lib: add stream_rewind_getp()

stream_forward_getp() cannot be used with negative numbers due to the
size_t argument, we'll end up doing overflow arithmetic.

Signed-off-by: Quentin Young <qlyoung@nvidia.com>

2 files changed, 32 insertions, 0 deletions

diff --git a/lib/stream.c b/lib/stream.c
index 6e62e11380..dc207c16a4 100644
--- a/lib/stream.c
+++ b/lib/stream.c
@@ -268,6 +268,30 @@ bool stream_forward_getp2(struct stream *s, size_t size)
 	return true;
 }
 
+void stream_rewind_getp(struct stream *s, size_t size)
+{
+	STREAM_VERIFY_SANE(s);
+
+	if (size > s->getp || !GETP_VALID(s, s->getp - size)) {
+		STREAM_BOUND_WARN(s, "rewind getp");
+		return;
+	}
+
+	s->getp -= size;
+}
+
+bool stream_rewind_getp2(struct stream *s, size_t size)
+{
+	STREAM_VERIFY_SANE(s);
+
+	if (size > s->getp || !GETP_VALID(s, s->getp - size))
+		return false;
+
+	s->getp -= size;
+
+	return true;
+}
+
 void stream_forward_endp(struct stream *s, size_t size)
 {
 	STREAM_VERIFY_SANE(s);
diff --git a/lib/stream.h b/lib/stream.h
index f2c16b3486..e78006fbcc 100644
--- a/lib/stream.h
+++ b/lib/stream.h
@@ -174,6 +174,8 @@ extern void stream_set_getp(struct stream *, size_t);
 extern void stream_set_endp(struct stream *, size_t);
 extern void stream_forward_getp(struct stream *, size_t);
 extern bool stream_forward_getp2(struct stream *, size_t);
+extern void stream_rewind_getp(struct stream *s, size_t size);
+extern bool stream_rewind_getp2(struct stream *s, size_t size);
 extern void stream_forward_endp(struct stream *, size_t);
 extern bool stream_forward_endp2(struct stream *, size_t);
 
@@ -461,6 +463,12 @@ static inline const uint8_t *ptr_get_be32(const uint8_t *ptr, uint32_t *out)
 			goto stream_failure;                                   \
 	} while (0)
 
+#define STREAM_REWIND_GETP(STR, SIZE)                                          \
+	do {                                                                   \
+		if (!stream_rewind_getp2((STR), (SIZE)))                       \
+			goto stream_failure;                                   \
+	} while (0)
+
 #define STREAM_FORWARD_ENDP(STR, SIZE)                                         \
 	do {                                                                   \
 		if (!stream_forward_endp2((STR), (SIZE)))                      \