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 <a...@arndb.de>
Acked-by: Eric Garver <e...@erig.me>