On 06.07.2012 15:03, Laurent Bercot wrote:
>> As I stated in my initial email, when running awk from a subshell
>> it works. I know PID=1 is special, and for one, it can't be straced
>> or debugged.
>
> Actually, on some Linux versions (I'm not certain which ones though,
> but it should work on modern kernels), process 1 *can* be straced:
> somehow get a shell running and do "strace -p 1". If you're not afraid
> of dirty hacks, you could even run that from your /init, if everything
> is properly mounted:
> ( strace -p 1 </dev/null >/dev/null 2>/tmp/strace-file ) &
> or even replace /tmp/strace-file with /dev/tty for a live performance.
I tried this right away, but failed: strace does not show anything
at all in that case, even if the process 1 actually does something.
No, I'm not afraid at all, since it is trivial to experiment in
a virtual machine.
And I don't think strace will do anything interesting here: I
looked at what happens when busybox ash spawns awk in a regular
case, there, it just forks (with a pipe open) and does one or
two read/writes -- it is ofcourse not enough information to
understand what's going on. It may help a bit ofcourse, if
I'll add a few debug printfs here and there.
Ideally it'd be nice to get a coredump, but for some reason
the process being fork'ed in this context isn't dumpable.
>> And finally, it isn't a solution to run it from a subshell,
>> since running awk from within /init in initramfs is a valid usage
>> case.
>
> But are you really running awk as process 1 ?
> I doubt your /init script is *executing* awk. If it was, then after
> performing its work, awk would die, and your kernel would panic.
> I suspect that instead, your /init script looks like
>
> #!/bin/sh
> ...
> awk something
> ...
> exec chroot foobar /sbin/init
>
> in which case you are *not* running awk as process 1: your shell is
> forking first, then executing awk as a child. The question then becomes,
> why is there a problem when awk is run directly as process 1's child.
Well. My $subject is inaccurate. I meant to say when awk is
spawned from pid=1 as NOEXEC applet. The code like
rootfs=`awk '$2=="/"{print $1}`
or anything of this theme. This works:
rootfs=`/bin/awk '$2=="/"{print $1}`
rootfs=`busybox awk '$2=="/"{print $1}`
and this
rootfs=`awk '$2=="/"{print $1}`
segfaults, just like a bare 'awk' call:
awk
(in normal case it produces an error message
about invalid usage).
Thank you!
/mjt
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