objtool: Improve detection of BUG() and other dead ends

The BUG() macro's use of __builtin_unreachable() via the unreachable()
macro tells gcc that the instruction is a dead end, and that it's safe
to assume the current code path will not execute past the previous
instruction.

On x86, the BUG() macro is implemented with the 'ud2' instruction.  When
objtool's branch analysis sees that instruction, it knows the current
code path has come to a dead end.

Peter Zijlstra has been working on a patch to change the WARN macros to
use 'ud2'.  That patch will break objtool's assumption that 'ud2' is
always a dead end.

Generally it's best for objtool to avoid making those kinds of
assumptions anyway.  The more ignorant it is of kernel code internals,
the better.

So create a more generic way for objtool to detect dead ends by adding
an annotation to the unreachable() macro.  The annotation stores a
pointer to the end of the unreachable code path in an '__unreachable'
section.  Objtool can read that section to find the dead ends.

Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/41a6d33971462ebd944a1c60ad4bf5be86c17b77.1487712920.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 0444b1336268..8ea159fc489d 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -195,6 +195,17 @@
 #endif
 #endif
 
+#ifdef CONFIG_STACK_VALIDATION
+#define annotate_unreachable() ({                                       \
+        asm("1:\t\n"                                                    \
+            ".pushsection __unreachable, \"a\"\t\n"                     \
+            ".long 1b\t\n"                                              \
+            ".popsection\t\n");                                         \
+})
+#else
+#define annotate_unreachable()
+#endif
+
 /*
  * Mark a position in code as unreachable.  This can be used to
  * suppress control flow warnings after asm blocks that transfer
@@ -204,7 +215,7 @@
  * this in the preprocessor, but we can live with this because they're
  * unreleased.  Really, we need to have autoconf for the kernel.
  */
-#define unreachable() __builtin_unreachable()
+#define unreachable() annotate_unreachable(); __builtin_unreachable()
 
 /* Mark a function definition as prohibited from being cloned. */
 #define __noclone       __attribute__((__noclone__, __optimize__("no-tracer")))