vfs: show unreachable paths in getcwd and proc

Prepend "(unreachable)" to path strings if the path is not reachable
from the current root.

Two places updated are
 - the return string from getcwd()
 - and symlinks under /proc/$PID.

Other uses of d_path() are left unchanged (we know that some old
software crashes if /proc/mounts is changed).

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

4 files changed, 57 insertions, 5 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 7fccb00f498d..166d35d56868 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2019,6 +2019,11 @@ static int path_with_deleted(const struct path *path, struct path *root,
         return prepend_path(path, root, buf, buflen);
 }
 
+static int prepend_unreachable(char **buffer, int *buflen)
+{
+        return prepend(buffer, buflen, "(unreachable)", 13);
+}
+
 /**
  * d_path - return the path of a dentry
  * @path: path to report
@@ -2064,6 +2069,39 @@ char *d_path(const struct path *path, char *buf, int buflen)
 }
 EXPORT_SYMBOL(d_path);
 
+/**
+ * d_path_with_unreachable - return the path of a dentry
+ * @path: path to report
+ * @buf: buffer to return value in
+ * @buflen: buffer length
+ *
+ * The difference from d_path() is that this prepends "(unreachable)"
+ * to paths which are unreachable from the current process' root.
+ */
+char *d_path_with_unreachable(const struct path *path, char *buf, int buflen)
+{
+        char *res = buf + buflen;
+        struct path root;
+        struct path tmp;
+        int error;
+
+        if (path->dentry->d_op && path->dentry->d_op->d_dname)
+                return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
+
+        get_fs_root(current->fs, &root);
+        spin_lock(&dcache_lock);
+        tmp = root;
+        error = path_with_deleted(path, &tmp, &res, &buflen);
+        if (!error && !path_equal(&tmp, &root))
+                error = prepend_unreachable(&res, &buflen);
+        spin_unlock(&dcache_lock);
+        path_put(&root);
+        if (error)
+                res =  ERR_PTR(error);
+
+        return res;
+}
+
 /*
  * Helper function for dentry_operations.d_dname() members
  */
@@ -2173,15 +2211,23 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
         if (!d_unlinked(pwd.dentry)) {
                 unsigned long len;
                 struct path tmp = root;
-                char * cwd;
+                char *cwd = page + PAGE_SIZE;
+                int buflen = PAGE_SIZE;
 
-                cwd = __d_path(&pwd, &tmp, page, PAGE_SIZE);
+                prepend(&cwd, &buflen, "\0", 1);
+                error = prepend_path(&pwd, &tmp, &cwd, &buflen);
                 spin_unlock(&dcache_lock);
 
-                error = PTR_ERR(cwd);
-                if (IS_ERR(cwd))
+                if (error)
                         goto out;
 
+                /* Unreachable from current root */
+                if (!path_equal(&tmp, &root)) {
+                        error = prepend_unreachable(&cwd, &buflen);
+                        if (error)
+                                goto out;
+                }
+
                 error = -ERANGE;
                 len = PAGE_SIZE + page - cwd;
                 if (len <= size) {
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 4cf5abf77ddf..a1c43e7c8a7b 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1526,7 +1526,7 @@ static int do_proc_readlink(struct path *path, char __user *buffer, int buflen)
         if (!tmp)
                 return -ENOMEM;
 
-        pathname = d_path(path, tmp, PAGE_SIZE);
+        pathname = d_path_with_unreachable(path, tmp, PAGE_SIZE);
         len = PTR_ERR(pathname);
         if (IS_ERR(pathname))
                 goto out;
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index d23be0386e2d..6a4aea30aa09 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -315,6 +315,7 @@ extern char *dynamic_dname(struct dentry *, char *, int, const char *, ...);
 
 extern char *__d_path(const struct path *path, struct path *root, char *, int);
 extern char *d_path(const struct path *, char *, int);
+extern char *d_path_with_unreachable(const struct path *, char *, int);
 extern char *__dentry_path(struct dentry *, char *, int);
 extern char *dentry_path(struct dentry *, char *, int);
 
diff --git a/include/linux/path.h b/include/linux/path.h
index 915e0c382a51..edc98dec6266 100644
--- a/include/linux/path.h
+++ b/include/linux/path.h
@@ -12,4 +12,9 @@ struct path {
 extern void path_get(struct path *);
 extern void path_put(struct path *);
 
+static inline int path_equal(const struct path *path1, const struct path *path2)
+{
+        return path1->mnt == path2->mnt && path1->dentry == path2->dentry;
+}
+
 #endif  /* _LINUX_PATH_H */