Re: [RFC][PATCH] Replacing the /proc/<pid|self>/exe symlink code

[Serge E. Hallyn]

Quoting Matt Helsley (matthltc@xxxxxxxxxx):
> On Wed, 2007-05-30 at 13:09 -0500, Serge E. Hallyn wrote:
> > Quoting Matt Helsley (matthltc@xxxxxxxxxx):
> > > This patch avoids holding the mmap semaphore while walking VMAs in response to
> > > programs which read or follow the /proc/<pid|self>/exe symlink. This also allows us
> > > to merge mmu and nommu proc_exe_link() functions. The costs are holding a separate
> > > reference to the executable file stored in the task struct and increased code in
> > > fork, exec, and exit paths.
> > > 
> > > Signed-off-by: Matt Helsley <matthltc@xxxxxxxxxx>
> > > ---
> > > 
> > > Compiled and passed simple tests for regressions when patched against a 2.6.20
> > > and 2.6.22-rc2-mm1 kernel.
> > > 
> > >  fs/exec.c             |    5 +++--
> > >  fs/proc/base.c        |   20 ++++++++++++++++++++
> > >  fs/proc/internal.h    |    1 -
> > >  fs/proc/task_mmu.c    |   34 ----------------------------------
> > >  fs/proc/task_nommu.c  |   34 ----------------------------------
> > >  include/linux/sched.h |    1 +
> > >  kernel/exit.c         |    2 ++
> > >  kernel/fork.c         |   10 +++++++++-
> > >  8 files changed, 35 insertions(+), 72 deletions(-)
> 
> <snip>
> 
> > > Index: linux-2.6.22-rc2-mm1/kernel/exit.c
> > > ===================================================================
> > > --- linux-2.6.22-rc2-mm1.orig/kernel/exit.c
> > > +++ linux-2.6.22-rc2-mm1/kernel/exit.c
> > > @@ -924,10 +924,12 @@ fastcall void do_exit(long code)
> > >  	if (unlikely(tsk->audit_context))
> > >  		audit_free(tsk);
> > >  
> > >  	taskstats_exit(tsk, group_dead);
> > >  
> > > +	if (tsk->exe_file)
> > > +		fput(tsk->exe_file);
> > 
> > Hi,
> > 
> > just taking a cursory look so I may be missing something, but doesn't
> > this leave the possibility that right here, with tsk->exe_file being
> > put, another task would try to look at tsk's /proc/tsk->pid/exe?
> > 
> > thanks,
> > -serge
> >
> >       exit_mm(tsk);
> >
>   
> <snip>
> 
> Good question. To be precise, I think the problem doesn't exist here but
> after the exit_mm() because there's a VMA that holds a reference to the
> same file.
> 
> The existing code appears to solve the race between
> reading/following /proc/tsk->pid/exe and exit_mm() in the exit path by
> returning -ENOENT for the case where there is no executable VMA with a
> reference to the file backing it.
> 
> So I need to put NULL in the exe_file field and adjust the return value
> to be -ENOENT instead of -ENOSYS.
> 
> Thanks for the review!

Ok, I had to think about this a bit, but so you're saying you set it to
NULL in do_exit(), and anyone who has just dereferenced tsk->exe_file
before the fput in do_exit() should be ok because the vma hasn't yet
been put?

Should the 
	if (!task->exe_file)
		goto out;
	*mnt = mntget(task->exe_file->f_path.mnt);
	*dentry = dget(task->exe_file->f_path.dentry);

also go inside an preempt_disable to prevent sleeping and maybe become

	exef = task->exe_file;  /* to prevent task->exe_file being set
			to NULL before we've grabbed the path */
	if (!exef)
		goto out;
	get_file(exef);  /* to prevent the mm somehow being put before
				we've grabbed the path? */
	*mnt = mntget(task->exe_file->f_path.mnt);
	*dentry = dget(task->exe_file->f_path.dentry);
	put_file(exef);  /* ? */

?

Or am I being overly paranoid?

thanks,
-serge
-
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