win32: interpret absolute paths as relative to %SYSTEMDRIVE%

BusyBox contains hardcoded references to absolute paths which
are unique in the *nix world but on Microsoft Windows are
interpreted as being on the current drive.  To make these unique
again consider them to be relative to %SYSTEMDRIVE%.

Support this by adding functions to:

- determine the system drive (not using the environment variable);
- change a process's current directory to the root of the system drive;
- make relative paths absolute before changing directory (if needed).

The following applications have been modified:

- ash references /etc/profile from the system drive;
- dpkg places its data store on and installs files to the system drive;
- rpm installs files to the system drive;
- man looks for configuration files and man pages on the system drive.

See GitHub issue #158.

1 files changed, 56 insertions, 0 deletions

diff --git a/win32/mingw.c b/win32/mingw.c
index 3ee1a2496..1d5644f92 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -1644,3 +1644,59 @@ int root_len(const char *path)
                 return 2;
         return unc_root_len(path);
 }
+
+char *get_system_drive(void)
+{
+        struct passwd *pwd;
+        char *drive = NULL;
+        int len;
+
+        pwd = getpwuid(0);
+        if (pwd != NULL && (len=root_len(pwd->pw_dir))) {
+                drive = xstrdup(pwd->pw_dir);
+                drive[len] = '\0';
+        }
+
+        return drive;
+}
+
+int chdir_system_drive(void)
+{
+        char *sd = get_system_drive();
+        int ret = -1;
+
+        if (sd) {
+                strcat(sd, "/");
+                ret = chdir(sd);
+        }
+        free(sd);
+        return ret;
+}
+
+/*
+ * This function is used to make relative paths absolute before a call
+ * to chdir_system_drive().  It's unlikely to be useful in other cases.
+ *
+ * If the argument is an absolute path or a relative path which resolves
+ * to a path on the system drive return 'path'.  If it's a relative path
+ * which resolves to a path that isn't on the system drive return an
+ * allocated string containing the resolved path.  Die on failure,
+ * which is most likely because the file doesn't exist.
+ */
+char *xabsolute_path(char *path)
+{
+        char *rpath, *sd;
+
+        if (root_len(path) != 0)
+                return path;    // absolute path
+        rpath = xmalloc_realpath(path);
+        if (rpath) {
+                sd = auto_string(get_system_drive());
+                if (sd && is_prefixed_with_case(rpath, sd)) {
+                        free(rpath);
+                        return path;    // resolved path is on system drive
+                }
+                return rpath;
+        }
+        bb_perror_msg_and_die("can't open '%s'", path);
+}