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

From: "Gustavo A. R. Silva" <[email-masked]>
Date: Mon, 27 Jun 2022 20:35:13 +0200

On Mon, Jun 27, 2022 at 08:27:37PM +0200, Daniel Borkmann wrote:
> 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/[email-masked]/
> > Signed-off-by: Gustavo A. R. Silva <[email-masked]>
> > ---
> > Hi all!
> > 
> > JFYI: I'm adding this to my -next tree. :)
> 
> Fyi, this breaks BPF CI:

Thanks for the report! It seems the 0-day robot didn't catch that one.
I'll fix it up right away. :)

--
Gustavo

> 
> 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.