objtool: Prevent GCC from merging annotate_unreachable()

0-day bot reported some new objtool warnings which were caused by the
new annotate_unreachable() macro:

  fs/afs/flock.o: warning: objtool: afs_do_unlk()+0x0: duplicate frame pointer save
  fs/afs/flock.o: warning: objtool: afs_do_unlk()+0x0: frame pointer state mismatch
  fs/btrfs/delayed-inode.o: warning: objtool: btrfs_delete_delayed_dir_index()+0x0: duplicate frame pointer save
  fs/btrfs/delayed-inode.o: warning: objtool: btrfs_delete_delayed_dir_index()+0x0: frame pointer state mismatch
  fs/dlm/lock.o: warning: objtool: _grant_lock()+0x0: duplicate frame pointer save
  fs/dlm/lock.o: warning: objtool: _grant_lock()+0x0: frame pointer state mismatch
  fs/ocfs2/alloc.o: warning: objtool: ocfs2_mv_path()+0x0: duplicate frame pointer save
  fs/ocfs2/alloc.o: warning: objtool: ocfs2_mv_path()+0x0: frame pointer state mismatch

It turns out that, for older versions of GCC, if a function has multiple
BUG() incantations, GCC will sometimes merge the corresponding
annotate_unreachable() inline asm statements into a single block.  That
has the undesirable effect of removing one of the entries in the
__unreachable section, confusing objtool greatly.

A workaround for this issue is to ensure that each instance of the
inline asm statement uses a different label, so that GCC sees the
statements are unique and leaves them alone.  The inline asm ‘%=’ token
could be used for that, but unfortunately older versions of GCC don't
support it.  So I implemented a poor man's version of it with the
__LINE__ macro.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
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>
Fixes: d1091c7fa3d5 ("objtool: Improve detection of BUG() and other dead ends")
Link: http://lkml.kernel.org/r/0c14b00baf9f68d1b0221ddb6c88b925181c8be8.1487997036.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 8ea159f..de47134 100644 (file)
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -197,10 +197,10 @@
 
 #ifdef CONFIG_STACK_VALIDATION
 #define annotate_unreachable() ({                                      \
-       asm("1:\t\n"                                                    \
+       asm("%c0:\t\n"                                                  \
            ".pushsection __unreachable, \"a\"\t\n"                     \
-           ".long 1b\t\n"                                              \
-           ".popsection\t\n");                                         \
+           ".long %c0b\t\n"                                            \
+           ".popsection\t\n" : : "i" (__LINE__));                      \
 })
 #else
 #define annotate_unreachable()