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SubjectRE: setproctitle
On Wed, 18 Aug 2004, Robert White wrote:

> -----Original Message-----
> From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-owner@vger.kernel.org]
> On Behalf Of William Lee Irwin III
> Sent: Wednesday, August 18, 2004 1:59 AM
> To: DervishD
> Cc: Linux-kernel
> Subject: Re: setproctitle
>
> > The command-line arguments are being fetched from the process address
> > space, i.e. simply editing argv[] in userspace will have the desired
> > effect. Though this code is butt ugly.
>
> What prevents overrun when updating arg[]?
>
> What happens to all the little ps (etc.) programs when I munge together a *really*
> *long* title?
>
> Can the entirety of arg[] be moved to a newly allocated region, if so how? (e.g.
> wouldn't I have to have access to overwrite mm->arg_start etc?
>
> I'd prefer a setthreadtitle(char * new_title) such that the individual threads in a
> process (including the master thread, and so setproctitle() function is covered)
> could be re-titled to declare their purposes. It would make debugging and logging a
> lot easier and/or more meaningful sometimes. 8-)
>
> It would also let the system preserve the original invocation and args for the
> lifetime of the process to prevent masquerading. You know, by default the title is
> the args, but the set operation would build the new title in a new kernel-controlled
> place and move the pointer.
>
> I'd be willing to work on this if there is interest.
>
> Rob.
>

Currently the *argv[], the args themselves, and the environment
are on the stack when _start is called.

#
# This is the entry point, usually the first thing in the text
# segment. The SVR4/i386 ABI (pages 3-31, 3-32) says that upon
# entry most registers' values are unspecified, except for:
#
# %edx Contains a function pointer to be registered with `atexit'.
# This is how the dynamic linker arranges to have DT_FINI
# functions called for shared libraries that have been loaded
# before this code runs.
#
# %esp The stack contains the arguments and environment:
# (%esp) argc
# 4(%esp) argv[0]
# ...
# (4*argc)(%esp) NULL
# (4*(argc+1))(%esp) envp[0]
# ...
# NULL

So, overwriting the arguments will destroy the task's copy of
the environment. Fortunately _start code doesn't execute
a return. If it did, the destroyed stack would not contain
anything like a return address. Instead, _start executes the
"exit" system-call after executing any 'atexit' procedures.


Cheers,
Dick Johnson
Penguin : Linux version 2.4.26 on an i686 machine (5570.56 BogoMips).
Note 96.31% of all statistics are fiction.


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