objtool: Generic annotation infrastructure

Avoid endless .discard.foo sections for each annotation, create a
single .discard.annotate_insn section that takes an annotation type along
with the instruction.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Peter Zijlstra

include/linux/objtool.h

@@ -57,6 +57,13 @@
 	".long 998b\n\t"						\
 	".popsection\n\t"
 
+#define ASM_ANNOTATE(type)						\
+	"911:\n\t"							\
+	".pushsection .discard.annotate_insn,\"M\",@progbits,8\n\t"	\
+	".long 911b - .\n\t"						\
+	".long " __stringify(type) "\n\t"				\
+	".popsection\n\t"
+
 #else /* __ASSEMBLY__ */
 
 /*
@@ -146,6 +153,14 @@
 	.popsection
 .endm
 
+.macro ANNOTATE type:req
+.Lhere_\@:
+	.pushsection .discard.annotate_insn,"M",@progbits,8
+	.long	.Lhere_\@ - .
+	.long	\type
+	.popsection
+.endm
+
 #endif /* __ASSEMBLY__ */
 
 #else /* !CONFIG_OBJTOOL */
@@ -155,6 +170,7 @@
 #define UNWIND_HINT(type, sp_reg, sp_offset, signal) "\n\t"
 #define STACK_FRAME_NON_STANDARD(func)
 #define STACK_FRAME_NON_STANDARD_FP(func)
+#define ASM_ANNOTATE(type)
 #define ANNOTATE_NOENDBR
 #define ASM_REACHABLE
 #else
@@ -167,6 +183,8 @@
 .endm
 .macro REACHABLE
 .endm
+.macro ANNOTATE type:req
+.endm
 #endif
 
 #endif /* CONFIG_OBJTOOL */

tools/objtool/check.c

@@ -2373,6 +2373,49 @@ static int read_unwind_hints(struct objtool_file *file)
 	return 0;
 }
 
+static int read_annotate(struct objtool_file *file, void (*func)(int type, struct instruction *insn))
+{
+	struct section *sec;
+	struct instruction *insn;
+	struct reloc *reloc;
+	int type;
+
+	sec = find_section_by_name(file->elf, ".discard.annotate_insn");
+	if (!sec)
+		return 0;
+
+	if (!sec->rsec)
+		return 0;
+
+	if (sec->sh.sh_entsize != 8) {
+		static bool warned = false;
+		if (!warned) {
+			WARN("%s: dodgy linker, sh_entsize != 8", sec->name);
+			warned = true;
+		}
+		sec->sh.sh_entsize = 8;
+	}
+
+	for_each_reloc(sec->rsec, reloc) {
+		type = *(u32 *)(sec->data->d_buf + (reloc_idx(reloc) * sec->sh.sh_entsize) + 4);
+
+		insn = find_insn(file, reloc->sym->sec,
+				 reloc->sym->offset + reloc_addend(reloc));
+		if (!insn) {
+			WARN("bad .discard.annotate_insn entry: %d of type %d", reloc_idx(reloc), type);
+			return -1;
+		}
+
+		func(type, insn);
+	}
+
+	return 0;
+}
+
+static void __annotate_nop(int type, struct instruction *insn)
+{
+}
+
 static int read_noendbr_hints(struct objtool_file *file)
 {
 	struct instruction *insn;
@@ -2670,6 +2713,8 @@ static int decode_sections(struct objtool_file *file)
 	if (ret)
 		return ret;
 
+	read_annotate(file, __annotate_nop);
+
 	/*
 	 * Must be before read_unwind_hints() since that needs insn->noendbr.
 	 */