Quoting Matt Helsley ([EMAIL PROTECTED]):
> On Wed, 2007-05-30 at 13:09 -0500, Serge E. Hallyn wrote:
> > Quoting Matt Helsley ([EMAIL PROTECTED]):
> > > 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 <[EMAIL PROTECTED]>
> > > ---
> > >
> > > 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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