[RFC PATCH v3 3/6] objtool: Fix "can't find starting instruction" warnings on vmlinux

[Sathvika Vasireddy <sv@linux.ibm.com>]

Objtool throws a lot of can't find starting instruction warnings
when run on vmlinux with --ftr-fixup option.

These warnings are seen because find_insn() function looks for
instructions at offsets that are relative to the start of the section.
In case of individual object files (.o), there are no can't find
starting instruction warnings seen because the actual offset
associated with an instruction is itself a relative offset since the
sections start at offset 0x0.

However, in case of vmlinux, find_insn() function fails to find
instructions at the actual offset associated with an instruction
since the sections in vmlinux do not start at offset 0x0. Due to
this, find_insn() will look for absolute offset and not the relative
offset. This is resulting in a lot of can't find starting instruction
warnings when objtool is run on vmlinux.

To fix this, pass offset that is relative to the start of the section
to find_insn().

find_insn() is also looking for symbols of size 0. But, objtool does
not store empty STT_NOTYPE symbols in the rbtree. Due to this,
for empty symbols, objtool is throwing can't find starting
instruction warnings. Fix this by ignoring symbols that are of
size 0 since objtool does not add them to the rbtree.

Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
---
 tools/objtool/check.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 667d95431793..92fe5dc05cdb 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -492,7 +492,11 @@ static int decode_instructions(struct objtool_file *file)
 			if (func->embedded_insn || func->alias != func)
 				continue;
 
-			if (!find_insn(file, sec, func->offset)) {
+			if (func->len == 0)
+				continue;
+
+			if (!find_insn(file, sec, opts.ftr_fixup ?
+					func->offset - sec->sym->offset : func->offset)) {
 				ERROR("%s(): can't find starting instruction", func->name);
 				return -1;
 			}
-- 
2.43.0