: <63b74bdc9a6e0_c81f029445@dwillia2-xfh.jf.intel.com.notmuch>
Date:   Thu, 5 Jan 2023 14:14:52 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Alex Sierra <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Shiyang Ruan <ruansy.fnst@...itsu.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        <linux-mm@...ck.org>, Alistair Popple <apopple@...dia.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-hardening@...r.kernel.org>
Subject: RE: [PATCH] memremap: Replace 0-length array with flexible array

Kees Cook wrote:
> Zero-length arrays are deprecated[1]. Replace struct ethtool_rxnfc's
> "rule_locs" 0-length array with a flexible array. Detected with GCC 13,
> using -fstrict-flex-arrays=3:
> 
> In file included from include/asm-generic/memory_model.h:5,
>                  from arch/x86/include/asm/page.h:86,
>                  from arch/x86/include/asm/thread_info.h:12,
>                  from include/linux/thread_info.h:60,
>                  from arch/x86/include/asm/preempt.h:9,
>                  from include/linux/preempt.h:78,
>                  from include/linux/spinlock.h:56,
>                  from include/linux/mmzone.h:8,
>                  from include/linux/gfp.h:7,
>                  from include/linux/mm.h:7,
>                  from mm/sparse-vmemmap.c:21:
> In function 'reuse_compound_section',
>     inlined from 'vmemmap_populate_compound_pages' at mm/sparse-vmemmap.c:407:6,
>     inlined from '__populate_section_memmap' at mm/sparse-vmemmap.c:463:7:
> mm/sparse-vmemmap.c:376:39: warning: array subscript <unknown> is outside array bounds of 'struct range[0]' [-Warray-bounds=]
>   376 |                 PHYS_PFN(pgmap->ranges[pgmap->nr_range].start);
>       |                          ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
> include/linux/pfn.h:22:43: note: in definition of macro 'PHYS_PFN'
>    22 | #define PHYS_PFN(x)     ((unsigned long)((x) >> PAGE_SHIFT))
>       |                                           ^
> In file included from include/linux/mm.h:31:
> include/linux/memremap.h: In function '__populate_section_memmap':
> include/linux/memremap.h:138:30: note: while referencing 'ranges'
>   138 |                 struct range ranges[0];
>       |                              ^~~~~~
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Alex Sierra <alex.sierra@....com>
> Cc: Felix Kuehling <Felix.Kuehling@....com>
> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> Cc: Shiyang Ruan <ruansy.fnst@...itsu.com>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: linux-mm@...ck.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Looks good to me:

Reviewed-by: Dan Williams <dan.j.williams@...el.com>

> ---
>  include/linux/memremap.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 7fcaf3180a5b..1314d9c5f05b 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -135,7 +135,7 @@ struct dev_pagemap {
>  	int nr_range;
>  	union {
>  		struct range range;
> -		struct range ranges[0];
> +		DECLARE_FLEX_ARRAY(struct range, ranges);
>  	};
>  };
>  
> -- 
> 2.34.1
>