lkml.org 
[lkml]   [2022]   [Sep]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN
Date
From: YingChi Long
> Sent: 27 September 2022 17:44
>
> > Interesting - what justification do they give?
> > Linux kernel requires that the compiler add no unnecessary padding
> > so that structure definitions are well defined.
>
> Yes, that's a clarification given in 2019.
>
> > So using a type definition inside offsetof() won't give a
> > useful value - but it still isn't really UB.
>
> WG14 may worry about commas and the scope of new definitions. So they provide
> new words into the standard and said:
>
> > If the specified type defines a new type or if the specified member is a
> > bit-field, the behavior is undefined.
>
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm

Except that the kernel requires it to be defined.

Did they clarify the clause that required offsetof() to return
a compile-time constant?
That stops you doing offsetof(struct x, member->array[expression]).
(Oh and the compiler for a common OS disallows any version of that
even when expression in an integer constant!)

>
> I've provided this link in the patch.
>
> > Has that ever worked?
> > Given:
> > struct foo {
> > int a;
> > char b;
> > char c;
> > };
>
> TYPE_ALIGN(struct foo) evaluates to 4 in the previous approach (based on
> offsetof). _Align(struct foo) evaluates to the same value.
>
> See https://godbolt.org/z/sqebhEnsq
>
> > I think CHECK_MEMBER_AT_END_OF_TYPE(struct foo, b) is true.
>
> Hmm, both the previous version and after this patch the macro gives me
> false. (See the godbolt link).

See https://godbolt.org/z/95shMx44j

It return 1 for a and 0 for b and c (and char d,e following b).
NFI what it is trying to do!

David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

\
 
 \ /
  Last update: 2022-09-28 10:10    [W:0.062 / U:0.732 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site