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Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexib

To: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
Subject: Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexible-array members
Cc: Kees Cook <keescook@xxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, dm-devel@xxxxxxxxxx, linux-m68k@xxxxxxxxxxxxxxxxxxxx, linux-mips@xxxxxxxxxxxxxxx, linux-s390@xxxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx, intel-gfx@xxxxxxxxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, bpf@xxxxxxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, linux-can@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux1394-devel@xxxxxxxxxxxxxxxxxxxxx, io-uring@xxxxxxxxxxxxxxx, lvs-devel@xxxxxxxxxxxxxxx, linux-mtd@xxxxxxxxxxxxxxxxxxx, kasan-dev@xxxxxxxxxxxxxxxx, linux-mmc@xxxxxxxxxxxxxxx, nvdimm@xxxxxxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxx, coreteam@xxxxxxxxxxxxx, linux-perf-users@xxxxxxxxxxxxxxx, linux-raid@xxxxxxxxxxxxxxx, linux-sctp@xxxxxxxxxxxxxxx, linux-stm32@xxxxxxxxxxxxxxxxxxxxxxxxxxxx, linux-arm-kernel@xxxxxxxxxxxxxxxxxxx, linux-scsi@xxxxxxxxxxxxxxx, target-devel@xxxxxxxxxxxxxxx, linux-usb@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, v9fs-developer@xxxxxxxxxxxxxxxxxxxxx, linux-rdma@xxxxxxxxxxxxxxx, alsa-devel@xxxxxxxxxxxxxxxx, linux-hardening@xxxxxxxxxxxxxxx
From: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
Date: Mon, 27 Jun 2022 12:53:43 -0700
On Mon, 27 Jun 2022 20:04:32 +0200
"Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx> wrote:

> There is a regular need in the kernel to provide a way to declare
> having a dynamically sized set of trailing elements in a structure.
> Kernel code should always use “flexible array members”[1] for these
> cases. The older style of one-element or zero-length arrays should
> no longer be used[2].
> 
> This code was transformed with the help of Coccinelle:
> (linux-5.19-rc2$ spatch --jobs $(getconf _NPROCESSORS_ONLN) --sp-file 
> script.cocci --include-headers --dir . > output.patch)
> 
> @@
> identifier S, member, array;
> type T1, T2;
> @@
> 
> struct S {
>   ...
>   T1 member;
>   T2 array[
> - 0
>   ];
> };
> 
> -fstrict-flex-arrays=3 is coming and we need to land these changes
> to prevent issues like these in the short future:
> 
> ../fs/minix/dir.c:337:3: warning: 'strcpy' will always overflow; destination 
> buffer has size 0,
> but the source string has length 2 (including NUL byte) [-Wfortify-source]
>               strcpy(de3->name, ".");
>               ^
> 
> Since these are all [0] to [] changes, the risk to UAPI is nearly zero. If
> this breaks anything, we can use a union with a new member name.
> 
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] 
> https://www.kernel.org/doc/html/v5.16/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Link: https://github.com/KSPP/linux/issues/78
> Build-tested-by: 
> https://lore.kernel.org/lkml/62b675ec.wKX6AOZ6cbE71vtF%25lkp@xxxxxxxxx/
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

Thanks this fixes warning with gcc-12 in iproute2.
In function ‘xfrm_algo_parse’,
    inlined from ‘xfrm_state_modify.constprop’ at xfrm_state.c:573:5:
xfrm_state.c:162:32: warning: writing 1 byte into a region of size 0 
[-Wstringop-overflow=]
  162 |                         buf[j] = val;
      |                         ~~~~~~~^~~~~

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