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Date: Fri, 1 Sep 2023 00:19:17 +0000
From: Joel Fernandes <joel@...lfernandes.org>
To: Uladzislau Rezki <urezki@...il.com>
Cc: linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Christoph Hellwig <hch@...radead.org>,
Lorenzo Stoakes <lstoakes@...il.com>,
Zhen Lei <thunder.leizhen@...weicloud.com>,
"Paul E . McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
Zqiang <qiang.zhang1211@...il.com>,
Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/2] mm/vmalloc: Add a safer version of find_vm_area()
for debug
On Thu, Aug 31, 2023 at 09:47:52PM +0200, Uladzislau Rezki wrote:
> On Thu, Aug 31, 2023 at 05:18:25PM +0000, Joel Fernandes (Google) wrote:
> > It is unsafe to dump vmalloc area information when trying to do so from
> > some contexts. Add a safer trylock version of the same function to do a
> > best-effort VMA finding and use it from vmalloc_dump_obj().
> >
> > [apply test robot feedback on unused function fix.]
> >
> > Reported-by: Zhen Lei <thunder.leizhen@...weicloud.com>
> > Cc: Paul E. McKenney <paulmck@...nel.org>
> > Cc: rcu@...r.kernel.org
> > Cc: Zqiang <qiang.zhang1211@...il.com>
> > Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> > v1->v2: Apply review tags and test robot feedback.
> >
> > mm/vmalloc.c | 39 ++++++++++++++++++++++++++++++++++++++-
> > 1 file changed, 38 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 93cf99aba335..f09e882ae3b8 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1865,6 +1865,20 @@ struct vmap_area *find_vmap_area(unsigned long addr)
> > return va;
> > }
> >
> > +#ifdef CONFIG_PRINTK
> > +static struct vmap_area *find_vmap_area_trylock(unsigned long addr)
> > +{
> > + struct vmap_area *va;
> > +
> > + if (!spin_trylock(&vmap_area_lock))
> > + return NULL;
> > + va = __find_vmap_area(addr, &vmap_area_root);
> > + spin_unlock(&vmap_area_lock);
> > +
> > + return va;
> > +}
> > +#endif
> > +
> > static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> > {
> > struct vmap_area *va;
> > @@ -2671,6 +2685,29 @@ struct vm_struct *find_vm_area(const void *addr)
> > return va->vm;
> > }
> >
> > +/**
> > + * try_to_find_vm_area - find a continuous kernel virtual area
> > + * @addr: base address
> > + *
> > + * This function is the same as find_vm_area() except that it is
> > + * safe to call if vmap_area_lock is already held and returns NULL
> > + * if it is. See comments in find_vmap_area() for other details.
> > + *
> > + * Return: the area descriptor on success or %NULL on failure.
> > + */
> > +#ifdef CONFIG_PRINTK
> > +static struct vm_struct *try_to_find_vm_area(const void *addr)
> > +{
> > + struct vmap_area *va;
> > +
> > + va = find_vmap_area_trylock((unsigned long)addr);
> > + if (!va)
> > + return NULL;
> > +
> > + return va->vm;
> > +}
> > +#endif
> > +
> > /**
> > * remove_vm_area - find and remove a continuous kernel virtual area
> > * @addr: base address
> > @@ -4277,7 +4314,7 @@ bool vmalloc_dump_obj(void *object)
> > struct vm_struct *vm;
> > void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> >
> > - vm = find_vm_area(objp);
> > + vm = try_to_find_vm_area(objp);
> > if (!vm)
> > return false;
> > pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
Hi Vlad,
Thanks for taking a look.
> I am not sure if this patch makes a lot of sense. I agree, this is a
> problem and it mitigates it. But it is broken in terms of once you drop
> the lock, the VA should not be accessed.
Just to note the lockless-access issue you are referring to is not introduced
by this patch but is rather in the existing code. Also just to note this is
debug code.
> Is that a real issue or it gets triggered due to some syntetic test case?
It is a real issue. See 2/2.
> If i were you, i would go with open-coded version of trylock. Because
> there is only one user so far.
Taking your open coding and locking suggestions, I came up with the below
which actually results in a smaller patch. Does it look good to you?
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 93cf99aba335..aaf6bad997a7 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -4274,14 +4274,31 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
#ifdef CONFIG_PRINTK
bool vmalloc_dump_obj(void *object)
{
+ void *caller, *objp = (void *)PAGE_ALIGN((unsigned long)object);
struct vm_struct *vm;
- void *objp = (void *)PAGE_ALIGN((unsigned long)object);
+ struct vmap_area *va;
+ unsigned long addr;
+ unsigned int nr_pages;
- vm = find_vm_area(objp);
- if (!vm)
+ if (!spin_trylock(&vmap_area_lock))
+ return false;
+ va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
+ if (!va) {
+ spin_unlock(&vmap_area_lock);
return false;
+ }
+
+ vm = va->vm;
+ if (!vm) {
+ spin_unlock(&vmap_area_lock);
+ return false;
+ }
+ addr = vm->addr;
+ caller = vm->caller;
+ nr_pages = vm->nr_pages;
+ spin_unlock(&vmap_area_lock);
pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
- vm->nr_pages, (unsigned long)vm->addr, vm->caller);
+ nr_pages, addr, caller);
return true;
}
#endif
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