[PATCH v2 5/9] tracing/probes: Support BTF field access from $retval

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Support BTF argument on '$retval' for function return events including
kretprobe and fprobe for accessing the return value.
This also allows user to access its fields if the return value is a
pointer of a data structure.

E.g.
 # echo 'f getname_flags%return +0($retval->name):string' \
   > dynamic_events
 # echo 1 > events/fprobes/getname_flags__exit/enable
 # ls > /dev/null
 # head -n 40 trace | tail
              ls-87      [000] ...1.  8067.616101: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./function_profile_enabled"
              ls-87      [000] ...1.  8067.616108: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./trace_stat"
              ls-87      [000] ...1.  8067.616115: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_graph_notrace"
              ls-87      [000] ...1.  8067.616122: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_graph_function"
              ls-87      [000] ...1.  8067.616129: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_ftrace_notrace"
              ls-87      [000] ...1.  8067.616135: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_ftrace_filter"
              ls-87      [000] ...1.  8067.616143: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./touched_functions"
              ls-87      [000] ...1.  8067.616237: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./enabled_functions"
              ls-87      [000] ...1.  8067.616245: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./available_filter_functions"
              ls-87      [000] ...1.  8067.616253: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_ftrace_notrace_pid"


Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 Changes in v2:
  - Use '$retval' instead of 'retval' because it is confusing.
---
 kernel/trace/trace_probe.c |   98 +++++++++++++++++---------------------------
 1 file changed, 37 insertions(+), 61 deletions(-)

diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
index dd646d35637d..4442ff9c2728 100644
--- a/kernel/trace/trace_probe.c
+++ b/kernel/trace/trace_probe.c
@@ -536,6 +536,22 @@ static int parse_btf_arg(char *varname,
 		return -EOPNOTSUPP;
 	}
 
+	if (ctx->flags & TPARG_FL_RETURN) {
+		if (strcmp(varname, "$retval") != 0) {
+			trace_probe_log_err(ctx->offset, NO_BTFARG);
+			return -ENOENT;
+		}
+		/* Check whether the function return type is not void */
+		type = btf_find_func_proto(btf, ctx->funcname);
+		if (!IS_ERR_OR_NULL(type) && type->type == 0) {
+			trace_probe_log_err(ctx->offset, NO_RETVAL);
+			return -ENOENT;
+		}
+		code->op = FETCH_OP_RETVAL;
+		tid = type->type;
+		goto found;
+	}
+
 	if (!ctx->params) {
 		params = find_btf_func_param(ctx->funcname, &ctx->nr_params,
 					     ctx->flags & TPARG_FL_TPOINT);
@@ -556,7 +572,6 @@ static int parse_btf_arg(char *varname,
 				code->param = i + 1;
 			else
 				code->param = i;
-
 			tid = params[i].type;
 			goto found;
 		}
@@ -581,7 +596,7 @@ static int parse_btf_arg(char *varname,
 	return 0;
 }
 
-static const struct fetch_type *parse_btf_arg_type(
+static const struct fetch_type *find_fetch_type_from_btf_type(
 					struct traceprobe_parse_context *ctx)
 {
 	struct btf *btf = traceprobe_get_btf();
@@ -593,26 +608,6 @@ static const struct fetch_type *parse_btf_arg_type(
 	return find_fetch_type(typestr, ctx->flags);
 }
 
-static const struct fetch_type *parse_btf_retval_type(
-					struct traceprobe_parse_context *ctx)
-{
-	struct btf *btf = traceprobe_get_btf();
-	const char *typestr = NULL;
-	const struct btf_type *type;
-	s32 tid;
-
-	if (btf && ctx->funcname) {
-		type = btf_find_func_proto(btf, ctx->funcname);
-		if (!IS_ERR_OR_NULL(type)) {
-			type = btf_type_skip_modifiers(btf, type->type, &tid);
-			if (!IS_ERR_OR_NULL(type))
-				typestr = fetch_type_from_btf_type(btf, type, ctx);
-		}
-	}
-
-	return find_fetch_type(typestr, ctx->flags);
-}
-
 static int parse_btf_bitfield(struct fetch_insn **pcode,
 			      struct traceprobe_parse_context *ctx)
 {
@@ -635,20 +630,6 @@ static int parse_btf_bitfield(struct fetch_insn **pcode,
 	return 0;
 }
 
-static bool is_btf_retval_void(const char *funcname)
-{
-	struct btf *btf = traceprobe_get_btf();
-	const struct btf_type *t;
-
-	if (!btf)
-		return false;
-
-	t = btf_find_func_proto(btf, funcname);
-	if (IS_ERR_OR_NULL(t))
-		return false;
-
-	return t->type == 0;
-}
 #else
 static struct btf *traceprobe_get_btf(void)
 {
@@ -676,24 +657,23 @@ static int parse_btf_bitfield(struct fetch_insn **pcode,
 	return -EOPNOTSUPP;
 }
 
-#define parse_btf_arg_type(ctx)		\
-	find_fetch_type(NULL, ctx->flags)
-
-#define parse_btf_retval_type(ctx)		\
+#define find_fetch_type_from_btf_type(ctx)		\
 	find_fetch_type(NULL, ctx->flags)
 
-#define is_btf_retval_void(funcname)	(false)
-
 #endif
 
 #define PARAM_MAX_STACK (THREAD_SIZE / sizeof(unsigned long))
 
-static int parse_probe_vars(char *arg, const struct fetch_type *t,
-			    struct fetch_insn *code,
+/* Parse $vars. @orig_arg points '$', which syncs to @ctx->offset */
+static int parse_probe_vars(char *orig_arg, const struct fetch_type *t,
+			    struct fetch_insn **pcode,
+			    struct fetch_insn *end,
 			    struct traceprobe_parse_context *ctx)
 {
-	unsigned long param;
+	struct fetch_insn *code = *pcode;
 	int err = TP_ERR_BAD_VAR;
+	char *arg = orig_arg + 1;
+	unsigned long param;
 	int ret = 0;
 	int len;
 
@@ -712,18 +692,17 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
 		goto inval;
 	}
 
-	if (strcmp(arg, "retval") == 0) {
-		if (ctx->flags & TPARG_FL_RETURN) {
-			if ((ctx->flags & TPARG_FL_KERNEL) &&
-			    is_btf_retval_void(ctx->funcname)) {
-				err = TP_ERR_NO_RETVAL;
-				goto inval;
-			}
+	if (str_has_prefix(arg, "retval")) {
+		if (!(ctx->flags & TPARG_FL_RETURN)) {
+			err = TP_ERR_RETVAL_ON_PROBE;
+			goto inval;
+		}
+		if (!(ctx->flags & TPARG_FL_KERNEL) ||
+		    !IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS)) {
 			code->op = FETCH_OP_RETVAL;
 			return 0;
 		}
-		err = TP_ERR_RETVAL_ON_PROBE;
-		goto inval;
+		return parse_btf_arg(orig_arg, pcode, end, ctx);
 	}
 
 	len = str_has_prefix(arg, "stack");
@@ -825,7 +804,7 @@ parse_probe_arg(char *arg, const struct fetch_type *type,
 
 	switch (arg[0]) {
 	case '$':
-		ret = parse_probe_vars(arg + 1, type, code, ctx);
+		ret = parse_probe_vars(arg, type, pcode, end, ctx);
 		break;
 
 	case '%':	/* named register */
@@ -1122,12 +1101,9 @@ static int traceprobe_parse_probe_arg_body(const char *argv, ssize_t *size,
 		goto fail;
 
 	/* Update storing type if BTF is available */
-	if (IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS) && !t) {
-		if (ctx->last_type)
-			parg->type = parse_btf_arg_type(ctx);
-		else if (ctx->flags & TPARG_FL_RETURN)
-			parg->type = parse_btf_retval_type(ctx);
-	}
+	if (IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS) &&
+	    !t && ctx->last_type)
+		parg->type = find_fetch_type_from_btf_type(ctx);
 
 	ret = -EINVAL;
 	/* Store operation */