lib/vfscore: Fix create through broken symlink

Previously a vfs syscall on a broken symlink would fail with ENOENT
instead of attempting the syscall on the symlink target. This would make
open(O_CREAT) to fail instead of creating the missing target file.
This change fixes this behavior, bringing it in line with expectations.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>

lib/vfscore/lookup.c

@@ -96,13 +96,14 @@ namei_follow_link(struct dentry *dp, char *node, char *name, char *fp, size_t mo
 	return (0);
 }
 /*
- * Convert a pathname into a pointer to a dentry
+ * Resolve a pathname into a pointer to a dentry and a realpath.
  *
  * @path: full path name.
  * @dpp:  dentry to be returned.
+ * @realpath: if not NULL, return path after resolving all symlinks
  */
 int
-namei(const char *path, struct dentry **dpp)
+namei_resolve(const char *path, struct dentry **dpp, char *realpath)
 {
 	char *p;
 	char node[PATH_MAX];
@@ -122,14 +123,16 @@ namei(const char *path, struct dentry **dpp)
 	memset(fp, 0, PATH_MAX);
 	strlcpy(fp, path, PATH_MAX);
 
+	error = 0;
 	do {
 		need_continue = 0;
 		/*
 		 * Convert a full path name to its mount point and
 		 * the local node in the file system.
 		 */
 		if (vfs_findroot(fp, &mp, &p)) {
-			return ENOTDIR;
+			error = ENOTDIR;
+			goto out;
 		}
 
 		size_t mountpoint_len = p - fp;
@@ -140,7 +143,7 @@ namei(const char *path, struct dentry **dpp)
 		if (dp) {
 			/* vnode is already active. */
 			*dpp = dp;
-			return 0;
+			goto out;
 		}
 		/*
 		 * Find target vnode, started from root directory.
@@ -189,7 +192,7 @@ namei(const char *path, struct dentry **dpp)
 				if (error) {
 					vn_unlock(dvp);
 					drele(ddp);
-					return error;
+					goto out;
 				}
 
 				dp = dentry_alloc(ddp, vp, node);
@@ -198,7 +201,8 @@ namei(const char *path, struct dentry **dpp)
 				if (!dp) {
 					vn_unlock(dvp);
 					drele(ddp);
-					return ENOMEM;
+					error = ENOMEM;
+					goto out;
 				}
 			}
 			vn_unlock(dvp);
@@ -209,7 +213,7 @@ namei(const char *path, struct dentry **dpp)
 				error = namei_follow_link(dp, node, name, fp, mountpoint_len);
 				if (error) {
 					drele(dp);
-					return (error);
+					goto out;
 				}
 
 				drele(dp);
@@ -223,21 +227,37 @@ namei(const char *path, struct dentry **dpp)
 				node[0] = 0;
 
 				if (++links_followed >= MAXSYMLINKS) {
-					return (ELOOP);
+					error = ELOOP;
+					goto out;
 				}
 				need_continue = 1;
 				break;
 			}
 
 			if (*p == '/' && ddp->d_vnode->v_type != VDIR) {
 				drele(ddp);
-				return ENOTDIR;
+				error = ENOTDIR;
+				goto out;
 			}
 		}
 	} while (need_continue);
 
 	*dpp = dp;
-	return 0;
+out:
+	if (realpath)
+		strlcpy(realpath, fp, PATH_MAX);
+	return error;
+}
+/*
+ * Convert a pathname into a pointer to a dentry
+ *
+ * @path: full path name.
+ * @dpp:  dentry to be returned.
+ */
+int
+namei(const char *path, struct dentry **dpp)
+{
+	return namei_resolve(path, dpp, NULL);
 }
 
 /*

lib/vfscore/syscalls.c

@@ -114,17 +114,18 @@ sys_open(char *path, int flags, mode_t mode, struct vfscore_file **fpp)
 	struct dentry *dp, *ddp;
 	struct vnode *vp;
 	char *filename;
+	char realpath[PATH_MAX];
 	int error;
 
 	DPRINTF(VFSDB_SYSCALL, ("sys_open: path=%s flags=%x mode=%x\n",
 				path, flags, mode));
 
 	flags = vfscore_fflags(flags);
 	if (flags & O_CREAT) {
-		error = namei(path, &dp);
+		error = namei_resolve(path, &dp, realpath);
 		if (error == ENOENT) {
 			/* Create new struct vfscore_file. */
-			if ((error = lookup(path, &ddp, &filename)) != 0)
+			if ((error = lookup(realpath, &ddp, &filename)) != 0)
 				return error;
 
 			vn_lock(ddp->d_vnode);

lib/vfscore/vfs.h

@@ -684,6 +684,21 @@ int sec_vnode_permission(char *path);
  */
 int namei(const char *path, struct dentry **dpp);
 
+/**
+ * Resolve a pathname into a pointer to a dentry and a realpath.
+ *
+ * @param path
+ *	The full path name
+ * @param[out] dpp
+ *	Dentry to be returned
+ * @param[out] realpath
+ *	if not NULL, return path after resolving all symlinks
+ * @return
+ *	- (0):  Completed successfully
+ *	- (<0): Negative value with error code
+ */
+int namei_resolve(const char *path, struct dentry **dpp, char *realpath);
+
 /**
  * Converts the last component in the path to a pointer to a dentry.
  *