From 3bba89d97f74d5556f18d20b9aad27f4a30c465b Mon Sep 17 00:00:00 2001
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Tue, 6 Sep 2022 11:57:58 -0400
Subject: [PATCH] Implement lttng_msgpack_write_user_str

Implement lttng_msgpack_write_user_str to allow safely capturing
user-space strings.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I0354382cdd599b041fd20e59bb673fda7d72b2be
---
 include/lttng/msgpack.h   |   2 +
 src/lib/msgpack/msgpack.c | 105 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 107 insertions(+)

diff --git a/include/lttng/msgpack.h b/include/lttng/msgpack.h
index 0c63f1ba..f667db6c 100644
--- a/include/lttng/msgpack.h
+++ b/include/lttng/msgpack.h
@@ -50,6 +50,8 @@ int lttng_msgpack_write_signed_integer(
 		struct lttng_msgpack_writer *writer, int64_t value);
 int lttng_msgpack_write_str(struct lttng_msgpack_writer *writer,
 		const char *value);
+int lttng_msgpack_write_user_str(struct lttng_msgpack_writer *writer,
+		const char __user *value);
 int lttng_msgpack_begin_map(struct lttng_msgpack_writer *writer, size_t count);
 int lttng_msgpack_end_map(struct lttng_msgpack_writer *writer);
 int lttng_msgpack_begin_array(
diff --git a/src/lib/msgpack/msgpack.c b/src/lib/msgpack/msgpack.c
index fbbd96f0..e7ff1e39 100644
--- a/src/lib/msgpack/msgpack.c
+++ b/src/lib/msgpack/msgpack.c
@@ -57,6 +57,7 @@
 #include <asm/byteorder.h>
 
 #include <lttng/msgpack.h>
+#include <lttng/probe-user.h>
 
 #define INT8_MIN		(-128)
 #define INT16_MIN		(-32767-1)
@@ -113,6 +114,33 @@ end:
 	return ret;
 }
 
+static inline int lttng_msgpack_append_user_buffer(
+		struct lttng_msgpack_writer *writer,
+		const uint8_t __user *ubuf,
+		size_t length)
+{
+	int ret = 0;
+
+	lttng_msgpack_assert(ubuf);
+
+	/* Ensure we are not trying to write after the end of the buffer. */
+	if (writer->write_pos + length > writer->end_write_pos) {
+		ret = -1;
+		goto end;
+	}
+
+	if (lttng_copy_from_user_check_nofault(writer->write_pos, ubuf, length)) {
+		/*
+		 * After a successful strlen user, a page fault on copy is handled by
+		 * considering the string as empty, returning a success.
+		 */
+		goto end;
+	}
+	writer->write_pos += length;
+end:
+	return ret;
+}
+
 static inline int lttng_msgpack_append_u8(
 		struct lttng_msgpack_writer *writer, uint8_t value)
 {
@@ -281,6 +309,53 @@ end:
 	return ret;
 }
 
+static inline int lttng_msgpack_encode_user_fixstr(
+		struct lttng_msgpack_writer *writer,
+		const char __user *ustr,
+		uint8_t len)
+{
+	int ret;
+
+	lttng_msgpack_assert(len <= MSGPACK_FIXSTR_MAX_LENGTH);
+
+	ret = lttng_msgpack_append_u8(writer, MSGPACK_FIXSTR_ID_MASK | len);
+	if (ret)
+		goto end;
+
+	ret = lttng_msgpack_append_user_buffer(writer, (uint8_t __user *) ustr, len);
+	if (ret)
+		goto end;
+
+end:
+	return ret;
+}
+
+static inline int lttng_msgpack_encode_user_str16(
+		struct lttng_msgpack_writer *writer,
+		const char __user *ustr,
+		uint16_t len)
+{
+	int ret;
+
+	lttng_msgpack_assert(len > MSGPACK_FIXSTR_MAX_LENGTH);
+
+	ret = lttng_msgpack_append_u8(writer, MSGPACK_STR16_ID);
+	if (ret)
+		goto end;
+
+	ret = lttng_msgpack_append_u16(writer, len);
+	if (ret)
+		goto end;
+
+	ret = lttng_msgpack_append_user_buffer(writer, (uint8_t __user *) ustr, len);
+	if (ret)
+		goto end;
+
+end:
+	return ret;
+}
+
+
 int lttng_msgpack_begin_map(struct lttng_msgpack_writer *writer, size_t count)
 {
 	int ret;
@@ -354,6 +429,36 @@ end:
 	return ret;
 }
 
+/*
+ * Provide the same behavior on lttng_strlen_user_inatomic page fault as the
+ * lttng ring buffer: truncate the last string character.
+ */
+int lttng_msgpack_write_user_str(struct lttng_msgpack_writer *writer,
+		const char __user *ustr)
+{
+	int ret;
+	size_t length = max_t(size_t, lttng_strlen_user_inatomic(ustr), 1);
+
+	if (length >= (1 << 16)) {
+		ret = -1;
+		goto end;
+	}
+
+	/*
+	 * Handle empty string and strlen user page fault as empty string.
+	 */
+	if (length == 1)
+		return lttng_msgpack_write_str(writer, "");
+
+	if (length <= MSGPACK_FIXSTR_MAX_LENGTH)
+		ret = lttng_msgpack_encode_user_fixstr(writer, ustr, length);
+	else
+		ret = lttng_msgpack_encode_user_str16(writer, ustr, length);
+
+end:
+	return ret;
+}
+
 int lttng_msgpack_write_nil(struct lttng_msgpack_writer *writer)
 {
 	return lttng_msgpack_append_u8(writer, MSGPACK_NIL_ID);
-- 
2.34.1