[PATCH v4 2/3] mac80211: Prevent build failure with CONFIG_UBSAN=y

[PATCH v4 2/3] mac80211: Prevent build failure with CONFIG_UBSAN=y

[Andrey Ryabinin]

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>
---
 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 abbdff0..3e24d0d 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),
@@ -126,9 +126,6 @@ static const char *hw_flag_names[NUM_IEEE80211_HW_FLAGS + 1] = {
 	FLAG(SUPPORTS_AMSDU_IN_AMPDU),
 	FLAG(BEACON_TX_STATUS),
 	FLAG(NEEDS_UNIQUE_STA_ADDR),
-
-	/* keep last for the build bug below */
-	(void *)0x1
 #undef FLAG
 };
 
@@ -148,7 +145,7 @@ 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))
-- 
2.4.10

Re: [PATCH v4 2/3] mac80211: Prevent build failure with CONFIG_UBSAN=y

[Johannes Berg]

On Thu, 2015-12-03 at 18:50 +0300, Andrey Ryabinin wrote:
> 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.
> 
Seems fine, would you want to take it through some other tree together
with UBSAN, or do you expect that to still take long enough to allow
this to trickle through our trees?

johannes

Re: [PATCH v4 2/3] mac80211: Prevent build failure with CONFIG_UBSAN=y

[Andrey Ryabinin]

2015-12-03 20:05 GMT+03:00 Johannes Berg <johannes@sipsolutions.net>:
> On Thu, 2015-12-03 at 18:50 +0300, Andrey Ryabinin wrote:
>> 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.
>>
> Seems fine, would you want to take it through some other tree together
> with UBSAN, or do you expect that to still take long enough to allow
> this to trickle through our trees?
>

I expect that Andrew will take it with UBSAN for 4.5