Re: [PATCH] skbuff: suppress clang object-size-mismatch error

[Tadeusz Struk]

Hi Marco,
On 11/11/21 01:51, Marco Elver wrote:
> On Thu, 11 Nov 2021 at 01:36, Tadeusz Struk<tadeusz.struk@xxxxxxxxxx>  wrote:
> > Kernel throws a runtime object-size-mismatch error in skbuff queue
> > helpers like in [1]. This happens every time there is a pattern
> > like the below:
> >
> > int skbuf_xmit(struct sk_buff *skb)
> > {
> >          struct sk_buff_head list;
> >
> >          __skb_queue_head_init(&list);
> >          __skb_queue_tail(&list, skb); <-- offending call
> >
> >          return do_xmit(net, &list);
> > }
> >
> > and the kernel is build with clang and -fsanitize=undefined flag set.
> > The reason is that the functions __skb_queue_[tail|head]() access the
> > struct sk_buff_head object via a pointer to struct sk_buff, which is
> > much bigger in size than the sk_buff_head. This could cause undefined
> > behavior and clang is complaining:
> >
> > UBSAN: object-size-mismatch in ./include/linux/skbuff.h:2023:28
> > member access within address ffffc90000cb71c0 with insufficient space
> > for an object of type 'struct sk_buff'
> The config includes CONFIG_UBSAN_OBJECT_SIZE, right? Normally that's
> disabled by default, probably why nobody has noticed these much.

Right, in all the defconfigs CONFIG_UBSAN_OBJECT_SIZE is not set.

> > Suppress the error with __attribute__((no_sanitize("undefined")))
> > in the skb helpers.
> Isn't there a better way, because doing this might also suppress other
> issues wholesale. __no_sanitize_undefined should be the last resort.

The other way to fix it would be to make the struct sk_buff_head
equal in size with struct sk_buff:

 struct sk_buff_head {
-       /* These two members must be first. */
-       struct sk_buff  *next;
-       struct sk_buff  *prev;
+       union {
+               struct {
+                       /* These two members must be first. */
+                       struct sk_buff  *next;
+                       struct sk_buff  *prev;

-       __u32           qlen;
-       spinlock_t      lock;
+                       __u32           qlen;
+                       spinlock_t      lock;
+               };
+               struct sk_buff  __prv;
+       };
 };

but that's much more invasive, and I don't even have means to
quantify this in terms of final binary size and performance
impact. I think that would be a flat out no go.

From the other hand if you look at the __skb_queue functions
they don't do much and at all so there is no much room for
other issues really. I followed the suggestion in [1]:

"if your function deliberately contains possible ..., you can
 use __attribute__((no_sanitize... "

[1] https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html

--
Thanks,
Tadeusz