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.

Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 305183f30a3331bde79b5bfd690314a0ed8dd06f..fccf72cbd343bfac7e6c354d4022340eb3eb4f32 100644 (file)
--- 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;