On Wednesday 31 January 2007 19:42:51 Alexey Dobriyan wrote:
> On Wed, Jan 31, 2007 at 11:54:35AM +0100, Duncan Sands wrote:
> > Can read_proc still be executing when remove_proc_entry returns?
> >
> > In my driver [*] I allocate some data and create a proc entry using
> > create_proc_entry. My read method reads from my allocated data. When
> > shutting down, I call remove_proc_entry and immediately free the data.
> > If some call to read_proc is still executing at this point then it will
> > be accessing freed memory. Can this happen? I've been rummaging around
> > in fs/proc to see what prevents it, but didn't find anything yet.
>
> This should be fixed by the following patch (in -mm currently):
> http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.20-rc6/2.6.20-rc6-mm3/broken-out/fix-rmmod-read-write-races-in-proc-entries.patch
>
> Tell me if you're unsure it will.
Excellent! But tell me,
+ atomic_inc(&dp->pde_users);
+ if (!dp->proc_fops)
don't you need a memory barrier between these two? Also a corresponding
one where proc_fops is set to NULL.
+ /*
+ * Stop accepting new readers/writers. If you're dynamically
+ * allocating ->proc_fops, save a pointer somewhere.
+ */
+ de->proc_fops = NULL;
+ /* Wait until all readers/writers are done. */
+ if (atomic_read(&de->pde_users) > 0) {
+ spin_unlock(&proc_subdir_lock);
+ msleep(1);
+ goto again;
+ }
I don't understand how this is supposed to work. Consider
CPU1 CPU2
atomic_inc(&dp->pde_users);
if (dp->proc_fops)
de->proc_fops = NULL;
use_proc_fops <= BOOM
if (atomic_read(&de->pde_users) > 0) {
what prevents dereference of a NULL proc_fops value?
Best wishes,
Duncan.
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