Re: Patch for man unix(7)

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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@xxxxxxxxxxxxxxxxxxx> wrote:
>  From f388eedbdc0b099bb9f36ab007f9370432abb300 Mon Sep 17 00:00:00 2001
>  From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> 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
         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@xxxxxxxxxxxxxxxxxxx>
> ---
>  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/
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