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Date: Fri, 18 Aug 2000 12:31:49 -0700 (PDT)
From: Mikko Tyolajarvi <mikko@dynas.se>
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To: kpielorz@tdx.co.uk
Cc: freebsd-hackers@freebsd.org
Subject: Re: Critical (or equivalent) section in Userland?
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Karl Pielorz wrote:

>Warner Losh wrote:

>> If advisory locks won't work (and they almost always will for things
>> like this), then you could walk the process tree.  For all processes
>> that aren't suspended or yourself, send a SIGSTOP, keep a list.

>I don't think advisory locks will work - the other process is sendmail... I
>have to keep it from opening any of it's config files, whilst I 'rename' out
>of place the old ones (keeping any fd's to them intact) and rename in the new
>ones...

Warning, here be dragons...

You could try replacing sendmail (using mailer.conf) with a script
that sets LD_PRELOAD and then execs sendmail.  Then you have to write
a little shared lib to wrap some system calls. If you are lucky,
wrapping open() will be sufficient.  In your wrapper function, you
should have the opportunity to use any of a number of mutual exclusion
schemes, including advisory locking.

Some sessions with truss/ktrace and some studying of the sendmail
source may be necessary to get it right, but this is something I'd
definitely check out.

A wrapper for open could look like:

  #define open	__hide_open_prototype
  #include <sys/types.h>
  #include <fcntl.h>
  #include <unistd.h>
  #undef open
  
  int
  open(const char *path, int flags, mode_t mode)
  {
	  if (/* path is a file to be protected */) {
	     /* do something */
	     /* and beware of calling open() recursively */
	  }
	  return _open(path, flags, mode);
  } 

Compile with "cc -shared -o open.so -fpic open.c"

Of course, this still may not help much when sendmail has opened some
of its files, and you then change all of them, which might lead to
inconsistencies.

 	$.02,
	/Mikko

-- 
 Mikko Työläjärvi_______________________________________mikko@rsasecurity.com
 RSA Security


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