On Thu, 22 Apr 2021 at 12:17, Marco Elver [off-list ref] wrote:

On Thu, 22 Apr 2021 at 11:48, David Laight [off-list ref] wrote:

quoted

From: Marco Elver

quoted

Sent: 22 April 2021 07:45

On some architectures, like Arm, the alignment of a structure is that of
its largest member.

That is true everywhere.
(Apart from obscure ABI where structure have at least 4 byte alignment!)

For instance, x86 didn't complain, nor did m68k. Both of them have
compile-time checks for the layout (I'm adding those for Arm
elsewhere).

[...]

quoted

Much as I hate __packed, you could add __packed to the
definition of the structure member _perf.
The compiler will remove the padding before it and will
assume it has the alignment of the previous item.

So it will never use byte accesses.

Sure __packed works for Arm. But I think there's no precedent using
this on siginfo_t, possibly for good reasons? I simply can't find
evidence that this is portable on *all* architectures and for *all*
possible definitions of siginfo_t, including those that live in things
like glibc.

Can we confirm that __packed is fine to add to siginfo_t on *all*
architectures for *all* possible definitions of siginfo_t? I currently
can't. And given it's outside the scope of the C standard (as of C11
we got _Alignas, but that doesn't help I think), I'd vote to not
venture too far for code that should be portable especially things as
important as siginfo_t, and has definitions *outside* the kernel (I
know we do lots of non-standard things, but others might not).

After thinking about this all afternoon, you convinced me that the
commit message wasn't great, and this should be in the commit message,
too: https://lkml.kernel.org/r/20210422191823.79012-1-elver@google.com

Thanks,
-- Marco

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