tracing/probes: Support typecast for various probe events

Support BTF typecast feature on other probe events, but only if it is
kernel function entry or return, and must use function parameter name
or $retval. This means you can do:

  (STRUCT)PARAM->MEMBER

Note: you can not use other variables like $stackN, %reg etc. That
needs nesting support.

To support other probe events, we just need to use last_struct type
when we find a function parameter in parse_btf_arg().

This also updates <tracefs>/README file to show struct typecast.

Link: https://lore.kernel.org/all/178271362928.1176915.12235759508786922490.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Documentation/trace/fprobetrace.rst

@@ -57,6 +57,9 @@ Synopsis of fprobe-events
                   (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
                   (x8/x16/x32/x64), "char", "string", "ustring", "symbol", "symstr"
                   and bitfield are supported.
+  (STRUCT)FIELD->MEMBER[->MEMBER] : If BTF is supported, typecast FIELD to
+                  a pointer to STRUCT and then derference the pointer defined by
+                  ->MEMBER.
 
   (\*1) This is available only when BTF is enabled.
   (\*2) only for the probe on function entry (offs == 0). Note, this argument access

Documentation/trace/kprobetrace.rst

@@ -61,6 +61,10 @@ Synopsis of kprobe_events
 		  (x8/x16/x32/x64), VFS layer common type(%pd/%pD), "char",
                   "string", "ustring", "symbol", "symstr" and bitfield are
                   supported.
+  (STRUCT)FIELD->MEMBER[->MEMBER] : If BTF is supported, typecast FIELD to
+                  a pointer to STRUCT and then derference the pointer defined by
+                  ->MEMBER. Note that this is available only when the probe is
+		   on function entry.
 
   (\*1) only for the probe on function entry (offs == 0). Note, this argument access
         is best effort, because depending on the argument type, it may be passed on

kernel/trace/trace.c

@@ -4322,7 +4322,7 @@ static const char readme_msg[] =
 #ifdef CONFIG_HAVE_FUNCTION_ARG_ACCESS_API
 	"\t           $stack<index>, $stack, $retval, $comm, $arg<N>,\n"
 #ifdef CONFIG_PROBE_EVENTS_BTF_ARGS
-	"\t           <argname>[->field[->field|.field...]],\n"
+	"\t           [(structname)]<argname>[->field[->field|.field...]],\n"
 #endif
 #else
 	"\t           $stack<index>, $stack, $retval, $comm,\n"

kernel/trace/trace_probe.c

@@ -699,7 +699,7 @@ static int parse_btf_arg(char *varname,
 
 	if (ctx->flags & TPARG_FL_RETURN && !strcmp(varname, "$retval")) {
 		code->op = FETCH_OP_RETVAL;
-		/* Check whether the function return type is not void */
+		/* Check whether the function return type is not void, even with typecast. */
 		if (query_btf_context(ctx) == 0) {
 			if (ctx->proto->type == 0) {
 				trace_probe_log_err(ctx->offset, NO_RETVAL);
@@ -708,6 +708,13 @@ static int parse_btf_arg(char *varname,
 			tid = ctx->proto->type;
 			goto found;
 		}
+		/*
+		 * Even if we can not find appropriate BTF info, we can still access
+		 * the field via typecast.
+		 */
+		if (ctx->struct_btf)
+			goto found;
+
 		if (field) {
 			trace_probe_log_err(ctx->offset + field - varname,
 					    NO_BTF_ENTRY);
@@ -752,7 +759,10 @@ static int parse_btf_arg(char *varname,
 	return -ENOENT;
 
 found:
-	type = btf_type_skip_modifiers(ctx->btf, tid, NULL);
+	if (ctx->struct_btf)
+		type = ctx->last_struct;
+	else
+		type = btf_type_skip_modifiers(ctx->btf, tid, NULL);
 found_type:
 	if (!type) {
 		trace_probe_log_err(ctx->offset, BAD_BTF_TID);
@@ -829,10 +839,11 @@ static int handle_typecast(char *arg, struct fetch_insn **pcode,
 	char *tmp;
 	int ret;
 
-	/* Currently this only works for eprobes */
-	if (!(ctx->flags & TPARG_FL_TEVENT)) {
-		trace_probe_log_err(ctx->offset, TYPECAST_NOT_EVENT);
-		return -EINVAL;
+	if (!(tparg_is_event_probe(ctx->flags) ||
+	      tparg_is_function_entry(ctx->flags) ||
+	      tparg_is_function_return(ctx->flags))) {
+		trace_probe_log_err(ctx->offset, NOSUP_BTFARG);
+		return -EOPNOTSUPP;
 	}
 
 	tmp = strchr(arg, ')');

kernel/trace/trace_probe.h

@@ -429,6 +429,11 @@ static inline bool tparg_is_function_return(unsigned int flags)
 	return (flags & TPARG_FL_LOC_MASK) == (TPARG_FL_KERNEL | TPARG_FL_RETURN);
 }
 
+static inline bool tparg_is_event_probe(unsigned int flags)
+{
+	return !!(flags & TPARG_FL_TEVENT);
+}
+
 struct traceprobe_parse_context {
 	struct trace_event_call *event;
 	/* BTF related parameters */