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Sun, 05 Feb 2023 07:30:22 -0800 (PST) From: Alejandro Colomar X-Google-Original-From: Alejandro Colomar To: linux-man@vger.kernel.org Cc: Alejandro Colomar , GCC , glibc , =?UTF-8?q?Bastien=20Roucari=C3=A8s?= , Stefan Puiu , Igor Sysoev , Rich Felker , Andrew Clayton , Richard Biener , Zack Weinberg , Florian Weimer , Joseph Myers , Jakub Jelinek , Eric Blake Subject: [PATCH] sockaddr.3type: BUGS: Document that libc should be fixed using a union Date: Sun, 5 Feb 2023 16:28:36 +0100 Message-Id: <20230205152835.17413-1-alx@kernel.org> X-Mailer: git-send-email 2.39.1 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-10.4 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: As discussed before, and Bastien and I seem to agree, ideally we should define the following types: struct sockaddr_storage { union { struct { sa_family_t ss_family; }; struct sockaddr_in sin; struct sockaddr_in6 sin6; struct sockaddr_un sun; // ... }; }; struct [[deprecated]] sockaddr { sa_family_t sa_family; }; union [[gnu::transparent_union]] sockaddr_ptr { struct sockaddr_storage *ss; struct sockaddr *sa; }; And then we could define APIs like: int bind(int sockfd, const union sockaddr_ptr *addr, socklen_t len); Link: Link: Cc: GCC Cc: glibc Cc: Bastien Roucariès Cc: Stefan Puiu Cc: Igor Sysoev Cc: Rich Felker Cc: Andrew Clayton Cc: Richard Biener Cc: Zack Weinberg Cc: Florian Weimer Cc: Joseph Myers Cc: Jakub Jelinek Cc: Eric Blake Signed-off-by: Alejandro Colomar --- man3type/sockaddr.3type | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/man3type/sockaddr.3type b/man3type/sockaddr.3type index 319a5e552..239e836fc 100644 --- a/man3type/sockaddr.3type +++ b/man3type/sockaddr.3type @@ -120,6 +120,26 @@ .SH NOTES .I and .IR . +.SH BUGS +.I sockaddr_storage +was designed back when strict aliasing wasn't a problem. +Back then, +one would define a variable of that type, +and then access it as any of the other +.IR sockaddr_ * +types, +depending on the value of the first member. +This is Undefined Behavior. +However, there is no way to use these APIs without invoking Unedfined Behavior, +either in the user program or in libc, +so it is still recommended to use this method. +The only correct way to use different types in an API is through a union. +However, +that union must be implemented in the library, +since the type must be shared between the library and user code, +so libc should be fixed by implementing +.I sockaddr_storage +as a union. .SH SEE ALSO .BR accept (2), .BR bind (2), -- 2.39.1