[3.12] gh-106242: Fix path truncation in os.path.normpath (GH-106816) (#107981)

* gh-106242: Fix path truncation in os.path.normpath (GH-106816)
* gh-106242: Minor fixup to avoid compiler warnings

---------

Co-authored-by: Finn Womack <flan313@gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

1 files changed, 21 insertions, 8 deletions

diff --git a/Python/fileutils.c b/Python/fileutils.c
index f137ee9..268ffa3 100644
--- a/Python/fileutils.c
+++ b/Python/fileutils.c
@@ -2377,12 +2377,14 @@ _Py_find_basename(const wchar_t *filename)
    path, which will be within the original buffer. Guaranteed to not
    make the path longer, and will not fail. 'size' is the length of
    the path, if known. If -1, the first null character will be assumed
-   to be the end of the path. */
+   to be the end of the path. 'normsize' will be set to contain the
+   length of the resulting normalized path. */
 wchar_t *
-_Py_normpath(wchar_t *path, Py_ssize_t size)
+_Py_normpath_and_size(wchar_t *path, Py_ssize_t size, Py_ssize_t *normsize)
 {
     assert(path != NULL);
-    if (!path[0] || size == 0) {
+    if ((size < 0 && !path[0]) || size == 0) {
+        *normsize = 0;
         return path;
     }
     wchar_t *pEnd = size >= 0 ? &path[size] : NULL;
@@ -2431,11 +2433,7 @@ _Py_normpath(wchar_t *path, Py_ssize_t size)
                 *p2++ = lastC = *p1;
             }
         }
-        if (sepCount) {
-            minP2 = p2;      // Invalid path
-        } else {
-            minP2 = p2 - 1;  // Absolute path has SEP at minP2
-        }
+        minP2 = p2 - 1;
     }
 #else
     // Skip past two leading SEPs
@@ -2495,13 +2493,28 @@ _Py_normpath(wchar_t *path, Py_ssize_t size)
         while (--p2 != minP2 && *p2 == SEP) {
             *p2 = L'\0';
         }
+    } else {
+        --p2;
     }
+    *normsize = p2 - path + 1;
 #undef SEP_OR_END
 #undef IS_SEP
 #undef IS_END
     return path;
 }
 
+/* In-place path normalisation. Returns the start of the normalized
+   path, which will be within the original buffer. Guaranteed to not
+   make the path longer, and will not fail. 'size' is the length of
+   the path, if known. If -1, the first null character will be assumed
+   to be the end of the path. */
+wchar_t *
+_Py_normpath(wchar_t *path, Py_ssize_t size)
+{
+    Py_ssize_t norm_length;
+    return _Py_normpath_and_size(path, size, &norm_length);
+}
+
 
 /* Get the current directory. buflen is the buffer size in wide characters
    including the null character. Decode the path from the locale encoding.