objtool: Disassemble instruction on warning or backtrace

When an instruction warning (WARN_INSN) or backtrace (BT_INSN) is issued,
disassemble the instruction to provide more context.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://patch.msgid.link/20251121095340.464045-8-alexandre.chartre@oracle.com

tools/objtool/check.c

@@ -4792,11 +4792,34 @@ static void free_insns(struct objtool_file *file)
 		free(chunk->addr);
 }
 
+static struct disas_context *objtool_disas_ctx;
+
+const char *objtool_disas_insn(struct instruction *insn)
+{
+	struct disas_context *dctx = objtool_disas_ctx;
+
+	if (!dctx)
+		return "";
+
+	disas_insn(dctx, insn);
+	return disas_result(dctx);
+}
+
 int check(struct objtool_file *file)
 {
-	struct disas_context *disas_ctx;
+	struct disas_context *disas_ctx = NULL;
 	int ret = 0, warnings = 0;
 
+	/*
+	 * If the verbose or backtrace option is used then we need a
+	 * disassembly context to disassemble instruction or function
+	 * on warning or backtrace.
+	 */
+	if (opts.verbose || opts.backtrace) {
+		disas_ctx = disas_context_create(file);
+		objtool_disas_ctx = disas_ctx;
+	}
+
 	arch_initial_func_cfi_state(&initial_func_cfi);
 	init_cfi_state(&init_cfi);
 	init_cfi_state(&func_cfi);
@@ -4936,11 +4959,12 @@ int check(struct objtool_file *file)
 	if (opts.verbose) {
 		if (opts.werror && warnings)
 			WARN("%d warning(s) upgraded to errors", warnings);
-		disas_ctx = disas_context_create(file);
-		if (disas_ctx) {
-			disas_warned_funcs(disas_ctx);
-			disas_context_destroy(disas_ctx);
-		}
+		disas_warned_funcs(disas_ctx);
+	}
+
+	if (disas_ctx) {
+		disas_context_destroy(disas_ctx);
+		objtool_disas_ctx = NULL;
 	}
 
 	free_insns(file);

tools/objtool/disas.c

@@ -303,7 +303,7 @@ void disas_context_destroy(struct disas_context *dctx)
 	free(dctx);
 }
 
-static char *disas_result(struct disas_context *dctx)
+char *disas_result(struct disas_context *dctx)
 {
 	return dctx->result;
 }
@@ -311,8 +311,7 @@ static char *disas_result(struct disas_context *dctx)
 /*
  * Disassemble a single instruction. Return the size of the instruction.
  */
-static size_t disas_insn(struct disas_context *dctx,
-			 struct instruction *insn)
+size_t disas_insn(struct disas_context *dctx, struct instruction *insn)
 {
 	disassembler_ftype disasm = dctx->disassembler;
 	struct disassemble_info *dinfo = &dctx->info;

tools/objtool/include/objtool/check.h

@@ -141,4 +141,6 @@ struct instruction *next_insn_same_sec(struct objtool_file *file, struct instruc
 	     insn && insn->offset < sym->offset + sym->len;		\
 	     insn = next_insn_same_sec(file, insn))
 
+const char *objtool_disas_insn(struct instruction *insn);
+
 #endif /* _CHECK_H */

tools/objtool/include/objtool/disas.h

@@ -17,6 +17,8 @@ void disas_warned_funcs(struct disas_context *dctx);
 int disas_info_init(struct disassemble_info *dinfo,
 		    int arch, int mach32, int mach64,
 		    const char *options);
+size_t disas_insn(struct disas_context *dctx, struct instruction *insn);
+char *disas_result(struct disas_context *dctx);
 
 #else /* DISAS */
 
@@ -38,6 +40,17 @@ static inline int disas_info_init(struct disassemble_info *dinfo,
 	return -1;
 }
 
+static inline size_t disas_insn(struct disas_context *dctx,
+				struct instruction *insn)
+{
+	return -1;
+}
+
+static inline char *disas_result(struct disas_context *dctx)
+{
+	return NULL;
+}
+
 #endif /* DISAS */
 
 #endif /* _DISAS_H */

tools/objtool/include/objtool/warn.h

@@ -77,9 +77,11 @@ static inline char *offstr(struct section *sec, unsigned long offset)
 #define WARN_INSN(insn, format, ...)					\
 ({									\
 	struct instruction *_insn = (insn);				\
-	if (!_insn->sym || !_insn->sym->warned)				\
+	if (!_insn->sym || !_insn->sym->warned)	{			\
 		WARN_FUNC(_insn->sec, _insn->offset, format,		\
 			  ##__VA_ARGS__);				\
+		BT_INSN(_insn, "");					\
+	}								\
 	if (_insn->sym)							\
 		_insn->sym->warned = 1;					\
 })
@@ -87,10 +89,14 @@ static inline char *offstr(struct section *sec, unsigned long offset)
 #define BT_INSN(insn, format, ...)				\
 ({								\
 	if (opts.verbose || opts.backtrace) {			\
-		struct instruction *_insn = (insn);		\
-		char *_str = offstr(_insn->sec, _insn->offset); \
-		WARN("  %s: " format, _str, ##__VA_ARGS__);	\
-		free(_str);					\
+		struct instruction *__insn = (insn);		\
+		char *_str = offstr(__insn->sec, __insn->offset); \
+		const char *_istr = objtool_disas_insn(__insn);	\
+		int _len;					\
+		_len = snprintf(NULL, 0, "  %s: " format,  _str, ##__VA_ARGS__);	\
+		_len = (_len < 50) ? 50 - _len : 0;		\
+		WARN("  %s: " format "  %*s%s", _str, ##__VA_ARGS__, _len, "", _istr); \
+		free(_str);						\
 	}							\
 })