[PATCH] objtool: prevent gcc from merging annotate_unreachable()

[Josh Poimboeuf]

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 â%=â 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.

Fixes: d1091c7fa3d5 ("objtool: Improve detection of BUG() and other dead ends")
Reported-by: kbuild test robot <fengguang.wu@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 include/linux/compiler-gcc.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 8ea159f..de47134 100644
--- 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()
-- 
2.7.4