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List:       busybox
Subject:    Re: Is it a bug of killall/pidof?
From:       Denis Vlasenko <vda.linux () googlemail ! com>
Date:       2007-06-23 14:12:35
Message-ID: 200706231612.35459.vda.linux () googlemail ! com
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On Friday 22 June 2007 01:43, john bigZ wrote:
>   I was wondering why my kill script didn't work. the reason I found
>   that the 'killall' failed with 'no process killed', was 'killall'
>   called 'pidof', 'pidof' didn't return a process id for the given
>   process name. After through investigation of the busybox sourcecode,
>   I found out, that pidof doesn't parse /proc/$PID/cmdline
>   for the process name instead it cuts out the text between the round
>   braces in /proc/$PID/stat. This text usually contains the name of argv[0]
>   and somehow this text contains "exe" instead of the real process name
>   itself. pidof/killall will then not be able to find the PID to the given
>   process name (you may try "pidof exe" instead, if you don't believe me).
>   I don't know how to fix this problem, can anyone help me or give me some
>   hints? I'd appreciate your help!

This is complex. In Linux we have three ways to determine "process name":
1. /proc/PID/stat has "...(name)...", among other things. It's so-called "comm" field.
2. /proc/PID/cmdline's first NUL-terminated string. It's argv[0] from exec syscall.
3. /proc/PID/exe symlink. Points to the running executable file.

kernel threads:
 comm: thread name
 cmdline: empty
 exe: <readlink fails>

executable
 comm: first 15 chars of base name
 (if executable is a symlink, then first 15 chars of symlink name are used)
 cmdline: argv[0] from exec syscall
 exe: points to executable (resolves symlink, unlike comm)

script (an executable with #!/path/to/interpreter):
 comm: first 15 chars of base name (symlinks are not resolved)
 cmdline: /path/to/interpreter (symlinks are not resolved)
 exe: points to interpreter's executable (symlinks are resolved)

>   and somehow this text contains "exe"

This is because you probably have FEATURE_PREFER_APPLETS=y (and maybe
FEATURE_SH_STANDALONE too), and therefore some commands started
from busybox shell, xargs or find are started by
execXXX("/proc/self/exe", applet_name, params....)
and therefore comm field contains "exe".

In busybox, pidof and killall are using find_pid_by_name, which is looking at
comm field only:

pid_t* find_pid_by_name(const char* procName)
{
        pid_t* pidList;
        int i = 0;
        procps_status_t* p = NULL;

        pidList = xmalloc(sizeof(*pidList));
        while ((p = procps_scan(p, PSSCAN_PID|PSSCAN_COMM))) {
                if (strncmp(p->comm, procName, sizeof(p->comm)-1) == 0) {
                        pidList = xrealloc(pidList, sizeof(*pidList) * (i+2));
                        pidList[i++] = p->pid;
                }
        }

        pidList[i] = 0;
        return pidList;
}

Ok, I explained how it works, now: do we want it to be fixed? and how exactly?
--
vda
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