Re: sockaddr_storage and GCC 6.1

[Rich Felker <dalias@libc.org>]

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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

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diff --git a/include/sys/socket.h b/include/sys/socket.h
index 6788375..d2bd5df 100644
--- a/include/sys/socket.h
+++ b/include/sys/socket.h
@@ -286,7 +286,7 @@ struct sockaddr
 
 struct sockaddr_storage
 {
-	sa_family_t ss_family;
+	sa_family_t ss_family, __ss_family_pad;
 	unsigned long __ss_align;
 	char __ss_padding[128-2*sizeof(unsigned long)];
 };