Re: Patch for man unix(7)

On Mon, Apr 16, 2012 at 9:42 AM, Michael Kerrisk (man-pages)
[off-list ref] wrote:

Hello Tetsuao Handa

[Thanks for the text program that you sent more recently]

On Wed, Nov 24, 2010 at 1:59 AM, Tetsuo Handa
[off-list ref] wrote:

quoted

 From f388eedbdc0b099bb9f36ab007f9370432abb300 Mon Sep 17 00:00:00 2001
 From: Tetsuo Handa [off-list ref]
Date: Tue, 23 Nov 2010 21:34:25 +0900
Subject: [PATCH] unix.7: Fix description of "pathname" sockets

Since unix_mkname() in net/unix/af_unix.c does

 ((char *)sunaddr)[len] = 0;

rather than

 ((char *)sunaddr)[len - 1] = 0;

, sunaddr->sun_path may not be terminated with a null byte if
len == sizeof(*sunaddr).

Therefore, the caller of getsockname(), getpeername(), accept() must not assume
that sunaddr->sun_path contains a null-terminated pathname even if the returned
addrlen is greater than sizeof(sa_family_t) and sun_path[0] != '\0'.

Thanks. I see what you mean. However, I'm wondering, is the kernel
behavior simply a bug that should be fixed, so that a null terminator
is always placed in sun_path?

I realize that's an ABI change, but:
a) I suspect most sane applications would never create a sun_path that
didn't contain a null terminator within sizeof(sun_path) bytes.
b) Considering these two sets:
  1. [applications that would break if the assumption that there
        is no null terminator inside sizeof(sun_path) bytes doesn't

Sorry! Small thinko in previous line:

"is no null terminator" ==> "is a null terminator"

Thanks,

Michael

        hold true]
  2. [applications that would break if the kernel behavior changed]
  I suspect that set 1 is much larger than set 2.

Your thoughts?

Thanks,

Michael

quoted

Signed-off-by: Tetsuo Handa <penguin-kernel-JPay3/Yim36HaxMnTkn67Xf5DAMn2ifp@public.gmane.org>
---
 man7/unix.7 |   19 ++++++++++++++++---
 1 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/man7/unix.7 b/man7/unix.7
index b53328b..7b0b47c 100644
--- a/man7/unix.7
+++ b/man7/unix.7

@@ -80,10 +80,23 @@ When the address of the socket is returned by

 and
 .BR accept (2),
 its length is
-.IR "offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1" ,
+.IR "offsetof(struct sockaddr_un, sun_path) + strlen(sun_path) + 1".
+Note that this length can be one byte larger than
+.IR "sizeof(struct sockaddr_un)"
+because
+.BR bind (2)
+accepts
+.IR sun_path
+which is not terminated with a null byte ('\\0').
+Therefore, you must not use string manipulation functions (e.g. strlen(),
+printf("%s")) against
+.IR sun_path
+because
+.BR getsockname (2),
+.BR getpeername (2),
 and
-.I sun_path
-contains the null-terminated pathname.
+.BR accept (2)
+may not have stored a null-terminated string.
 .IP *
 .IR unnamed :
 A stream socket that has not been bound to a pathname using
--
1.6.1

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
--
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