[PATCH] e1000: Distribute switch variables for initialization

[PATCH] e1000: Distribute switch variables for initialization

[Kees Cook]

Variables declared in a switch statement before any case statements
cannot be automatically initialized with compiler instrumentation (as
they are not part of any execution flow). With GCC's proposed automatic
stack variable initialization feature, this triggers a warning (and they
don't get initialized). Clang's automatic stack variable initialization
(via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
doesn't initialize such variables[1]. Note that these warnings (or silent
skipping) happen before the dead-store elimination optimization phase,
so even when the automatic initializations are later elided in favor of
direct initializations, the warnings remain.

To avoid these problems, move such variables into the "case" where
they're used or lift them up into the main function body.

drivers/net/ethernet/intel/e1000/e1000_main.c: In function ‘e1000_xmit_frame’:
drivers/net/ethernet/intel/e1000/e1000_main.c:3143:18: warning: statement will never be executed [-Wswitch-unreachable]
 3143 |     unsigned int pull_size;
      |                  ^~~~~~~~~

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/net/ethernet/intel/e1000/e1000_main.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/e1000/e1000_main.c b/drivers/net/ethernet/intel/e1000/e1000_main.c
index 2bced34c19ba..a540d0206129 100644
--- a/drivers/net/ethernet/intel/e1000/e1000_main.c
+++ b/drivers/net/ethernet/intel/e1000/e1000_main.c
@@ -3140,8 +3140,9 @@ static netdev_tx_t e1000_xmit_frame(struct sk_buff *skb,
 		hdr_len = skb_transport_offset(skb) + tcp_hdrlen(skb);
 		if (skb->data_len && hdr_len == len) {
 			switch (hw->mac_type) {
+			case e1000_82544: {
 				unsigned int pull_size;
-			case e1000_82544:
+
 				/* Make sure we have room to chop off 4 bytes,
 				 * and that the end alignment will work out to
 				 * this hardware's requirements
@@ -3162,6 +3163,7 @@ static netdev_tx_t e1000_xmit_frame(struct sk_buff *skb,
 				}
 				len = skb_headlen(skb);
 				break;
+			}
 			default:
 				/* do nothing */
 				break;

RE: [Intel-wired-lan] [PATCH] e1000: Distribute switch variables for initialization

[Brown, Aaron F]

> From: Intel-wired-lan <intel-wired-lan-bounces@osuosl.org> On Behalf Of
> Kees Cook
> Sent: Wednesday, February 19, 2020 10:23 PM
> To: Kirsher, Jeffrey T <jeffrey.t.kirsher@intel.com>
> Cc: intel-wired-lan@lists.osuosl.org; Alexander Potapenko
> <glider@google.com>; Kees Cook <keescook@chromium.org>; linux-
> kernel@vger.kernel.org
> Subject: [Intel-wired-lan] [PATCH] e1000: Distribute switch variables for
> initialization
> 
> Variables declared in a switch statement before any case statements
> cannot be automatically initialized with compiler instrumentation (as
> they are not part of any execution flow). With GCC's proposed automatic
> stack variable initialization feature, this triggers a warning (and they
> don't get initialized). Clang's automatic stack variable initialization
> (via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
> doesn't initialize such variables[1]. Note that these warnings (or silent
> skipping) happen before the dead-store elimination optimization phase,
> so even when the automatic initializations are later elided in favor of
> direct initializations, the warnings remain.
> 
> To avoid these problems, move such variables into the "case" where
> they're used or lift them up into the main function body.
> 
> drivers/net/ethernet/intel/e1000/e1000_main.c: In function
> ‘e1000_xmit_frame’:
> drivers/net/ethernet/intel/e1000/e1000_main.c:3143:18: warning:
> statement will never be executed [-Wswitch-unreachable]
>  3143 |     unsigned int pull_size;
>       |                  ^~~~~~~~~
> 
> [1] https://bugs.llvm.org/show_bug.cgi?id=44916
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  drivers/net/ethernet/intel/e1000/e1000_main.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Tested-by: Aaron Brown <aaron.f.brown@intel.com>