bcachefs: Add safe versions of varint encode/decode

This adds safe versions of bch2_varint_(encode|decode) that don't read
or write past the end of the buffer, or varint being encoded.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/alloc_background.c

@@ -130,7 +130,7 @@ static int bch2_alloc_unpack_v2(struct bkey_alloc_unpacked *out,
 
 #define x(_name, _bits)							\
 	if (fieldnr < a.v->nr_fields) {					\
-		ret = bch2_varint_decode(in, end, &v);			\
+		ret = bch2_varint_decode_fast(in, end, &v);		\
 		if (ret < 0)						\
 			return ret;					\
 		in += ret;						\
@@ -166,7 +166,7 @@ static void bch2_alloc_pack_v2(struct bkey_alloc_buf *dst,
 	nr_fields++;							\
 									\
 	if (src._name) {						\
-		out += bch2_varint_encode(out, src._name);		\
+		out += bch2_varint_encode_fast(out, src._name);		\
 									\
 		last_nonzero_field = out;				\
 		last_nonzero_fieldnr = nr_fields;			\

fs/bcachefs/inode.c

@@ -137,7 +137,7 @@ static void bch2_inode_pack_v2(struct bkey_inode_buf *packed,
 	nr_fields++;							\
 									\
 	if (inode->_name) {						\
-		ret = bch2_varint_encode(out, inode->_name);		\
+		ret = bch2_varint_encode_fast(out, inode->_name);	\
 		out += ret;						\
 									\
 		if (_bits > 64)						\
@@ -246,13 +246,13 @@ static int bch2_inode_unpack_v2(struct bkey_s_c_inode inode,
 
 #define x(_name, _bits)							\
 	if (fieldnr < INODE_NR_FIELDS(inode.v)) {			\
-		ret = bch2_varint_decode(in, end, &v[0]);		\
+		ret = bch2_varint_decode_fast(in, end, &v[0]);		\
 		if (ret < 0)						\
 			return ret;					\
 		in += ret;						\
 									\
 		if (_bits > 64) {					\
-			ret = bch2_varint_decode(in, end, &v[1]);	\
+			ret = bch2_varint_decode_fast(in, end, &v[1]);	\
 			if (ret < 0)					\
 				return ret;				\
 			in += ret;					\

fs/bcachefs/varint.c

@@ -2,15 +2,80 @@
 
 #include <linux/bitops.h>
 #include <linux/math.h>
+#include <linux/string.h>
 #include <asm/unaligned.h>
 
 #include "varint.h"
 
+/**
+ * bch2_varint_encode - encode a variable length integer
+ * @out - destination to encode to
+ * @v	- unsigned integer to encode
+ *
+ * Returns the size in bytes of the encoded integer - at most 9 bytes
+ */
 int bch2_varint_encode(u8 *out, u64 v)
 {
 	unsigned bits = fls64(v|1);
 	unsigned bytes = DIV_ROUND_UP(bits, 7);
 
+	if (likely(bytes < 9)) {
+		v <<= bytes;
+		v |= ~(~0 << (bytes - 1));
+		v = cpu_to_le64(v);
+		memcpy(out, &v, bytes);
+	} else {
+		*out++ = 255;
+		bytes = 9;
+		put_unaligned_le64(v, out);
+	}
+
+	return bytes;
+}
+
+/**
+ * bch2_varint_decode - encode a variable length integer
+ * @in	- varint to decode
+ * @end	- end of buffer to decode from
+ * @out	- on success, decoded integer
+ *
+ * Returns the size in bytes of the decoded integer - or -1 on failure (would
+ * have read past the end of the buffer)
+ */
+int bch2_varint_decode(const u8 *in, const u8 *end, u64 *out)
+{
+	unsigned bytes = likely(in < end)
+		? ffz(*in & 255) + 1
+		: 1;
+	u64 v;
+
+	if (unlikely(in + bytes > end))
+		return -1;
+
+	if (likely(bytes < 9)) {
+		v = 0;
+		memcpy(&v, in, bytes);
+		v = le64_to_cpu(v);
+		v >>= bytes;
+	} else {
+		v = get_unaligned_le64(++in);
+	}
+
+	*out = v;
+	return bytes;
+}
+
+/**
+ * bch2_varint_encode_fast - fast version of bch2_varint_encode
+ *
+ * This version assumes it's always safe to write 8 bytes to @out, even if the
+ * encoded integer would be smaller.
+ */
+int bch2_varint_encode_fast(u8 *out, u64 v)
+{
+	unsigned bits = fls64(v|1);
+	unsigned bytes = DIV_ROUND_UP(bits, 7);
+
 	if (likely(bytes < 9)) {
 		v <<= bytes;
 		v |= ~(~0 << (bytes - 1));
@@ -23,7 +88,13 @@ int bch2_varint_encode(u8 *out, u64 v)
 	return bytes;
 }
 
-int bch2_varint_decode(const u8 *in, const u8 *end, u64 *out)
+/**
+ * bch2_varint_decode_fast - fast version of bch2_varint_decode
+ *
+ * This version assumes that it is safe to read at most 8 bytes past the end of
+ * @end (we still return an error if the varint extends past @end).
+ */
+int bch2_varint_decode_fast(const u8 *in, const u8 *end, u64 *out)
 {
 	u64 v = get_unaligned_le64(in);
 	unsigned bytes = ffz(v & 255) + 1;

fs/bcachefs/varint.h

@@ -5,4 +5,7 @@
 int bch2_varint_encode(u8 *, u64);
 int bch2_varint_decode(const u8 *, const u8 *, u64 *);
 
+int bch2_varint_encode_fast(u8 *, u64);
+int bch2_varint_decode_fast(const u8 *, const u8 *, u64 *);
+
 #endif /* _BCACHEFS_VARINT_H */