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Date: Tue, 24 May 2016 18:36:02 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: sockaddr_storage and GCC 6.1
On Tue, May 24, 2016 at 03:07:35PM -0700, William Ahern wrote:
> GCC 6.1 more aggressively decomposes aggregate assignments into a series of
> scalar member assignments. This has uncovered an issue with glibc's layout
> of struct sockaddr_storage, which has a padding hole from offsets 2 to 8,
> precisely where .sin_port and .sin_addr are in struct sockaddr_in.
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71120
>
> musl shares this same issue. Specifically, the __ss_align member with an
> 8-byte alignment on LP64 archs. You can track the glibc resolution at
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=20111
>
> Or not track it. Reasonable folks can disagree regarding many aspects of
> this issue, but I thought it worthwhile to bring to people's attention.
I maintain that it's a bug (violation of effective type rules) for a
program to attempt to copy sockaddr types using sockaddr_storage, but
this is a nasty application bug to track down (usually silent
breakage) that's worth avoiding since it's easy. Does the attached
patch work?
I don't think we should even consider the sorts of may_alias hacks
glibc/gcc folks are discussing, though. There's already a gcc option
for compiling broken code like that; it's called -fno-strict-aliasing.
Rich
View attachment "sockaddr_storage.diff" of type "text/plain" (357 bytes)
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