lib/ubsan: default UBSAN_ALIGNMENT to not set

When booting an allmodconfig kernel, there are a lot of false-positives.
With a message like this 'UBSAN: Undefined behaviour in...' with a call
trace that follows.

UBSAN warnings are a result of enabling noisy CONFIG_UBSAN_ALIGNMENT
which is disabled by default if HAVE_EFFICIENT_UNALIGNED_ACCESS=y.

It's noisy even if don't have efficient unaligned access, e.g.  people
often add __cacheline_aligned_in_smp in structs, but forget to align
allocations of such struct (kmalloc() give 8-byte alignment in worst
case).

Rework so that when building a allmodconfig kernel that turns everything
into '=m' or '=y' will turn off UBSAN_ALIGNMENT.

[[email protected]: changelog addition]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Anders Roxell <[email protected]>
Suggested-by: Arnd Bergmann <[email protected]>
Acked-by: Andrey Ryabinin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/Kconfig.ubsan

@@ -27,15 +27,19 @@ config UBSAN_SANITIZE_ALL
 	  Enabling this option will get kernel image size increased
 	  significantly.
 
-config UBSAN_ALIGNMENT
-	bool "Enable checking of pointers alignment"
+config UBSAN_NO_ALIGNMENT
+	bool "Disable checking of pointers alignment"
 	depends on UBSAN
-	default y if !HAVE_EFFICIENT_UNALIGNED_ACCESS
+	default y if HAVE_EFFICIENT_UNALIGNED_ACCESS
 	help
-	  This option enables detection of unaligned memory accesses.
-	  Enabling this option on architectures that support unaligned
+	  This option disables the check of unaligned memory accesses.
+	  This option should be used when building allmodconfig.
+	  Disabling this option on architectures that support unaligned
 	  accesses may produce a lot of false positives.
 
+config UBSAN_ALIGNMENT
+	def_bool !UBSAN_NO_ALIGNMENT
+
 config TEST_UBSAN
 	tristate "Module for testing for undefined behavior detection"
 	depends on m && UBSAN