Update ino_path for the large cursor

Previously we could iterate over backref items with a small u64.  Now we
need a larger opaque buffer.

Signed-off-by: Zach Brown <zab@versity.com>

utils/src/ino_path.c

@@ -18,7 +18,8 @@ static int ino_path_cmd(int argc, char **argv)
 {
 	struct scoutfs_ioctl_ino_path args;
 	char *endptr = NULL;
-	char *path;
+	char *path = NULL;
+	char *curs = NULL;
 	u64 ino;
 	int ret;
 	int fd;
@@ -51,9 +52,18 @@ static int ino_path_cmd(int argc, char **argv)
 		goto out;
 	}
 
+	curs = calloc(1, SCOUTFS_IOC_INO_PATH_CURSOR_BYTES);
+	if (!curs) {
+		fprintf(stderr, "couldn't allocate %ld byte cursor\n",
+			SCOUTFS_IOC_INO_PATH_CURSOR_BYTES);
+		ret = -ENOMEM;
+		goto out;
+	}
+
 	args.ino = ino;
-	args.ctr = 0;
+	args.cursor_ptr = (intptr_t)curs;
 	args.path_ptr = (intptr_t)path;
+	args.cursor_bytes = SCOUTFS_IOC_INO_PATH_CURSOR_BYTES;
 	args.path_bytes = PATH_MAX;
 	do {
 		ret = ioctl(fd, SCOUTFS_IOC_INO_PATH, &args);
@@ -68,6 +78,7 @@ static int ino_path_cmd(int argc, char **argv)
 	}
 out:
 	free(path);
+	free(curs);
 	close(fd);
 	return ret;
 };

utils/src/ioctl.h

@@ -1,6 +1,8 @@
 #ifndef _SCOUTFS_IOCTL_H_
 #define _SCOUTFS_IOCTL_H_
 
+#include "format.h"
+
 /* XXX I have no idea how these are chosen. */
 #define SCOUTFS_IOCTL_MAGIC 's'
 
@@ -24,18 +26,61 @@ struct scoutfs_ioctl_inodes_since {
 #define SCOUTFS_IOC_INODES_SINCE _IOW(SCOUTFS_IOCTL_MAGIC, 1, \
 				      struct scoutfs_ioctl_inodes_since)
 
-/* returns bytes of path buffer set starting at _off, including null */
+/*
+ * Fill the path buffer with the next path to the target inode.  An
+ * iteration cursor is stored in the cursor buffer which advances
+ * through the paths to the inode at each call.
+ *
+ * @ino: The target ino that we're finding paths to.  Constant across
+ * all the calls that make up an iteration over all the inode's paths.
+ *
+ * @cursor_ptr: A pointer to the buffer that will hold the iteration
+ * cursor.  It must be initialized to 0 before iterating.  Each call
+ * modifies it to skip past the result of that call.
+ *
+ * @cusur_bytes: The length of the cursor buffer.  Must be
+ * SCOUTFS_IOC_INO_PATH_CURSOR_BYTES.
+ *
+ * @path_ptr: The buffer to store each found path.
+ *
+ * @path_bytes: The size of the buffer that will the found path
+ * including null termination.  (PATH_MAX is a solid choice.)
+ *
+ * This only walks back through full hard links.  None of the returned
+ * paths will reflect symlinks to components in the path.
+ *
+ * This doesn't ensure that the caller has permissions to traverse the
+ * returned paths to the inode.  It requires CAP_DAC_READ_SEARCH which
+ * bypasses permissions checking.
+ *
+ * ENAMETOOLONG is returned when the next path found from the cursor
+ * doesn't fit in the path buffer.
+ *
+ * This call is not serialized with any modification (create, rename,
+ * unlink) of the path components.  It will return all the paths that
+ * were stable both before and after the call.  It may or may not return
+ * paths which are created or unlinked during the call.
+ *
+ * The number of bytes in the path, including the null terminator, are
+ * returned when a path is found.  0 is returned when there are no more
+ * paths to the link to the inode from the cursor.
+ */
 struct scoutfs_ioctl_ino_path {
 	__u64 ino;
-	__u64 ctr;		/* init to 0, set to next */
+	__u64 cursor_ptr;
 	__u64 path_ptr;
-	__u16 path_bytes;	/* total buffer space, including null term */
+	__u16 cursor_bytes;
+	__u16 path_bytes;
 } __packed;
 
+#define SCOUTFS_IOC_INO_PATH_CURSOR_BYTES \
+	(sizeof(u64) + SCOUTFS_NAME_LEN + 1)
+
 /* Get a single path from the root to the given inode number */
 #define SCOUTFS_IOC_INO_PATH _IOW(SCOUTFS_IOCTL_MAGIC, 2, \
 				      struct scoutfs_ioctl_ino_path)
 
+
 /* XXX might as well include a seq?  0 for current behaviour? */
 struct scoutfs_ioctl_find_xattr {
 	__u64 first_ino;