Re: [PATCH 1/4] mm: add new mmgrab() helper

[Vegard Nossum]

On 12/16/2016 10:56 AM, Peter Zijlstra wrote:
> On Fri, Dec 16, 2016 at 09:21:59AM +0100, Vegard Nossum wrote:
> > Apart from adding the helper function itself, the rest of the kernel is
> > converted mechanically using:
> >
> >   git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)->mm_count);/mmgrab\(\1\);/'
> >   git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)\.mm_count);/mmgrab\(\&\1\);/'
> >
> > This is needed for a later patch that hooks into the helper, but might be
> > a worthwhile cleanup on its own.
>
> Given the desire to replace all refcounting with a specific refcount
> type, this seems to make sense.
>
> FYI: http://www.openwall.com/lists/kernel-hardening/2016/12/07/8

If we're going that way eventually (replacing all reference counting
things with a generic interface), I wonder if we shouldn't consider a
generic mechanism for reference counting debugging too.

We could wrap all the 'type *' + 'type_ref' occurrences in a struct, so
that with debugging it boils down to just a pointer (like we have now):

struct ref {
    void *ptr;
#ifdef CONFIG_REF_DEBUG
    /* list_entry, pid, stacktrace, etc. */
#endif
};

Instead of calling refcount_inc() in most of the kernel code, that would
be considered a low-level detail and you'd have the main interface be
something like:

void ref_acquire(refcount_t *count, struct ref *old, struct ref *new)
{
    refcount_inc(&count);
    new->ptr = old->ptr;
#ifdef CONFIG_REF_DEBUG
    /* extra code for debugging case */
#endif
}

So if you had old code that did (for example):

struct task_struct {
    struct mm_struct *mm;
    ...
};

int proc_pid_cmdline_read(struct task_struct *task)
{
    struct mm_struct *mm;

    task_lock(task);
    mm = task->mm;
    atomic_inc(&mm->mm_users);
    task_unlock(task);

    ...

    mmput(mm);
}

you'd instead have:

struct task_struct {
    struct ref mm;
};

int proc_pid_cmdline_read(struct task_struct *task)
{
    REF(mm);

    task_lock(task);
    ref_acquire(&mm->mm_users, &task->mm, &mm)
    task_unlock(task);

    ...

    ref_release(&mm->mm_users, &mm);
}

Of course you'd define a 'struct ref' per type using a macro or
something to keep it type safe (maybe even wrap the counter itself in
there, e.g. mm_users in the example above, so you wouldn't have to pass
it explicitly).

Functions that don't touch reference counts (because the caller holds
one) can just take a plain pointer as usual.

In the example above, you could also have ref_release() set mm->ptr =
NULL; as the pointer should not be considered usable after it has been
released anyway for added safety/debugability.

Best of both worlds?


Vegard