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

[Gustavo A. R. Silva]

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