Fix readlink() for non-PostgreSQL junction points on Windows.

macdice · adunstan · commit 2c4a532c96a9 · 2025-01-18T09:33:49.000-05:00
Since commit c5cb8f3 taught stat() to follow symlinks, and since initdb uses pg_mkdir_p(), and that examines parent directories, our humble readlink() implementation can now be exposed to junction points not of PostgreSQL origin. Those might be corrupted by our naive path mangling, which doesn't really understand NT paths in general. Simply decline to transform paths that don't look like a drive absolute path. That means that readlink() returns the NT path directly when checking a parent directory of PGDATA that happen to point to a drive using "rooted" format. That works for the purposes of our stat() emulation. Reported-by: Roman Zharkov <r.zharkov@postgrespro.ru> Reviewed-by: Roman Zharkov <r.zharkov@postgrespro.ru> Discussion: https://postgr.es/m/4590c37927d7b8ee84f9855d83229018%40postgrespro.ru Discussion: https://postgr.es/m/CA%2BhUKG%2BajSQ_8eu2AogTncOnZ5me2D-Cn66iN_-wZnRjLN%2Bicg%40mail.gmail.com Backpatched commit f71007f as above by Thomas Munro into releases 13 thru 15 Discussion: https://postgr.es/m/CA+hUKGLbnv+pe3q1fYOVkLD3pMra7GuihfMxUN-1831YH9RYQg@mail.gmail.com
diff --git a/src/port/dirmod.c b/src/port/dirmod.c
@@ -351,10 +351,21 @@ pgreadlink(const char *path, char *buf, size_t size)
 	}
 
 	/*
-	 * If the path starts with "\??\", which it will do in most (all?) cases,
-	 * strip those out.
+	 * If the path starts with "\??\" followed by a "drive absolute" path
+	 * (known to Windows APIs as RtlPathTypeDriveAbsolute), then strip that
+	 * prefix.  This undoes some of the transformation performed by
+	 * pqsymlink(), to get back to a format that users are used to seeing.  We
+	 * don't know how to transform other path types that might be encountered
+	 * outside PGDATA, so we just return them directly.
 	 */
-	if (r > 4 && strncmp(buf, "\\??\\", 4) == 0)
+	if (r >= 7 &&
+		buf[0] == '\\' &&
+		buf[1] == '?' &&
+		buf[2] == '?' &&
+		buf[3] == '\\' &&
+		isalpha(buf[4]) &&
+		buf[5] == ':' &&
+		buf[6] == '\\')
 	{
 		memmove(buf, buf + 4, strlen(buf + 4) + 1);
 		r -= 4;
