netdev - Re: Patch for man unix(7)

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Message-ID: <CAKgNAki82=5=SkYkfXuF08YMMZ4dpYvORmWQ5OSkQx6Uhahqdw@mail.gmail.com>
Date:	Mon, 16 Apr 2012 10:40:09 +1200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	linux-man@...r.kernel.org, netdev@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: Patch for man unix(7)

On Mon, Apr 16, 2012 at 9:42 AM, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> 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
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
>>  From f388eedbdc0b099bb9f36ab007f9370432abb300 Mon Sep 17 00:00:00 2001
>>  From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>> 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
>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>> ---
>>  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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