Add an installd command to link files

Given a pair of absolute paths {from_path, to_path} , check that
they are both valid apk subpaths (eg. /data/app/package/foo) and
link(2) to_path to from_path. This is required by staged installs,
where we link existing apks and oat files to their staging location.

bug: 20889739
Change-Id: I3b5e3b43677af68be59308121a4409caaa6a72f0

cmds/installd/commands.cpp

@@ -1778,6 +1778,31 @@ int rm_package_dir(const char* apk_path)
     return delete_dir_contents(apk_path, 1 /* also_delete_dir */ , NULL /* exclusion_predicate */);
 }
 
+int link_file(const char* relative_path, const char* from_base, const char* to_base) {
+    char from_path[PKG_PATH_MAX];
+    char to_path[PKG_PATH_MAX];
+    snprintf(from_path, PKG_PATH_MAX, "%s/%s", from_base, relative_path);
+    snprintf(to_path, PKG_PATH_MAX, "%s/%s", to_base, relative_path);
+
+    if (validate_apk_path_subdirs(from_path)) {
+        ALOGE("invalid app data sub-path '%s' (bad prefix)\n", from_path);
+        return -1;
+    }
+
+    if (validate_apk_path_subdirs(to_path)) {
+        ALOGE("invalid app data sub-path '%s' (bad prefix)\n", to_path);
+        return -1;
+    }
+
+    const int ret = link(from_path, to_path);
+    if (ret < 0) {
+        ALOGE("link(%s, %s) failed : %s", from_path, to_path, strerror(errno));
+        return -1;
+    }
+
+    return 0;
+}
+
 int calculate_oat_file_path(char path[PKG_PATH_MAX], const char *oat_dir, const char *apk_path,
         const char *instruction_set) {
     char *file_name_start;

cmds/installd/installd.cpp

@@ -179,6 +179,12 @@ static int do_rm_package_dir(char **arg, char reply[REPLY_MAX] __unused)
     return rm_package_dir(arg[0]);
 }
 
+static int do_link_file(char **arg, char reply[REPLY_MAX] __unused)
+{
+    /* relative_path, from_base, to_base */
+    return link_file(arg[0], arg[1], arg[2]);
+}
+
 struct cmdinfo {
     const char *name;
     unsigned numargs;
@@ -209,7 +215,8 @@ struct cmdinfo cmds[] = {
     { "idmap",                3, do_idmap },
     { "restorecondata",       4, do_restorecon_data },
     { "createoatdir",         2, do_create_oat_dir },
-    { "rmpackagedir",         1, do_rm_package_dir},
+    { "rmpackagedir",         1, do_rm_package_dir },
+    { "linkfile",             3, do_link_file }
 };
 
 static int readx(int s, void *_buf, int count)

cmds/installd/installd.h

@@ -203,6 +203,7 @@ int get_path_from_string(dir_rec_t* rec, const char* path);
 int copy_and_append(dir_rec_t* dst, const dir_rec_t* src, const char* suffix);
 
 int validate_apk_path(const char *path);
+int validate_apk_path_subdirs(const char *path);
 
 int append_and_increment(char** dst, const char* src, size_t* dst_size);
 
@@ -254,3 +255,4 @@ int calculate_oat_file_path(char path[PKG_PATH_MAX], const char *oat_dir, const
                             const char *instruction_set);
 int move_package_dir(char path[PKG_PATH_MAX], const char *oat_dir, const char *apk_path,
                             const char *instruction_set);
+int link_file(const char *relative_path, const char *from_base, const char *to_base);

cmds/installd/utils.cpp

@@ -1043,15 +1043,13 @@ int copy_and_append(dir_rec_t* dst, const dir_rec_t* src, const char* suffix) {
 }
 
 /**
- * Check whether path points to a valid path for an APK file. Only one level of
- * subdirectory names is allowed. Returns -1 when an invalid path is encountered
- * and 0 when a valid path is encountered.
+ * Check whether path points to a valid path for an APK file. The path must
+ * begin with a whitelisted prefix path and must be no deeper than |maxSubdirs| within
+ * that path. Returns -1 when an invalid path is encountered and 0 when a valid path
+ * is encountered.
  */
-int validate_apk_path(const char *path)
-{
+static int validate_apk_path_internal(const char *path, int maxSubdirs) {
     const dir_rec_t* dir = NULL;
-    int maxSubdirs = 1;
-
     if (!strncmp(path, android_app_dir.path, android_app_dir.len)) {
         dir = &android_app_dir;
     } else if (!strncmp(path, android_app_private_dir.path, android_app_private_dir.len)) {
@@ -1060,7 +1058,9 @@ int validate_apk_path(const char *path)
         dir = &android_asec_dir;
     } else if (!strncmp(path, android_mnt_expand_dir.path, android_mnt_expand_dir.len)) {
         dir = &android_mnt_expand_dir;
-        maxSubdirs = 2;
+        if (maxSubdirs < 2) {
+            maxSubdirs = 2;
+        }
     } else {
         return -1;
     }
@@ -1068,6 +1068,14 @@ int validate_apk_path(const char *path)
     return validate_path(dir, path, maxSubdirs);
 }
 
+int validate_apk_path(const char* path) {
+    return validate_apk_path_internal(path, 1 /* maxSubdirs */);
+}
+
+int validate_apk_path_subdirs(const char* path) {
+    return validate_apk_path_internal(path, 3 /* maxSubdirs */);
+}
+
 int append_and_increment(char** dst, const char* src, size_t* dst_size) {
     ssize_t ret = strlcpy(*dst, src, *dst_size);
     if (ret < 0 || (size_t) ret >= *dst_size) {