Messages in this thread | | | Date | Tue, 25 Oct 2016 10:11:12 +0200 | From | Jiri Pirko <> | Subject | Re: [PATCH v3 net] flow_dissector: fix vlan tag handling |
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Mon, Oct 24, 2016 at 11:40:30PM CEST, arnd@arndb.de 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@arndb.de>
Acked-by: Jiri Pirko <jiri@mellanox.com>
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