objtool: Validate disassembler headers in libopcodes probegrafted

commit 3f2de814c059 ("objtool: Fix libopcodes linking with static libraries")
tested for libopcodes availability by linking a test snippet with a forward
declaration of disassemble_init_for_target().

However, testing symbol linkage with an extern declaration only verifies
the presence of the library (.so/.a) and bypasses checking for development
headers (binutils-dev). On systems where libopcodes is present without
development headers installed, the probe succeeds, enabling BUILD_DISAS.
Subsequent compilation of objtool then fails:

  fatal error: 'bfd.h' file not found
    113 | #include <bfd.h>

Additionally, the probe invokes $(HOSTCC) without $(HOSTCFLAGS), ignoring
any sysroot or include flags specified for the host compiler.

Fix this by including <bfd.h> and <dis-asm.h> directly in the test snippet,
passing $(HOSTCFLAGS) so host compiler options are respected, and defining
PACKAGE="objtool" to satisfy the configuration check in <bfd.h>.

Fixes: 3f2de814c059 ("objtool: Fix libopcodes linking with static libraries")
Fixes: 436326bc525d ("objtool: fix build failure due to missing libopcodes check")
Reported-by: Alice Ryhl <aliceryhl@google.com>
Assisted-by: Antigravity:Gemini-Next
Signed-off-by: Ulises Mendez Martinez <umendez@google.com>
Link: https://patch.msgid.link/20260904150710.2997558-1-umendez@google.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

1 files changed, 45 insertions, 0 deletions

diff --git a/scripts/objdump-func b/scripts/objdump-func
new file mode 100755
index 000000000..7b15b873d
--- /dev/null
+++ b/scripts/objdump-func
@@ -0,0 +1,45 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Disassemble a single function.
+#
+# usage: objdump-func <file> <func> [<func> ...]
+
+set -o errexit
+set -o nounset
+
+OBJDUMP="${CROSS_COMPILE:-}objdump"
+
+command -v gawk >/dev/null 2>&1 || die "gawk isn't installed"
+
+usage() {
+	echo "usage: objdump-func <file> <func> [<func> ...]" >&2
+	exit 1
+}
+
+[[ $# -lt 2 ]] && usage
+
+OBJ=$1; shift
+FUNCS=("$@")
+
+${OBJDUMP} -wdr $OBJ | gawk -M -v _funcs="${FUNCS[*]}" '
+	BEGIN { split(_funcs, funcs); }
+	/^$/ { func_match=0; }
+	/<.*>:/ {
+		f = gensub(/.*<(.*)>:/, "\\1", 1);
+		for (i in funcs) {
+			# match compiler-added suffixes like ".cold", etc
+			if (f ~ "^" funcs[i] "(\\..*)?") {
+				func_match = 1;
+				base = strtonum("0x" $1);
+				break;
+			}
+		}
+	}
+	{
+		if (func_match) {
+			addr = strtonum("0x" $1);
+			printf("%04x ", addr - base);
+			print;
+		}
+	}'