Re: [PATCH] socket: Use may_alias on sockaddr structs (bug 19622)

[Sam James <sam@gentoo.org>]

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Sam James <sam@gentoo.org> writes:

> Florian Weimer <fweimer@redhat.com> writes:
>
>> This supports common coding paddings.  GCC before version 8
>> rejects the may_alias attribute on a struct definition if it was
>> not present in a previous forward declaration, so this attribute
>> can only be conditionally applied.
>>
>> Tested on i686-linux-gnu, x86_64-linux-gnu.  Built with
>> build-many-glibcs.py.
>>
>> Suggested-by: Marek Polacek <polacek@redhat.com>
>> Suggested-by: Jakub Jelinek <jakub@redhat.com>
>
> Reviewed-by: Sam James <sam@gentoo.org>

Also, on reflection, consider citing the POSIX change I mentioned on
Bugzilla in the commit message too.

I think it's useful in making clear this isn't (just) a QoI change?

(https://austingroupbugs.net/view.php?id=1641)

>
>>
>> ---
>>  bits/socket.h                           | 4 ++--
>>  inet/netinet/in.h                       | 8 +++++---
>>  misc/sys/cdefs.h                        | 9 +++++++++
>>  socket/sys/un.h                         | 2 +-
>>  sysdeps/mach/hurd/bits/socket.h         | 4 ++--
>>  sysdeps/unix/sysv/linux/bits/socket.h   | 4 ++--
>>  sysdeps/unix/sysv/linux/net/if_packet.h | 2 +-
>>  sysdeps/unix/sysv/linux/netash/ash.h    | 2 +-
>>  sysdeps/unix/sysv/linux/neteconet/ec.h  | 2 +-
>>  sysdeps/unix/sysv/linux/netiucv/iucv.h  | 2 +-
>>  10 files changed, 25 insertions(+), 14 deletions(-)
>>
>> diff --git a/bits/socket.h b/bits/socket.h
>> index 13de124bac..772074d52a 100644
>> --- a/bits/socket.h
>> +++ b/bits/socket.h
>> @@ -149,7 +149,7 @@ enum __socket_type
>>  #include <bits/sockaddr.h>
>>  
>>  /* Structure describing a generic socket address.  */
>> -struct sockaddr
>> +struct __attribute_struct_may_alias__ sockaddr
>>    {
>>      __SOCKADDR_COMMON (sa_);	/* Common data: address family and length.  */
>>      char sa_data[14];		/* Address data.  */
>> @@ -166,7 +166,7 @@ struct sockaddr
>>  #define _SS_PADSIZE \
>>    (_SS_SIZE - __SOCKADDR_COMMON_SIZE - sizeof (__ss_aligntype))
>>  
>> -struct sockaddr_storage
>> +struct __attribute_struct_may_alias__ sockaddr_storage
>>    {
>>      __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
>>      char __ss_padding[_SS_PADSIZE];
>> diff --git a/inet/netinet/in.h b/inet/netinet/in.h
>> index fa57b61079..f684be5beb 100644
>> --- a/inet/netinet/in.h
>> +++ b/inet/netinet/in.h
>> @@ -244,7 +244,7 @@ extern const struct in6_addr in6addr_loopback;   /* ::1 */
>>  
>>  
>>  /* Structure describing an Internet socket address.  */
>> -struct sockaddr_in
>> +struct __attribute_struct_may_alias__ sockaddr_in
>>    {
>>      __SOCKADDR_COMMON (sin_);
>>      in_port_t sin_port;			/* Port number.  */
>> @@ -257,9 +257,11 @@ struct sockaddr_in
>>  			   - sizeof (struct in_addr)];
>>    };
>>  
>> -#if !__USE_KERNEL_IPV6_DEFS
>> +#if __USE_KERNEL_IPV6_DEFS
>> +struct __attribute_struct_may_alias__ sockaddr_in6;
>> +#else
>>  /* Ditto, for IPv6.  */
>> -struct sockaddr_in6
>> +struct __attribute_struct_may_alias__ sockaddr_in6
>>    {
>>      __SOCKADDR_COMMON (sin6_);
>>      in_port_t sin6_port;	/* Transport layer port # */
>> diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
>> index 2e8279a2c7..e21a2a7080 100644
>> --- a/misc/sys/cdefs.h
>> +++ b/misc/sys/cdefs.h
>> @@ -874,4 +874,13 @@ _Static_assert (0, "IEEE 128-bits long double requires redirection on this platf
>>  # define __attribute_returns_twice__ /* Ignore.  */
>>  #endif
>>  
>> +/* Mark struct types as aliasable.  Restricted to compilers that
>> +   support forward declarations of structs in the presence of the
>> +   attribute.  */
>> +#if __GNUC_PREREQ (8, 0) || defined __clang__
>> +# define __attribute_struct_may_alias__ __attribute__ ((__may_alias__))
>> +#else
>> +# define __attribute_struct_may_alias__
>> +#endif
>
> Consider doing '/* Ignore. */' for consistency, even if I think it's a
> bit silly.
>
>> +
>>  #endif	 /* sys/cdefs.h */
>> diff --git a/socket/sys/un.h b/socket/sys/un.h
>> index bf03b7d6ce..ff9cbd6efa 100644
>> --- a/socket/sys/un.h
>> +++ b/socket/sys/un.h
>> @@ -26,7 +26,7 @@
>>  __BEGIN_DECLS
>>  
>>  /* Structure describing the address of an AF_LOCAL (aka AF_UNIX) socket.  */
>> -struct sockaddr_un
>> +struct __attribute_struct_may_alias__ sockaddr_un
>>    {
>>      __SOCKADDR_COMMON (sun_);
>>      char sun_path[108];		/* Path name.  */
>> diff --git a/sysdeps/mach/hurd/bits/socket.h b/sysdeps/mach/hurd/bits/socket.h
>> index 3e72f9fa93..b5eeac3731 100644
>> --- a/sysdeps/mach/hurd/bits/socket.h
>> +++ b/sysdeps/mach/hurd/bits/socket.h
>> @@ -153,7 +153,7 @@ enum __socket_type
>>  #include <bits/sockaddr.h>
>>  
>>  /* Structure describing a generic socket address.  */
>> -struct sockaddr
>> +struct __attribute_struct_may_alias__ sockaddr
>>    {
>>      __SOCKADDR_COMMON (sa_);	/* Common data: address family and length.  */
>>      char sa_data[14];		/* Address data.  */
>> @@ -170,7 +170,7 @@ struct sockaddr
>>  #define _SS_PADSIZE \
>>    (_SS_SIZE - __SOCKADDR_COMMON_SIZE - sizeof (__ss_aligntype))
>>  
>> -struct sockaddr_storage
>> +struct __attribute_struct_may_alias__ sockaddr_storage
>>    {
>>      __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
>>      char __ss_padding[_SS_PADSIZE];
>> diff --git a/sysdeps/unix/sysv/linux/bits/socket.h b/sysdeps/unix/sysv/linux/bits/socket.h
>> index 86444f0018..5ab19a8c08 100644
>> --- a/sysdeps/unix/sysv/linux/bits/socket.h
>> +++ b/sysdeps/unix/sysv/linux/bits/socket.h
>> @@ -181,7 +181,7 @@ typedef __socklen_t socklen_t;
>>  #include <bits/sockaddr.h>
>>  
>>  /* Structure describing a generic socket address.  */
>> -struct sockaddr
>> +struct __attribute_struct_may_alias__ sockaddr
>>    {
>>      __SOCKADDR_COMMON (sa_);	/* Common data: address family and length.  */
>>      char sa_data[14];		/* Address data.  */
>> @@ -194,7 +194,7 @@ struct sockaddr
>>  #define _SS_PADSIZE \
>>    (_SS_SIZE - __SOCKADDR_COMMON_SIZE - sizeof (__ss_aligntype))
>>  
>> -struct sockaddr_storage
>> +struct __attribute_struct_may_alias__ sockaddr_storage
>>    {
>>      __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
>>      char __ss_padding[_SS_PADSIZE];
>> diff --git a/sysdeps/unix/sysv/linux/net/if_packet.h b/sysdeps/unix/sysv/linux/net/if_packet.h
>> index 9ffb69b508..c17e1c23c5 100644
>> --- a/sysdeps/unix/sysv/linux/net/if_packet.h
>> +++ b/sysdeps/unix/sysv/linux/net/if_packet.h
>> @@ -26,7 +26,7 @@
>>     From Linux 2.1 the AF_PACKET interface is preferred and you should
>>     consider using it in place of this one.  */
>>  
>> -struct sockaddr_pkt
>> +struct __attribute_struct_may_alias__ sockaddr_pkt
>>    {
>>      __SOCKADDR_COMMON (spkt_);
>>      unsigned char spkt_device[14];
>> diff --git a/sysdeps/unix/sysv/linux/netash/ash.h b/sysdeps/unix/sysv/linux/netash/ash.h
>> index 7d885d17cc..7a6ff50b17 100644
>> --- a/sysdeps/unix/sysv/linux/netash/ash.h
>> +++ b/sysdeps/unix/sysv/linux/netash/ash.h
>> @@ -22,7 +22,7 @@
>>  #include <features.h>
>>  #include <bits/sockaddr.h>
>>  
>> -struct sockaddr_ash
>> +struct __attribute_struct_may_alias__ sockaddr_ash
>>    {
>>      __SOCKADDR_COMMON (sash_);		/* Common data: address family etc.  */
>>      int sash_ifindex;			/* Interface to use.  */
>> diff --git a/sysdeps/unix/sysv/linux/neteconet/ec.h b/sysdeps/unix/sysv/linux/neteconet/ec.h
>> index b07a107961..f3132f06ff 100644
>> --- a/sysdeps/unix/sysv/linux/neteconet/ec.h
>> +++ b/sysdeps/unix/sysv/linux/neteconet/ec.h
>> @@ -28,7 +28,7 @@ struct ec_addr
>>      unsigned char net;			/* Network number.  */
>>    };
>>  
>> -struct sockaddr_ec
>> +struct __attribute_struct_may_alias__ sockaddr_ec
>>    {
>>      __SOCKADDR_COMMON (sec_);
>>      unsigned char port;			/* Port number.  */
>> diff --git a/sysdeps/unix/sysv/linux/netiucv/iucv.h b/sysdeps/unix/sysv/linux/netiucv/iucv.h
>> index f5fad81751..27151e8bbe 100644
>> --- a/sysdeps/unix/sysv/linux/netiucv/iucv.h
>> +++ b/sysdeps/unix/sysv/linux/netiucv/iucv.h
>> @@ -23,7 +23,7 @@
>>  
>>  __BEGIN_DECLS
>>  
>> -struct sockaddr_iucv
>> +struct __attribute_struct_may_alias__ sockaddr_iucv
>>    {
>>      __SOCKADDR_COMMON (siucv_);
>>      unsigned short	siucv_port;		/* Reserved */
>>
>> base-commit: 4bbca1a44691a6e9adcee5c6798a707b626bc331

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