kasan: fix stack traces dependency for HW_TAGS

Currently, whether the alloc/free stack traces collection is enabled by
default for hardware tag-based KASAN depends on CONFIG_DEBUG_KERNEL.
The intention for this dependency was to only enable collection on slow
debug kernels due to a significant perf and memory impact.

As it turns out, CONFIG_DEBUG_KERNEL is not considered a debug option
and is enabled on many productions kernels including Android and Ubuntu.
As the result, this dependency is pointless and only complicates the
code and documentation.

Having stack traces collection disabled by default would make the
hardware mode work differently to to the software ones, which is
confusing.

This change removes the dependency and enables stack traces collection
by default.

Looking into the future, this default might makes sense for production
kernels, assuming we implement a fast stack trace collection approach.

Link: https://lkml.kernel.org/r/6678d77ceffb71f1cff2cf61560e2ffe7bb6bfe9.1612808820.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/kasan.rst

@@ -163,8 +163,7 @@ particular KASAN features.
 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
 
 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
-  traces collection (default: ``on`` for ``CONFIG_DEBUG_KERNEL=y``, otherwise
-  ``off``).
+  traces collection (default: ``on``).
 
 - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
   report or also panic the kernel (default: ``report``).

mm/kasan/hw_tags.c

@@ -134,11 +134,7 @@ void __init kasan_init_hw_tags(void)
 
 	switch (kasan_arg_stacktrace) {
 	case KASAN_ARG_STACKTRACE_DEFAULT:
-		/*
-		 * Default to enabling stack trace collection for
-		 * debug kernels.
-		 */
-		if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
+		/* Default to enabling stack trace collection. */
 		static_branch_enable(&kasan_flag_stacktrace);
 		break;
 	case KASAN_ARG_STACKTRACE_OFF: