linux-kernel - Re: [PATCH 01/17] kbuild: Disable -Waddress-of-packed-member for gcc 9

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Message-ID: <20190322165826.GE24002@tassilo.jf.intel.com>
Date:   Fri, 22 Mar 2019 09:58:26 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Andi Kleen <andi@...stfloor.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Subject: Re: [PATCH 01/17] kbuild: Disable -Waddress-of-packed-member for gcc
 9

> Lol... we're actively moving away from the C standard on many places.

Yes and also packed is not part of the C standard.

> Why does the silly compiler think it is a problem to take the address of
> a member of a packed structure? That sounds like something that's
> perfectly fine and useful.

Probably because a pointer reference doesn't do whatever magic
may be needed on architectures with poor misalignment handling.
In theory you would need memcpy().

In practice it's likely a lot of false positives, like
the architectures with poor misalignment handling are usually
in SOCs without PCI and they don't have the devices
with the problematic drivers.

-Andi