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Re: [Qemu-devel] [PATCH 0/2] slirp: fix -Waddress-of-packed-member warni

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 0/2] slirp: fix -Waddress-of-packed-member warnings
Date: Tue, 22 Jan 2019 13:27:32 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 1/22/19 12:18 PM, Peter Maydell wrote:
> These patches fix the clang -Waddress-of-packed-member warnings
> in the slirp code. For slirp, the structs marked as packed
> are already naturally packed/aligned, so adding the 'packed'
> attribute doesn't change their layout but means the compiler
> has to assume they might be misaligned. Instead, for the structs
> which map to on-the-wire packet formats, we add build time
> assertions that they have the sizes we think they should.
> The 'struct ipq' and 'struct ipasfrag' aren't on-the-wire
> packet representations, so it's not clear why they were
> marked packed in the first place. (They contain host pointers
> and so have a size that depends on the host.) Here we just
> assert the only thing that's documented as a layout restriction:
> that struct ipq's frag_link and struct ipasfrag's ipf_link
> are at the same offset.
> Together, this fixes all the -Waddress-of-packed-member warnings
> in slirp, which I think was the only set remaining without
> patches at least being on-list. (Some of the block/ warning fixes
> are still working their way through the relevant trees.)

How does this series play with the ongoing effort to make libslirp an
independent project?  I'm assuming your use of QEMU_BUILD_BUG_ON() will
have to be translated into whatever libslirp-specific mechanism we want
to split off into that project.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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