On Sep 1, 2023, at 8:48 AM, Uladzislau Rezki [off-list ref] wrote:

On Fri, Sep 01, 2023 at 12:33:21AM +0000, Joel Fernandes wrote:

quoted

quoted

On Fri, Sep 01, 2023 at 12:19:17AM +0000, Joel Fernandes wrote:
On Thu, Aug 31, 2023 at 09:47:52PM +0200, Uladzislau Rezki wrote:

quoted

On Thu, Aug 31, 2023 at 05:18:25PM +0000, Joel Fernandes (Google) wrote:

quoted

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 <redacted>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: rcu@vger.kernel.org
Cc: Zqiang <redacted>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Joel Fernandes (Google) <redacted>
---
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.

quoted

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.

quoted

Is that a real issue or it gets triggered due to some syntetic test case?

It is a real issue. See 2/2.

quoted

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

And with some trivial compiler errors fixed (sorry should have build tested
but wanted to just share the idea earlier):

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 93cf99aba335..2c6a0e2ff404 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c

@@ -4274,14 +4274,32 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)

#ifdef CONFIG_PRINTK
bool vmalloc_dump_obj(void *object)
{
-    struct vm_struct *vm;
   void *objp = (void *)PAGE_ALIGN((unsigned long)object);
+    const void *caller;
+    struct vm_struct *vm;
+    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)objp, &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 = (unsigned long)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

Looks good to me and thank you for fixing a locking issue :)
I think you will re-spin and resend it one more time?

Yes. May I add your Reviewed-by tag to both patches after re-spinning as mentioned above?

thanks!

 - Joel

--
Uladzislau Rezki