ubsan: Clarify Kconfig text for CONFIG_UBSAN_TRAP

Make it clearer in the one-line description and the verbose description
text that CONFIG_UBSAN_TRAP as currently implemented involves a tradeoff of
much less helpful oops messages in exchange for a smaller kernel image.
(With the additional effect of turning UBSAN warnings into crashes, which
may or may not be desired.)

Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20230705215128.486054-1-jannh@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

lib/Kconfig.ubsan

@@ -13,7 +13,7 @@ menuconfig UBSAN
 if UBSAN
 
 config UBSAN_TRAP
-	bool "On Sanitizer warnings, abort the running kernel code"
+	bool "Abort on Sanitizer warnings (smaller kernel but less verbose)"
 	depends on !COMPILE_TEST
 	help
 	  Building kernels with Sanitizer features enabled tends to grow
@@ -26,6 +26,14 @@ config UBSAN_TRAP
 	  the system. For some system builders this is an acceptable
 	  trade-off.
 
+	  Also note that selecting Y will cause your kernel to Oops
+	  with an "illegal instruction" error with no further details
+	  when a UBSAN violation occurs. (Except on arm64, which will
+	  report which Sanitizer failed.) This may make it hard to
+	  determine whether an Oops was caused by UBSAN or to figure
+	  out the details of a UBSAN violation. It makes the kernel log
+	  output less useful for bug reports.
+
 config CC_HAS_UBSAN_BOUNDS_STRICT
 	def_bool $(cc-option,-fsanitize=bounds-strict)
 	help