Re: [PATCH v3 net] flow_dissector: fix vlan tag handling

[Eric Garver]

On Mon, Oct 24, 2016 at 11:40:30PM +0200, Arnd Bergmann wrote:
> gcc warns about an uninitialized pointer dereference in the vlan
> priority handling:
> 
> net/core/flow_dissector.c: In function '__skb_flow_dissect':
> net/core/flow_dissector.c:281:61: error: 'vlan' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> 
> As pointed out by Jiri Pirko, the variable is never actually used
> without being initialized first as the only way it end up uninitialized
> is with skb_vlan_tag_present(skb)==true, and that means it does not
> get accessed.
> 
> However, the warning hints at some related issues that I'm addressing
> here:
> 
> - the second check for the vlan tag is different from the first one
>   that tests the skb for being NULL first, causing both the warning
>   and a possible NULL pointer dereference that was not entirely fixed.
> - The same patch that introduced the NULL pointer check dropped an
>   earlier optimization that skipped the repeated check of the
>   protocol type
> - The local '_vlan' variable is referenced through the 'vlan' pointer
>   but the variable has gone out of scope by the time that it is
>   accessed, causing undefined behavior
> 
> Caching the result of the 'skb && skb_vlan_tag_present(skb)' check
> in a local variable allows the compiler to further optimize the
> later check. With those changes, the warning also disappears.
> 
> Fixes: 3805a938a6c2 ("flow_dissector: Check skb for VLAN only if skb specified.")
> Fixes: d5709f7ab776 ("flow_dissector: For stripped vlan, get vlan info from skb->vlan_tci")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Acked-by: Eric Garver <e@xxxxxxx>