[PATCH 4.4] net/mac80211/debugfs.c: prevent build failure with CONFIG_UBSAN=y

[PATCH 4.4] net/mac80211/debugfs.c: prevent build failure with CONFIG_UBSAN=y

[Daniel Wagner]

From: Andrey Ryabinin <aryabinin@virtuozzo.com>

commit 68920c973254c5b71a684645c5f6f82d6732c5d6 upstream.

With upcoming CONFIG_UBSAN the following BUILD_BUG_ON in
net/mac80211/debugfs.c starts to trigger:

  BUILD_BUG_ON(hw_flag_names[NUM_IEEE80211_HW_FLAGS] != (void *)0x1);

It seems, that compiler instrumentation causes some code
deoptimizations.  Because of that GCC is not being able to resolve
condition in BUILD_BUG_ON() at compile time.

We could make size of hw_flag_names array unspecified and replace the
condition in BUILD_BUG_ON() with following:

  ARRAY_SIZE(hw_flag_names) != NUM_IEEE80211_HW_FLAGS

That will have the same effect as before (adding new flag without
updating array will trigger build failure) except it doesn't fail with
CONFIG_UBSAN.  As a bonus this patch slightly decreases size of
hw_flag_names array.

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[Daniel: backport to 4.4.]
Signed-off-by: Daniel Wagner <daniel.wagner@siemens.com>
---

Hi,

The only stable tree which is missing this fix is 4.4. 4.1 doesn't
have 30686bf7f5b3 ("mac80211: convert HW flags to unsigned long
bitmap") which makes gcc unhappy with allmodconfig. 4.9 contains the
fix.

Thanks,
Daniel

 net/mac80211/debugfs.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/net/mac80211/debugfs.c b/net/mac80211/debugfs.c
index 4d2aaebd4f97..e546a987a9d3 100644
--- a/net/mac80211/debugfs.c
+++ b/net/mac80211/debugfs.c
@@ -91,7 +91,7 @@ static const struct file_operations reset_ops = {
 };
 #endif
 
-static const char *hw_flag_names[NUM_IEEE80211_HW_FLAGS + 1] = {
+static const char *hw_flag_names[] = {
 #define FLAG(F)	[IEEE80211_HW_##F] = #F
 	FLAG(HAS_RATE_CONTROL),
 	FLAG(RX_INCLUDES_FCS),
@@ -125,9 +125,6 @@ static const char *hw_flag_names[NUM_IEEE80211_HW_FLAGS + 1] = {
 	FLAG(TDLS_WIDER_BW),
 	FLAG(SUPPORTS_AMSDU_IN_AMPDU),
 	FLAG(BEACON_TX_STATUS),
-
-	/* keep last for the build bug below */
-	(void *)0x1
 #undef FLAG
 };
 
@@ -147,7 +144,7 @@ static ssize_t hwflags_read(struct file *file, char __user *user_buf,
 	/* fail compilation if somebody adds or removes
 	 * a flag without updating the name array above
 	 */
-	BUILD_BUG_ON(hw_flag_names[NUM_IEEE80211_HW_FLAGS] != (void *)0x1);
+	BUILD_BUG_ON(ARRAY_SIZE(hw_flag_names) != NUM_IEEE80211_HW_FLAGS);
 
 	for (i = 0; i < NUM_IEEE80211_HW_FLAGS; i++) {
 		if (test_bit(i, local->hw.flags))
-- 
2.14.3

Re: [PATCH 4.4] net/mac80211/debugfs.c: prevent build failure with CONFIG_UBSAN=y

[Greg Kroah-Hartman]

On Tue, Jan 09, 2018 at 12:53:27PM +0100, Daniel Wagner wrote:
> From: Andrey Ryabinin <aryabinin@virtuozzo.com>
> 
> commit 68920c973254c5b71a684645c5f6f82d6732c5d6 upstream.
> 
> With upcoming CONFIG_UBSAN the following BUILD_BUG_ON in
> net/mac80211/debugfs.c starts to trigger:
> 
>   BUILD_BUG_ON(hw_flag_names[NUM_IEEE80211_HW_FLAGS] != (void *)0x1);
> 
> It seems, that compiler instrumentation causes some code
> deoptimizations.  Because of that GCC is not being able to resolve
> condition in BUILD_BUG_ON() at compile time.
> 
> We could make size of hw_flag_names array unspecified and replace the
> condition in BUILD_BUG_ON() with following:
> 
>   ARRAY_SIZE(hw_flag_names) != NUM_IEEE80211_HW_FLAGS
> 
> That will have the same effect as before (adding new flag without
> updating array will trigger build failure) except it doesn't fail with
> CONFIG_UBSAN.  As a bonus this patch slightly decreases size of
> hw_flag_names array.
> 
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Johannes Berg <johannes@sipsolutions.net>
> Cc: "David S. Miller" <davem@davemloft.net>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> [Daniel: backport to 4.4.]
> Signed-off-by: Daniel Wagner <daniel.wagner@siemens.com>
> ---
> 
> Hi,
> 
> The only stable tree which is missing this fix is 4.4. 4.1 doesn't
> have 30686bf7f5b3 ("mac80211: convert HW flags to unsigned long
> bitmap") which makes gcc unhappy with allmodconfig. 4.9 contains the
> fix.

Now queued up, thanks for the backport.

greg k-h