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

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Message-Id: <A64AEAB9-9C41-408E-8B51-F2F58882CFBE@coly.li>
Date: Thu, 17 Apr 2025 22:17:30 +0800
From: Coly Li <i@...y.li>
To: Ard Biesheuvel <ardb@...nel.org>,
 Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Kees Cook <kees@...nel.org>,
 Coly Li <colyli@...nel.org>,
 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



> 2025年4月17日 22:08，Ard Biesheuvel <ardb@...nel.org> 写道：
> 
> 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?

I feel the “static” here is an optimization to avoid initializing zero_uuid again ….

Kent,

Am I right?

Coly Li