linux-hardening - Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexible-array members

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Date:   Mon, 27 Jun 2022 20:27:37 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
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Subject: Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with
 flexible-array members

On 6/27/22 8:04 PM, Gustavo A. R. Silva 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@intel.com/
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
> Hi all!
> 
> JFYI: I'm adding this to my -next tree. :)

Fyi, this breaks BPF CI:

https://github.com/kernel-patches/bpf/runs/7078719372?check_suite_focus=true

   [...]
   progs/map_ptr_kern.c:314:26: error: field 'trie_key' with variable sized type 'struct bpf_lpm_trie_key' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
           struct bpf_lpm_trie_key trie_key;
                                   ^
   1 error generated.
   make: *** [Makefile:519: /tmp/runner/work/bpf/bpf/tools/testing/selftests/bpf/map_ptr_kern.o] Error 1
   make: *** Waiting for unfinished jobs....
   Error: Process completed with exit code 2.