* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Mar 3, 2016 at 8:53 AM, tip-bot for Dave Hansen
> <tipbot@xxxxxxxxx> wrote:
> >
> > If u64 has a natural alignment of 8 bytes (this is rare, most 32-bit
> > platforms align it to 4 bytes), then the leadup to the _sifields union
> > matters:
>
> Side note: I'm not sure that "this is rare" comment is necessarily correct.
>
> I think natural alignment is pretty common, even for 32-bit targets.
> x86-32 is I think the exception rather than the rule.
>
> There is some real odd case iirc - embedded m68k, which has some
> ridiculous alignment rules. I think it only ever aligns to 16-bit
> boundaries.
So I got curious about this, but couldn't find any good online documentation about
the alignment defaults of various architectures that GCC supports. So I reverted
the fix and added the new check from linux-next:
Revert "mm/pkeys: Fix siginfo ABI breakage caused by new u64 field"
kernel/signal.c: add compile-time check for __ARCH_SI_PREAMBLE_SIZE
... which does:
void __init signals_init(void)
{
+ /* If this check fails, the __ARCH_SI_PREAMBLE_SIZE value is wrong! */
+ BUILD_BUG_ON(__ARCH_SI_PREAMBLE_SIZE
+ != offsetof(struct siginfo, _sifields._pad));
+
and tested it on the -tip cross-arch build testing suite, which gave the following
result (only 32-bit architectures listed):
(warns) (warns)
testing x86-32: -git: pass ( 0), -tip: pass ( 0)
testing arm: -git: pass ( 1), -tip: FAIL .....
testing blackfin: -git: pass ( 0), -tip: pass ( 0)
testing cris: -git: pass ( 32), -tip: pass ( 32)
testing frv: -git: pass ( 1), -tip: FAIL .....
testing m32r: -git: pass ( 6), -tip: pass ( 6)
testing m68k: -git: pass ( 1), -tip: pass ( 1)
testing microblaze: -git: pass ( 0), -tip: pass ( 0)
testing mips: -git: pass ( 1), -tip: FAIL .....
testing openrisc: -git: pass ( 2), -tip: pass ( 2)
testing parisc: -git: pass ( 0), -tip: FAIL .....
testing sh: -git: pass ( 36), -tip: pass ( 36)
testing sparc: -git: pass ( 0), -tip: FAIL .....
testing tile: -git: pass ( 5), -tip: pass ( 5)
testing xtensa: -git: pass ( 0), -tip: FAIL .....
testing powerpc32: -git: pass ( 0), -tip: FAIL .....
so if my test is correct then it's 9 architectures that align u64 to 4 bytes, vs.
7 that align it to 8 bytes.
So naturally aligned u64 is definitely not 'rare' (so the characterisation in my
changelog is wrong), but it's not dominant either.
FWIIW: if we only list 'major' architectures then x86-32 is indeed the odd one
out...
> I do keep coming back to the fact that we should *probably* just do
> something like
>
> typedef unsigned long long __attribute__((aligned(8))) __u64;
>
> and then introduce a separate "u64_unaligned" type for all the legacy
> cases that depended on 32-bit alignment.
>
> It's horrendously nasty to test, though.
So in theory we could test most of it by comparing the disassembly of allyesconfig
builds, but comparing disassemblies is a pretty hard to use method in practice.
A more workable method would be to have a test .c file that includes all UAPI
structures in existence and defines a variable out of every single one, and then
generates a list of sizeof() values or so. But even that isn't perfect: a
structure might shift some fields forward, into a pre-existing hole, without
changing the sizeof? We'd need a list of all field offsets in all structures to be
really sure, and that's nasty.
Thanks,
Ingo
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