diff --git a/src/i_o.lua b/src/i_o.lua
index 07bb153..1456793 100644
--- a/src/i_o.lua
+++ b/src/i_o.lua
@@ -88,10 +88,19 @@ M.file_exists = function(path)
    return M.exit_code_cmd('stat '..path..' > /dev/null 2>&1') == 0
 end
 
+M.file_readable = function(path)
+   return M.exit_code_cmd('test -r '..path) == 0
+end
+
 M.assert_file_exists = function(path)
    M.assertf(M.file_exists(path), '%s does not exist', path)
 end
 
+M.assert_file_readable = function(path)
+   M.assertf(M.file_exists(path), '%s does not exist', path)
+   M.assertf(M.file_readable(path), '%s is not readable', path)
+end
+
 --------------------------------------------------------------------------------
 -- conky object execution
 
diff --git a/src/modules/filesystem.lua b/src/modules/filesystem.lua
index 4d18d56..5a27ffe 100644
--- a/src/modules/filesystem.lua
+++ b/src/modules/filesystem.lua
@@ -35,6 +35,8 @@ return function(config, main_state, common, width, point)
    local mk_bars = function(y)
       local paths = pure.map_keys('path', config.fs_paths)
       local names = pure.map_keys('name', config.fs_paths)
+      -- TODO this might not be enough (conky might actually need +x permissions
+      -- to decend into the dir and read its contents)
       impure.each(i_o.assert_file_exists, paths)
       local CONKY_CMDS = pure.map(
          pure.partial(string.format, '${fs_used_perc %s}', true),
diff --git a/src/modules/power.lua b/src/modules/power.lua
index d2e2e9e..c646454 100644
--- a/src/modules/power.lua
+++ b/src/modules/power.lua
@@ -110,7 +110,6 @@ return function(update_freq, config, common, width, point)
       point = point,
       width = width,
       set_state = nil,
-      -- TODO make sure these interfaces actually exist before trying to read them
       top = pure.concat(
          pure.map(mk_rate_blockspec, config.rapl_specs),
          {{mk_bat, config.battery ~= '', 0}}
diff --git a/src/sys.lua b/src/sys.lua
index 0f0cbce..4095960 100644
--- a/src/sys.lua
+++ b/src/sys.lua
@@ -84,7 +84,7 @@ local SYSFS_RAPL = '/sys/class/powercap'
 
 M.intel_powercap_reader = function(dev)
    local uj = __string_format('%s/%s/energy_uj', SYSFS_RAPL, dev)
-   i_o.assert_file_exists(uj)
+   i_o.assert_file_readable(uj)
    return function()
       return read_micro(uj)
    end
@@ -97,7 +97,7 @@ local SYSFS_POWER = '/sys/class/power_supply'
 
 local format_power_path = function(battery, property)
    local p = __string_format('%s/%s/%s', SYSFS_POWER, battery, property)
-   i_o.assert_file_exists(p)
+   i_o.assert_file_readable(p)
    return p
 end