Re: [PATCH v2] packet.7: Describe SOCK_PACKET netif name length issues and workarounds.

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

Hi Ralf,

On 9/16/21 3:38 PM, Ralf Baechle wrote:
> Describe the issues with SOCK_PACKET possibly truncating network interface
> names in results, solutions and possible workarounds.
> 
> While the issue is known for a long time it appears to have never been
> properly documented is has started to bite software antiques including
> the AX.25 userland badly since the introduction of Predictable Network
> Interface Names.  So let's document it.
> 
> Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Patch applied!

Thanks,

Alex

> ---
>   man7/packet.7 | 39 ++++++++++++++++++++++++++++++++++++---
>   1 file changed, 36 insertions(+), 3 deletions(-)
> 
> Changes in v2: Correct issues raised by Alejandro Colomar in review of v1.
> 
> diff --git a/man7/packet.7 b/man7/packet.7
> index 706efbb54..fa022bee8 100644
> --- a/man7/packet.7
> +++ b/man7/packet.7
> @@ -616,10 +616,10 @@ is the device name as a null-terminated string, for example, eth0.
>   .PP
>   This structure is obsolete and should not be used in new code.
>   .SH BUGS
> +.SS LLC header handling
>   The IEEE 802.2/803.3 LLC handling could be considered as a bug.
>   .PP
> -Socket filters are not documented.
> -.PP
> +.SS MSG_TRUNC issues
>   The
>   .B MSG_TRUNC
>   .BR recvmsg (2)
> @@ -627,6 +627,38 @@ extension is an ugly hack and should be replaced by a control message.
>   There is currently no way to get the original destination address of
>   packets via
>   .BR SOCK_DGRAM .
> +.PP
> +.SS spkt_device device name truncation
> +The
> +.I spkt_device
> +field of
> +.I sockaddr_pkt
> +has a size of 14 bytes which is less than the constant
> +.B IFNAMSIZ
> +defined in
> +.I <net/if.h>
> +which is 16 bytes and describes the system limit for a network interface name.
> +This means the names of network devices longer than 14 bytes will be truncated
> +to fit into
> +.IR spkt_device .
> +All these lengths include the terminating null byte (\(aq\e0\(aq)).
> +.PP
> +Issues from this with old code typically show up with very long interface
> +names used by the
> +.B Predictable Network Interface Names
> +feature enabled by default in many modern Linux distributions.
> +.PP
> +The preferred solution is to rewrite code to avoid
> +.BR SOCK_PACKET .
> +Possible user solutions are to disable
> +.B Predictable Network Interface Names
> +or to rename the interface to a name of at most 13 bytes, for example using
> +the
> +.BR ip (8)
> +tool.
> +.PP
> +.SS Documentation issues
> +Socket filters are not documented.
>   .\" .SH CREDITS
>   .\" This man page was written by Andi Kleen with help from Matthew Wilcox.
>   .\" AF_PACKET in Linux 2.2 was implemented
> @@ -637,7 +669,8 @@ packets via
>   .BR capabilities (7),
>   .BR ip (7),
>   .BR raw (7),
> -.BR socket (7)
> +.BR socket (7),
> +.BR ip (8),
>   .PP
>   RFC\ 894 for the standard IP Ethernet encapsulation.
>   RFC\ 1700 for the IEEE 802.3 IP encapsulation.
> 


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/