realpath: implement realpath(3) and enable realpath(1)realpath

The implementation of realpath(3) is based on code by Stuart Dootson
(studoot on GitHub).

3 files changed, 58 insertions, 5 deletions

diff --git a/configs/mingw32_defconfig b/configs/mingw32_defconfig
index b0bdb25cb..249ae2072 100644
--- a/configs/mingw32_defconfig
+++ b/configs/mingw32_defconfig
@@ -288,7 +288,7 @@ CONFIG_PRINTF=y
 CONFIG_PWD=y
 # CONFIG_READLINK is not set
 # CONFIG_FEATURE_READLINK_FOLLOW is not set
-# CONFIG_REALPATH is not set
+CONFIG_REALPATH=y
 CONFIG_RM=y
 CONFIG_RMDIR=y
 CONFIG_SEQ=y
diff --git a/configs/mingw64_defconfig b/configs/mingw64_defconfig
index 8cb8431d0..147e1e266 100644
--- a/configs/mingw64_defconfig
+++ b/configs/mingw64_defconfig
@@ -288,7 +288,7 @@ CONFIG_PRINTF=y
 CONFIG_PWD=y
 # CONFIG_READLINK is not set
 # CONFIG_FEATURE_READLINK_FOLLOW is not set
-# CONFIG_REALPATH is not set
+CONFIG_REALPATH=y
 CONFIG_RM=y
 CONFIG_RMDIR=y
 CONFIG_SEQ=y
diff --git a/win32/mingw.c b/win32/mingw.c
index 8141e45a5..b75565fc2 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -1,5 +1,6 @@
 #include "libbb.h"
 #include <userenv.h>
+#include "lazyload.h"
 
 #if defined(__MINGW64_VERSION_MAJOR)
 #if ENABLE_GLOBBING
@@ -886,10 +887,63 @@ int link(const char *oldpath, const char *newpath)
         return 0;
 }
 
+static char *resolve_symlinks(char *path)
+{
+        HANDLE h = INVALID_HANDLE_VALUE;
+        DECLARE_PROC_ADDR(kernel32.dll, DWORD, GetFinalPathNameByHandleA, HANDLE,
+                                                LPSTR, DWORD, DWORD);
+
+        if (!INIT_PROC_ADDR(GetFinalPathNameByHandleA)) {
+                errno = ENOSYS;
+                return NULL;
+        }
+
+        /* need a file handle to resolve symlinks */
+        h = CreateFileA(path, GENERIC_READ, FILE_SHARE_READ, NULL,
+                        OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL|FILE_FLAG_BACKUP_SEMANTICS,
+                        NULL);
+        if (h != INVALID_HANDLE_VALUE) {
+                /* normalize the path and return it on success */
+                DWORD status = GetFinalPathNameByHandleA(h, path, MAX_PATH,
+                                                        FILE_NAME_NORMALIZED|VOLUME_NAME_DOS);
+                CloseHandle(h);
+                if (status != 0 && status < MAX_PATH) {
+                        /* skip '\\?\' prefix */
+                        return (!strncmp(path, "\\\\?\\", 4)) ? (path + 4) : path;
+                }
+        }
+
+        errno = err_win_to_posix(GetLastError());
+        return NULL;
+}
+
+/*
+ * Emulate realpath in two stages:
+ *
+ * - _fullpath handles './', '../' and extra '/' characters.  The
+ *   resulting path may not refer to an actual file.
+ *
+ * - resolve_symlinks checks that the file exists (by opening it) and
+ *   resolves symlinks by calling GetFinalPathNameByHandleA.
+ */
 char *realpath(const char *path, char *resolved_path)
 {
-        /* FIXME: need normalization */
-        return strcpy(resolved_path, path);
+        char buffer[MAX_PATH];
+        char *real_path;
+
+        /* enforce glibc pre-2.3 behaviour */
+        if (path == NULL || resolved_path == NULL) {
+                errno = EINVAL;
+                return NULL;
+        }
+
+        if (_fullpath(buffer, path, MAX_PATH) &&
+                        (real_path=resolve_symlinks(buffer))) {
+                strcpy(resolved_path, real_path);
+                convert_slashes(resolved_path);
+                return resolved_path;
+        }
+        return NULL;
 }
 
 const char *get_busybox_exec_path(void)
@@ -1208,7 +1262,6 @@ off_t mingw_lseek(int fd, off_t offset, int whence)
 
 #if ENABLE_FEATURE_PS_TIME || ENABLE_FEATURE_PS_LONG
 #undef GetTickCount64
-#include "lazyload.h"
 
 ULONGLONG CompatGetTickCount64(void)
 {