Date: Tue, 27 Mar 2001 17:41:19 +0100 From: Brian Somers <brian@Awfulhak.org> To: David Malone <dwmalone@maths.tcd.ie> Cc: freebsd-hackers@freebsd.org, freebsd-alpha@freebsd.org, wpaul@freebsd.org, brian@freebsd.org, brian@Awfulhak.org Subject: Re: Control messages. Message-ID: <200103271641.f2RGfJL00975@hak.lan.Awfulhak.org> In-Reply-To: Message from David Malone <dwmalone@maths.tcd.ie> of "Tue, 27 Mar 2001 13:47:51 BST." <200103271347.aa40230@salmon.maths.tcd.ie>
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I can't see where in the kernel we're *not* using CMSG_DATA(). This
was fixed a while ago and tested ok on beast (for 3 descriptors
AFAIR). Are we looking at the same code (I'm looking in /sys/kern) ?
The only dodgy thing I see in there is the COMPAT_OLDSOCK stuff in
uipc_syscalls.c, and that has a very nice disclaimer:
/*
* We assume that old recvmsg calls won't receive access
* rights and other control info, esp. as control info
* is always optional and those options didn't exist in 4.3.
* If we receive rights, trim the cmsghdr; anything else
* is tossed.
*/
> I was looking at our implimention of passing descriptors and
> credentials over unix domain sockets (I want to add the ability to
> pass more than one message at a time). According to Steven's book
> you should use the CMSG_DATA macro to find the data in associated
> with a struct cmsghdr. We define this macro as:
>
> #define CMSG_DATA(cmsg) ((u_char *)(cmsg) + \
> _ALIGN(sizeof(struct cmsghdr)))
>
> However, the kernel seems to get the address of the data by looking
> at (void *)(&cmsg[1]). Some applications either find the data either
> using the kernel's method or by using:
>
> struct blah {
> struct cmsghdr header;
> sturct wibble data;
> };
>
> and taking the address of "data". Fortunately all these methods are
> the same on the i386 'cos the sizeof(sizeof(struct cmsghdr)) is a
> multiple of the alighment size. However on the alpha the alignment
> requirement is 8 bytes and so:
>
> CMSG_DATA(cmsg) != (&cmsg[1])
>
> If you use the struct method then offset of "data" will line up
> with one or the other of these depending on the alignment requirements
> of data.
>
> I'd like to change the code to always use the CMSG_DATA method,
> which will change the ABI on the alpha. This shouldn't cause too
> much disruption as descriptor and credential passing is rarely
> used. (I think ppp and maybe some rpc stuff use it - not much
> else does).
>
> (As far as I can tell descriptor passing for a large number of
> descriptors will panic an alpha running FreeBSD too. If anyone can
> give access to an alpha that it's OK for me to crash I can test
> this).
>
> David.
--
Brian <brian@Awfulhak.org> <brian@[uk.]FreeBSD.org>
<http://www.Awfulhak.org>; <brian@[uk.]OpenBSD.org>
Don't _EVER_ lose your sense of humour !
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