Re: a problem with linux 2.6.11 and sa

["George Georgalis" <george@galis.org>]

On Wed, Mar 09, 2005 at 01:06:11PM +0000, Nix wrote:

>> An interesting technique that allows a program (such as a log writer)
>> to run as an unprivileged user, while receiving privileged data. (taken
>> almost verbatim from Gerrit Pape's socklog)
>> 
>> #!/bin/sh
>> exec </proc/kmsg
>> exec 2>&1
>> exec softlimit -m 2000000 setuidgid nobody socklog ucspi
>> 
>> This script, run by root takes its stdin from /proc/kmsg then combines
>> its stdout and stderr, and exec-switches to the socklog program run
>> as an ucspi application listening to the domain stream socket, as
>> nobody:nogroup, with memory consumption limited to 2Mb. (and sends
>> log to stdout)
>
>This is definitely redirection, not piping. As far as I know the
>implementation of redirection in the kernel remains unchanged: certainly
>the need to buffer piped data doesn't exist in this case, and since the
>redesign was of the buffering, this is probably not your problem :)
>
>> It worked flawlessly until several kernel revs back when the kernel
>> started protecting kmsg and wouldn't allow the user program to receive
>> it,
>
>Indeed.
>
>>       result: nothing sent to the logging program and no error. The fix
>> was to run socklog as root instead of nobody.
>
>You should be able to open it as root and read from it as another user:
>i.e., your technique above shouldn't break. (I'd hope.)

Here is a nice proof that kmsg did become a problem around 2.6.0
http://article.gmane.org/gmane.comp.misc.pape.general/595
http://thread.gmane.org/gmane.comp.misc.pape.general/590


It (Gerrit Pape's technique) very defiantly stopped working a few revs
back (2.6.7?). I'm seeing a similar failed read from /dev/rtc and
mplayer with 2.6.10, now too.

http://lkml.org/lkml/2005/3/8/226

while read file; do mplayer $file ; done <mediafiles.txt

Failed to open /dev/rtc: Permission denied

for file in `cat mediafiles.txt`; do mplayer $file ; done

works.

// George

-- 
George Georgalis, systems architect, administrator Linux BSD IXOYE
http://galis.org/george/ cell:646-331-2027 mailto:george@galis.org