Re: PATCH Multithreaded core dumps for the 2.5.17 kernel was ....RE: PATCH Multithreaded core dump support for the 2.5.14 (and 15) kernel.

[Daniel Jacobowitz <dmj+@andrew.cmu.edu>]

On Wed, May 22, 2002 at 05:09:01PM -0400, Mark Gross wrote:
> On Wednesday 22 May 2002 07:32 pm, Daniel Jacobowitz wrote:
> >
> > What about other things which take mmap_sem?  I believe at least ptrace
> > is involved.  The notion of avoiding taking a semaphore like this is a
> > somewhat risky one.
> 
> Yes access_process_vm down_writes the mmap_sem.  However; it can only read 
> and write to existing user pages.  As long as it doesn't delete any of them 
> its not a problem.  It won't cause a dead lock or panic during the core dump 
> processing if this happens.  
> 
> The only process I know that could honestly use this ptrace function is GDB 
> doing live debugging.
> 
> >
> > Why shouldn't you take the semaphore as before in elf_core_dump, if you
> > know that no suspended process has it - which you do if you hold it
> > while suspending them?
> 
> For Ia64 those down_writes are just a pain.  If a user application is 
> crashing because someone is being rude with GDB corrupting its user pages 
> then I don't think its worth the hassle of protecting the core dumped user 
> page mm data from being messed up by a GDB user.
> 
> I would like to leave the down_write out of elf_core_dump, but it could be 
> put back if its felt that its needed.
> 
> Opinions? Comments?

I'm not worried about the application crashing.  I'm worried about
oopsing if someone is poking at the mmap_sem while we are pretending to
have it.  If that is not a valid concern, there should at least be a
big red flag saying so.

-- 
Daniel Jacobowitz                           Debian GNU/Linux Developer
MontaVista Software                         Carnegie Mellon University