objtool: Fix unreachable instruction warnings for weak functions

In the presence of both weak and strong function definitions, the
linker drops the weak symbol in favor of a strong symbol, but
leaves the code in place. Code in ignore_unreachable_insn() has
some heuristics to suppress the warning, but it does not work when
-ffunction-sections is enabled.

Suppose function foo has both strong and weak definitions.
Case 1: The strong definition has an annotated section name,
like .init.text. Only the weak definition will be placed into
.text.foo. But since the section has no symbols, there will be no
"hole" in the section.

Case 2: Both sections are without an annotated section name.
Both will be placed into .text.foo section, but there will be only one
symbol (the strong one). If the weak code is before the strong code,
there is no "hole" as it fails to find the right-most symbol before
the offset.

The fix is to use the first node to compute the hole if hole.sym
is empty. If there is no symbol in the section, the first node
will be NULL, in which case, -1 is returned to skip the whole
section.

Co-developed-by: Han Shen <[email protected]>
Signed-off-by: Han Shen <[email protected]>
Signed-off-by: Rong Xu <[email protected]>
Suggested-by: Sriraman Tallam <[email protected]>
Suggested-by: Krzysztof Pszeniczny <[email protected]>
Tested-by: Yonghong Song <[email protected]>
Tested-by: Yabin Cui <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>

Author: xur-llvm

tools/objtool/elf.c

@@ -224,12 +224,17 @@ int find_symbol_hole_containing(const struct section *sec, unsigned long offset)
 	if (n)
 		return 0; /* not a hole */
 
-	/* didn't find a symbol for which @offset is after it */
-	if (!hole.sym)
-		return 0; /* not a hole */
+	/*
+	 * @offset >= sym->offset + sym->len, find symbol after it.
+	 * When hole.sym is empty, use the first node to compute the hole.
+	 * If there is no symbol in the section, the first node will be NULL,
+	 * in which case, -1 is returned to skip the whole section.
+	 */
+	if (hole.sym)
+		n = rb_next(&hole.sym->node);
+	else
+		n = rb_first_cached(&sec->symbol_tree);
 
-	/* @offset >= sym->offset + sym->len, find symbol after it */
-	n = rb_next(&hole.sym->node);
 	if (!n)
 		return -1; /* until end of address space */