Allow to use potentially dangerous filenames from the root directory

* src/pch.c (cwd_is_root): New function to check if we are in the root
directory of a filename.
(name_is_valid): Allow to use potentially dangerous filenames when the current
working directory is the root directory: from there, those names are not
any more dangerous than other names.
* tests/bad-filenames: New test case.

2 files changed, 38 insertions, 0 deletions

diff --git a/src/pch.c b/src/pch.c
index 551099e..8d5f1c4 100644
--- a/src/pch.c
+++ b/src/pch.c
@@ -389,6 +389,29 @@ skip_hex_digits (char const *str)
   return s == str ? NULL : s;
 }
 
+/* Check if we are in the root of a particular filesystem namespace ("/" on
+   UNIX or a particular drive's root on DOS-like systems).  */
+static bool
+cwd_is_root (char const *name)
+{
+  unsigned int prefix_len = FILE_SYSTEM_PREFIX_LEN (name);
+  char root[prefix_len + 2];
+  struct stat st;
+  dev_t root_dev;
+  ino_t root_ino;
+
+  memcpy (root, name, prefix_len);
+  root[prefix_len] = '/';
+  root[prefix_len + 1] = 0;
+  if (lstat (root, &st))
+    return false;
+  root_dev = st.st_dev;
+  root_ino = st.st_ino;
+  if (lstat (".", &st))
+    return false;
+  return root_dev == st.st_dev && root_ino == st.st_ino;
+}
+
 static bool
 name_is_valid (char const *name)
 {
@@ -420,6 +443,10 @@ name_is_valid (char const *name)
 	  n++;
       }
 
+  /* Allow any filename if we are in the filesystem root.  */
+  if (! is_valid && cwd_is_root (name))
+    is_valid = true;
+
   if (! is_valid)
     {
       say ("Ignoring potentially dangerous file name %s\n", quotearg (name));
diff --git a/tests/bad-filenames b/tests/bad-filenames
index 5b4f205..121cfda 100644
--- a/tests/bad-filenames
+++ b/tests/bad-filenames
@@ -42,6 +42,17 @@ No file to patch.  Skipping patch.
 status: 1
 EOF
 
+for dir in "$TMPDIR" "$TMP" "$TEMP" "/tmp"; do
+    if test -n "$dir"; then
+	filename=$dir/patch-$$-$RANDOM
+	break
+    fi
+done
+
+check 'emit_patch $filename | (cd / && patch -f -p0 --dry-run) || echo status: $?' <<EOF
+patching file $filename
+EOF
+
 check 'emit_patch a/../z | patch -f -p0 --dry-run || echo status: $?' <<EOF
 Ignoring potentially dangerous file name a/../z
 can't find file to patch at input line 3