[PATCH v2 6/9] tracing/probes: Add string type check with BTF

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

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

Add a string type checking with BTF information if possible.
This will check whether the given BTF argument (and field) is
signed char array or pointer to signed char. If not, it reject
the 'string' type. If it is pointer to signed char, it adds
a dereference opration so that it can correctly fetch the
string data from memory.

 # echo 'f getname_flags%return retval->name:string' >> dynamic_events
 # echo 't sched_switch next->comm:string' >> dynamic_events

The above cases, 'struct filename::name' is 'char *' and
'struct task_struct::comm' is 'char []'. But in both case,
user can specify ':string' to fetch the string data.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/trace/trace_probe.c |   89 +++++++++++++++++++++++++++++++++++++++++++-
 kernel/trace/trace_probe.h |    3 +
 2 files changed, 89 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
index 4442ff9c2728..4d1dccb8724b 100644
--- a/kernel/trace/trace_probe.c
+++ b/kernel/trace/trace_probe.c
@@ -319,6 +319,77 @@ static u32 btf_type_int(const struct btf_type *t)
 	return *(u32 *)(t + 1);
 }
 
+static bool btf_type_is_char_ptr(struct btf *btf, const struct btf_type *type)
+{
+	const struct btf_type *real_type;
+	u32 intdata;
+	s32 tid;
+
+	real_type = btf_type_skip_modifiers(btf, type->type, &tid);
+	if (!real_type)
+		return false;
+
+	if (BTF_INFO_KIND(real_type->info) != BTF_KIND_INT)
+		return false;
+
+	intdata = btf_type_int(real_type);
+	return !(BTF_INT_ENCODING(intdata) & BTF_INT_SIGNED)
+		&& BTF_INT_BITS(intdata) == 8;
+}
+
+static bool btf_type_is_char_array(struct btf *btf, const struct btf_type *type)
+{
+	const struct btf_type *real_type;
+	const struct btf_array *array;
+	u32 intdata;
+	s32 tid;
+
+	if (BTF_INFO_KIND(type->info) != BTF_KIND_ARRAY)
+		return false;
+
+	array = (const struct btf_array *)(type + 1);
+
+	real_type = btf_type_skip_modifiers(btf, array->type, &tid);
+
+	intdata = btf_type_int(real_type);
+	return !(BTF_INT_ENCODING(intdata) & BTF_INT_SIGNED)
+		&& BTF_INT_BITS(intdata) == 8;
+}
+
+static int check_prepare_btf_string_fetch(char *typename,
+				struct fetch_insn **pcode,
+				struct traceprobe_parse_context *ctx)
+{
+	struct btf *btf = traceprobe_get_btf();
+
+	if (!btf || !ctx->last_type)
+		return 0;
+
+	/* char [] does not need any change. */
+	if (btf_type_is_char_array(btf, ctx->last_type))
+		return 0;
+
+	/* char * requires dereference the pointer. */
+	if (btf_type_is_char_ptr(btf, ctx->last_type)) {
+		struct fetch_insn *code = *pcode + 1;
+
+		if (code->op == FETCH_OP_END) {
+			trace_probe_log_err(ctx->offset, TOO_MANY_OPS);
+			return -E2BIG;
+		}
+		if (typename[0] == 'u')
+			code->op = FETCH_OP_UDEREF;
+		else
+			code->op = FETCH_OP_DEREF;
+		code->offset = 0;
+		*pcode = code;
+		return 0;
+	}
+	/* Other types are not available for string */
+	trace_probe_log_err(ctx->offset, BAD_TYPE4STR);
+	return -EINVAL;
+}
+
 static const char *fetch_type_from_btf_type(struct btf *btf,
 					const struct btf_type *type,
 					struct traceprobe_parse_context *ctx)
@@ -660,6 +731,13 @@ static int parse_btf_bitfield(struct fetch_insn **pcode,
 #define find_fetch_type_from_btf_type(ctx)		\
 	find_fetch_type(NULL, ctx->flags)
 
+static int check_prepare_btf_string_fetch(char *typename,
+				struct fetch_insn **pcode,
+				struct traceprobe_parse_context *ctx)
+{
+	return 0;
+}
+
 #endif
 
 #define PARAM_MAX_STACK (THREAD_SIZE / sizeof(unsigned long))
@@ -1102,8 +1180,15 @@ static int traceprobe_parse_probe_arg_body(const char *argv, ssize_t *size,
 
 	/* Update storing type if BTF is available */
 	if (IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS) &&
-	    !t && ctx->last_type)
-		parg->type = find_fetch_type_from_btf_type(ctx);
+	    ctx->last_type) {
+		if (!t) {
+			parg->type = find_fetch_type_from_btf_type(ctx);
+		} else if (strstr(t, "string")) {
+			ret = check_prepare_btf_string_fetch(t, &code, ctx);
+			if (ret)
+				goto fail;
+		}
+	}
 
 	ret = -EINVAL;
 	/* Store operation */
diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
index 050909aaaa1b..604d6fb9c5ff 100644
--- a/kernel/trace/trace_probe.h
+++ b/kernel/trace/trace_probe.h
@@ -504,7 +504,8 @@ extern int traceprobe_define_arg_fields(struct trace_event_call *event_call,
 	C(NOSUP_DAT_ARG,	"Non pointer structure/union argument is not supported."),\
 	C(BAD_HYPHEN,		"Failed to parse single hyphen. Forgot '>'?"),	\
 	C(NO_BTF_FIELD,		"This field is not found."),	\
-	C(BAD_BTF_TID,		"Failed to get BTF type info."),
+	C(BAD_BTF_TID,		"Failed to get BTF type info."),\
+	C(BAD_TYPE4STR,		"This type does not fit for string."),
 
 #undef C
 #define C(a, b)		TP_ERR_##a