[PATCH v2 51/53] objtool/klp: Fix kCFI prefix finding/cloning

With CFI+CALL_PADDING, Clang places .Ltmp labels at the start of the NOP
padding (offset 5) between the __cfi_ prefix and the function entry
point.  get_func_prefix() only checks the immediately previous symbol,
so the intervening .Ltmp label causes it to miss the __cfi_ prefix
symbol.

This results in klp-diff not cloning the kCFI type hash into the
livepatch module, causing a CFI failure at module load when calling
callback functions through indirect calls:

  CFI failure at __klp_enable_patch+0xab/0x140
    (target: pre_patch_callback+0x0/0x80 [livepatch_combined];
     expected type: 0xde073954)

Instead of walking backward through the section's symbol list, just use
find_func_containing() for the byte before the function.  This works now
that __cfi_ symbols are being grown by objtool to fill the padding.

Signed-off-by: Josh Poimboeuf <[email protected]>
---
 tools/objtool/include/objtool/elf.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 305183f30a33..fccf72cbd343 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -539,10 +539,10 @@ static inline struct symbol *get_func_prefix(struct symbol *func)
 {
 	struct symbol *prev;
 
-	if (!is_func_sym(func))
+	if (!is_func_sym(func) || !func->offset)
 		return NULL;
 
-	prev = sec_prev_sym(func);
+	prev = find_func_containing(func->sec, func->offset - 1);
 	if (prev && is_prefix_func(prev))
 		return prev;
 
-- 
2.53.0