libceph: add start en/decoding block helpers

Add ceph_start_encoding() and ceph_start_decoding(), the equivalent of
ENCODE_START and DECODE_START in the userspace ceph code.

This is based on a patch from Mike Christie <michaelc@cs.wisc.edu>.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

1 files changed, 54 insertions, 0 deletions

diff --git a/include/linux/ceph/decode.h b/include/linux/ceph/decode.h
index e83f3c81ef43..f990f2cc907a 100644
--- a/include/linux/ceph/decode.h
+++ b/include/linux/ceph/decode.h
@@ -218,6 +218,60 @@ static inline void ceph_encode_string(void **p, void *end,
         *p += len;
 }
 
+/*
+ * version and length starting block encoders/decoders
+ */
+
+/* current code version (u8) + compat code version (u8) + len of struct (u32) */
+#define CEPH_ENCODING_START_BLK_LEN 6
+
+/**
+ * ceph_start_encoding - start encoding block
+ * @struct_v: current (code) version of the encoding
+ * @struct_compat: oldest code version that can decode it
+ * @struct_len: length of struct encoding
+ */
+static inline void ceph_start_encoding(void **p, u8 struct_v, u8 struct_compat,
+                                       u32 struct_len)
+{
+        ceph_encode_8(p, struct_v);
+        ceph_encode_8(p, struct_compat);
+        ceph_encode_32(p, struct_len);
+}
+
+/**
+ * ceph_start_decoding - start decoding block
+ * @v: current version of the encoding that the code supports
+ * @name: name of the struct (free-form)
+ * @struct_v: out param for the encoding version
+ * @struct_len: out param for the length of struct encoding
+ *
+ * Validates the length of struct encoding, so unsafe ceph_decode_*
+ * variants can be used for decoding.
+ */
+static inline int ceph_start_decoding(void **p, void *end, u8 v,
+                                      const char *name, u8 *struct_v,
+                                      u32 *struct_len)
+{
+        u8 struct_compat;
+
+        ceph_decode_need(p, end, CEPH_ENCODING_START_BLK_LEN, bad);
+        *struct_v = ceph_decode_8(p);
+        struct_compat = ceph_decode_8(p);
+        if (v < struct_compat) {
+                pr_warn("got struct_v %d struct_compat %d > %d of %s\n",
+                        *struct_v, struct_compat, v, name);
+                return -EINVAL;
+        }
+
+        *struct_len = ceph_decode_32(p);
+        ceph_decode_need(p, end, *struct_len, bad);
+        return 0;
+
+bad:
+        return -ERANGE;
+}
+
 #define ceph_encode_need(p, end, n, bad)                        \
         do {                                                    \
                 if (!likely(ceph_has_room(p, end, n)))          \