From: Rich Felker <dalias@libc.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
linux-man <linux-man@vger.kernel.org>
Subject: Re: Access to CMSG_DATA
Date: Tue, 17 Dec 2019 15:47:51 -0500 [thread overview]
Message-ID: <20191217204751.GI1666@brightrain.aerifal.cx> (raw)
In-Reply-To: <CAK8P3a2Rv4uEW4acMm_byZQdsH8yNgfuy9qcmT6tmuPrQxvcHA@mail.gmail.com>
On Tue, Dec 17, 2019 at 09:00:08PM +0100, Arnd Bergmann wrote:
> On Tue, Dec 17, 2019 at 3:36 PM Rich Felker <dalias@libc.org> wrote:
> >
> > It came to my attention while reviewing possible breakage with move to
> > 64-bit time_t that some applications are dereferencing data in socket
> > control messages (particularly SCM_TIMESTAMP*) in-place as the message
> > type, rather than memcpy'ing it to appropriate storage. This
> > necessarily does not work and is not supportable if the message
> > contains data with greater alignment requirement than the header. In
> > particular, on 32-bit archs, cmsghdr has size 12 and alignment 4, but
> > struct timeval and timespec may have alignment requirement 8.
> >
> > I found at least ptpd, socat, and ssmping doing this via Debian Code
> > Search:
> >
> > https://sources.debian.org/src/ptpd/2.3.1-debian1-4/src/dep/net.c/?hl=1578#L1578
> > https://sources.debian.org/src/socat/1.7.3.3-2/xio-socket.c/?hl=1839#L1839
> > https://sources.debian.org/src/ssmping/0.9.1-3/ssmpngcl.c/?hl=307#L307
> >
> > and I suspect there are a good deal more out there. On most archs they
> > won't break, or will visibly break with SIGBUS, but in theory it's
> > possible that they silently read wrong data and this might happen on
> > some older and more tiny-embedded-oriented archs.
>
> Good find. I suppose this is going to be particularly annoying for
> architectures that are affected because all systems that are in
> widespread use are not affected:
>
> - x86, riscv, ppc and s390 always allow unaligned loads
> - ARMv6+ mostly allows unaligned loads. Some instructions such as ldrd
> require alignment of four bytes, which is ok, and ARMv5 requires natural
> alignment up to 32 bits, so this is also ok
Seems correct.
x
> - On MIPS I think that o32 is fine since there are no 64-bit loads, but
> n64 would likely be affected, if there are still users remaining (musl
> supports it, so I assume there are some users).
I think you mean n32. n64 is the full LP64 ABI. Indeed it seems like
n32 is likely affected unless the kernel traps and fixes up misaligned
accesses.
> - m68k only requires 16-bit alignment
> - For the other 32-bit architectures that musl supports (microblaze, sh,
^^^^^^^^^^^^^^^^^^
FWIW this isn't specific to musl; glibc is also affected, and uclibc
would be too if they ever implement time64.
> openrisc), none advertise unaligned-access capability to the kernel,
> but I also don't think any of them have a native 64-bit load instruction.
> armv5, microblaze, sh and nds32 fix up unaligned accesses in an
> exception handler; openrisc and csky require aligned accesses in user
> space.
This sounds correct. Presently J2 (open source SH2 ISA implementation)
has no unaligned trap; it just loads/stores the wrong value. But there
are no 64-bit load/store insns anyway and 32-bit alignment is met.
> > I think it's clear to someone who understands alignment and who's
> > thought about it that applications just can't do this, but it doesn't
> > seem to be documented, and an example in cmsg(3) even shows access to
> > int payload via *(int *)CMSG_DATA(cmsg) (of course int is safe because
> > its alignment is <= header alignment, but this is not mentioned).
> >
> > Could we add text, and perhaps change the example, to indicate that in
> > general memcpy needs to be used to copy the payload to/from a suitable
> > object?
>
> Yes, I think that would be a good idea.
How about adding to:
* CMSG_DATA() returns a pointer to the data portion of a cmsghdr.
"The pointer returned cannot be assumed to be suitably aligned for
accessing arbitrary payload data types. Applications should not cast
it to a pointer type matching the payload, but should use memcpy to
copy data to or from a suitably declared object."
and doing this in the examples? Are there other places it should be
mentioned to to make sure readers see it?
Rich
next prev parent reply other threads:[~2019-12-17 20:47 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-17 14:36 Access to CMSG_DATA Rich Felker
2019-12-17 20:00 ` Arnd Bergmann
2019-12-17 20:47 ` Rich Felker [this message]
2020-02-05 0:30 ` Michael Kerrisk (man-pages)
2020-02-05 0:40 ` Rich Felker
2020-02-05 8:08 ` [PATCH] cmsg.3: ffix Dmitry V. Levin
2020-02-07 15:17 ` Michael Kerrisk (man-pages)
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=20191217204751.GI1666@brightrain.aerifal.cx \
--to=dalias@libc.org \
--cc=arnd@arndb.de \
--cc=linux-man@vger.kernel.org \
--cc=mtk.manpages@gmail.com \
/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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
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).