/proc/*/cmdline corrupted

jan.kolar kolar@math.cas.cz
Mon Oct 17 10:41:00 GMT 2011



jan.kolar wrote:
> 
> For example sendmail likes to do that  (on Linux):
>        root      3051  sendmail: accepting connections
>        smmsp 3061  sendmail: Queue runner@00:01:00 for
> /var/spool/clientmqueue
>        root     14631 sendmail: server [1.46.244.248] cmd read
>        root     15254 sendmail: ./p9CDUban025571 mail3.cae3.com.: user
> open
> entries of sendmail are not nullterminated (!).
> Others have set (on Linux) a number of NULL pointers:
> 0000000   i   n   i   t       [   3   ]  \0  \0  \0  \0  \0  \0  \0  \0 
> \0  \0
> 0000000   l   p   d       W   a   i   t   i   n   g  \0  \0  \0
> 

This was on 
Linux host.a.b.c. 2.6.18-194.26.1.el5-ipx #1 SMP Wed Dec 8 20:08:05 CET 2010
x86_64 x86_64 x86_64 GNU/Linux

Corinna Vinschen-2  wrote
> On Linux, /proc/$PID/cmdline always contains the full command line as
> it has been when the process got started, irrespectively of changes
> after process startup.  It looks like the loader creates a copy of the
> argv array before calling main. 

Yes, I agree. A simple C program behaves like that. I did not know how
exactly
sendmail, lpd, init and other achieve the change. 
Also perl allows to set $0 with appropriate effect (but not $1).
So,   Q: how they do that ?    A:" It depends " :-)
See 
http://cvs.rutgers.edu/cgi-bin/viewvc.cgi/tags/start/postman1.11/PsTitle.cc?revision=1806&view=markup
where (probably) cygwin is SPT_CHANGEARGV and Linux is SPT_REUSEARGV.

(And blind xterm modifies its command line in the case SPT_CHANGEARGV. 
 Do the same other programs using XmParseCommand(), or do they first 
 make a working copy of argv pointer array?)

This works on Linux to change /proc/<cmd>/cmdline:
main (int argc, char **argv)  
{      int i;
        argv[0][0]='A';
        for (i=1; i<argc; i++)            argv[i][0]= 'A'+i ; // ! bad for
"", much bad if last arg is ""
        sleep(30);  }
 ./a.out 1 2 3 4 5 &
ps -fC a.out
A/a.out B C D E F


Thanks for the timeout on proc-IPC !

JK
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