From: "J. Hanne" <firstname.lastname@example.org> To: email@example.com Subject: Re: [musl] CMSG_LEN macro Date: Sun, 3 Oct 2021 10:14:59 +0200 (CEST) [thread overview] Message-ID: <20211003081459.34C1C2FC23EF@dd11108.kasserver.com> (raw) In-Reply-To: <20210905172745.GD3090@voyager> Hi, thanks for your thoughts. The NLMSG macros also already gave me similar headache some time ago. I personally find both APIs counter-intuitive because they use the term "ALIGN", when they mean "PAD", so that the *following* item is aligned. I would suggest the following patch now: Do not use CMSG_ALIGN on struct cmsghdr, because: - This has no effect on any architecture anyway, because sizeof(struct cmsghdr) == 16 on all archs - Using it contradicts with CMSG_DATA, which does NOT apply any padding after struct cmsghdr - This is consistent with the NLMSG_* macros --- diff -uNr a/include/sys/socket.h b/include/sys/socket.h --- a/include/sys/socket.h 2021-01-15 03:26:00.000000000 +0100 +++ b/include/sys/socket.h 2021-10-03 09:49:35.000000000 +0200 @@ -358,8 +358,8 @@ #define CMSG_FIRSTHDR(mhdr) ((size_t) (mhdr)->msg_controllen >= sizeof (struct cmsghdr) ? (struct cmsghdr *) (mhdr)->msg_control : (struct cmsghdr *) 0) #define CMSG_ALIGN(len) (((len) + sizeof (size_t) - 1) & (size_t) ~(sizeof (size_t) - 1)) -#define CMSG_SPACE(len) (CMSG_ALIGN (len) + CMSG_ALIGN (sizeof (struct cmsghdr))) -#define CMSG_LEN(len) (CMSG_ALIGN (sizeof (struct cmsghdr)) + (len)) +#define CMSG_SPACE(len) (CMSG_ALIGN (len) + sizeof (struct cmsghdr)) +#define CMSG_LEN(len) (sizeof (struct cmsghdr) + (len)) #define SCM_RIGHTS 0x01 #define SCM_CREDENTIALS 0x02 -- By the way, the question which led me to all this stuff is: How do I get the payload length of a received cmsg. Neither the man page nor an Internet search gave me any satisfactory answer. So my best guess was "do some arithmetic with CMSG_LEN": payloadlen = cmsghdr->cmsg_len - CMSG_LEN(0); However, when double-checking with musl source code, the CMSG_ALIGN on struct cmsghdr made me doubt my approach. Now, I will stick to it - as long as nobody else has a better idea? Regards, Johann Markus Wichmann schrieb am 05.09.2021 19:27 (GMT +02:00): > On Fri, Sep 03, 2021 at 09:12:13AM -0400, Rich Felker wrote: > > Anyone else have thoughts on this? > > > > Rich > > I noticed something similar about the NLMSG_* macros that allow for > padding where there can be none (in the interface). struct nlmsghdr has > alignment of 4, and the netlink message alignment is also 4, and that > can never be changed on any existing arch since it would break binary > compatibility. And for netlink, it is unlikely they would add > architecture specific alignment in future, given that today it is > arch-independent. > > I guess those are symptoms of overly general software design. The macros > must exist, but I concur with your conclusion that they can be > implemented without reference to CMSG_ALIGN. > > BTW, I just checked the implementation of the NLMSG_* macros in musl, > and they do assume the alignment of struct nlmsghdr. So for consistency, > we should probably do the same for the CMSG_* macros. > > Ciao, > Markus > > >
next prev parent reply other threads:[~2021-10-03 8:15 UTC|newest] Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-09-03 10:13 J. Hanne 2021-09-03 13:12 ` Rich Felker 2021-09-05 17:27 ` Markus Wichmann 2021-10-03 8:14 ` J. Hanne [this message] 2021-10-03 13:35 ` Rich Felker
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20211003081459.34C1C2FC23EF@dd11108.kasserver.com \ --firstname.lastname@example.org \ --email@example.com \ --subject='Re: [musl] CMSG_LEN macro' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
Code repositories for project(s) associated with this inbox: https://git.vuxu.org/mirror/musl/ This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).