linux-hardening - Re: [PATCH][next] xfs: Replace one-element arrays with flexible-array members

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Date:   Mon, 6 Feb 2023 13:17:14 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     "Darrick J. Wong" <djwong@...nel.org>, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH][next] xfs: Replace one-element arrays with flexible-array
 members



On 2/3/23 11:53, Kees Cook wrote:
> On Thu, Feb 02, 2023 at 07:24:50PM -0600, Gustavo A. R. Silva wrote:
>> One-element arrays are deprecated, and we are replacing them with flexible
>> array members instead. So, replace one-element arrays with flexible-array
>> members in structures xfs_attr_leaf_name_local and
>> xfs_attr_leaf_name_remote.
>>
>> The only binary differences reported after the changes are all like
>> these:
>>
>> fs/xfs/libxfs/xfs_attr_leaf.o
>> _@@ -435,7 +435,7 @@
>>        3b8:      movzbl 0x2(%rbx),%eax
>>        3bc:      rol    $0x8,%bp
>>        3c0:      movzwl %bp,%ebp
>> -     3c3:      lea    0x2(%rax,%rbp,1),%ebx
>> +     3c3:      lea    0x3(%rax,%rbp,1),%ebx
>>        3c7:      call   3cc <xfs_attr_leaf_entsize+0x8c>
>>                          3c8: R_X86_64_PLT32     __tsan_func_exit-0x4
>>        3cc:      or     $0x3,%ebx
>> _@@ -454,7 +454,7 @@
>>        3ea:      movzbl 0x8(%rbx),%ebx
>>        3ee:      call   3f3 <xfs_attr_leaf_entsize+0xb3>
>>                          3ef: R_X86_64_PLT32     __tsan_func_exit-0x4
>> -     3f3:      add    $0xa,%ebx
>> +     3f3:      add    $0xb,%ebx
>>        3f6:      or     $0x3,%ebx
>>        3f9:      add    $0x1,%ebx
>>        3fc:      mov    %ebx,%eax
>>
>> similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.
> 
> I usually turn off the sanitizers for the A/B build comparisons to make

Oh yes! that's a good point. I'll see that they are turned off next time. :)

> it easier to read the results. It looks like it _grew_ in size here,
> though?

Yep; I'm sorry I got it wrong. :/ I had it right in the beginning, then after
reading the code once again just before sending out a version of this patch
with only the flex-array transformations, I noticed the entsize functions and
the "sizeof(struct-with-one-element-array) - 1" and I forgot about the padding,
removed the "- 1" and got a bit confused with my build-tests.

I'll send v2 with my original changes... the flex-array transformations, only.

--
Gustavo