linux-kernel - Re: [PATCH] md/bcache: Mark _

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Message-ID: <CAMj1kXHwBKu1K=yjJprnEArZFKphvDPsfZzA5KgTtyo5L866=w@mail.gmail.com>
Date: Thu, 17 Apr 2025 16:08:22 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Coly Li <i@...y.li>
Cc: Kees Cook <kees@...nel.org>, Coly Li <colyli@...nel.org>, 
	Kent Overstreet <kent.overstreet@...ux.dev>, linux-bcache@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] md/bcache: Mark __nonstring look-up table

On Thu, 17 Apr 2025 at 15:12, Coly Li <i@...y.li> wrote:
>
>
>
> > 2025年4月17日 15:10，Kees Cook <kees@...nel.org> 写道：
> >
> >
> >
> > On April 16, 2025 11:16:45 PM PDT, Ard Biesheuvel <ardb@...nel.org> wrote:
> >> On Thu, 17 Apr 2025 at 00:01, Kees Cook <kees@...nel.org> wrote:
> >>>
> >>> GCC 15's new -Wunterminated-string-initialization notices that the 16
> >>> character lookup table "zero_uuid" (which is not used as a C-String)
> >>> needs to be marked as "nonstring":
> >>>
> >>> drivers/md/bcache/super.c: In function 'uuid_find_empty':
> >>> drivers/md/bcache/super.c:549:43: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (17 chars into 16 available) [-Wunterminated-string-initialization]
> >>>  549 |         static const char zero_uuid[16] = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
> >>>      |                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>>
> >>> Add the annotation to silence the GCC warning.
> >>>
> >>> Signed-off-by: Kees Cook <kees@...nel.org>
> >>> ---
> >>> Cc: Coly Li <colyli@...nel.org>
> >>> Cc: Kent Overstreet <kent.overstreet@...ux.dev>
> >>> Cc: linux-bcache@...r.kernel.org
> >>> ---
> >>> drivers/md/bcache/super.c | 2 +-
> >>> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> >>> index e42f1400cea9..577d048170fe 100644
> >>> --- a/drivers/md/bcache/super.c
> >>> +++ b/drivers/md/bcache/super.c
> >>> @@ -546,7 +546,7 @@ static struct uuid_entry *uuid_find(struct cache_set *c, const char *uuid)
> >>>
> >>> static struct uuid_entry *uuid_find_empty(struct cache_set *c)
> >>> {
> >>> -       static const char zero_uuid[16] = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
> >>> +       static const char zero_uuid[] __nonstring = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
> >>>
> >>
> >> Just
> >>
> >> static const char zero_uuid[16] = {};
> >>
> >> should work fine here too. No need for the initializer.
> >
> > 🤦 Yes. This is what I get for fixing dozens of these. I'll send a v2...
>
>
> Can we do this,
>
> static const char zero_uuid[16] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
>
> I like the explicit array element number 16, and the explicit uuid content by obvious zero (‘0’) symbols. They provide redundant information.
> Not sure whether GCC 15 complains or not.
>

Even the {} initializer is entirely redundant, given that the variable
has static linkage, and so C guarantees that it will be zero
initialized.

Could you use NULL_GUID and be done with it?