Kernel/SysFS: Provide a way to "truncate" and "set" mtime on inodes

Normally, trying to truncate a SysFSInode should result in EPERM error.
However, as suggested by Ali (@alimpfard), we can allow the PowerState
node to be "truncated" so one can open that file with O_TRUNC option.
Likewise, we also need to provide a way to set modified time on SysFS
inodes. For most inodes, we should return ENOTIMPL error, but for the
power state switch, we ignore the modified time setting and just return
KSuccess.

These fixes allow to do "echo -n 1 > /sys/firmware/power_state" in Shell
after gaining root permissions, to switch the power state.

1 files changed, 9 insertions, 0 deletions

diff --git a/Kernel/Firmware/PowerStateSwitch.cpp b/Kernel/Firmware/PowerStateSwitch.cpp
index 16f9e07101..e0f9b53dbe 100644
--- a/Kernel/Firmware/PowerStateSwitch.cpp
+++ b/Kernel/Firmware/PowerStateSwitch.cpp
@@ -30,6 +30,15 @@ UNMAP_AFTER_INIT PowerStateSwitchNode::PowerStateSwitchNode(FirmwareSysFSDirecto
 {
 }
 
+KResult PowerStateSwitchNode::truncate(u64 size)
+{
+    // Note: This node doesn't store any useful data anyway, so we can safely
+    // truncate this to zero (essentially ignoring the request without failing).
+    if (size != 0)
+        return EPERM;
+    return KSuccess;
+}
+
 KResultOr<size_t> PowerStateSwitchNode::write_bytes(off_t offset, size_t count, UserOrKernelBuffer const& data, OpenFileDescription*)
 {
     if (Checked<off_t>::addition_would_overflow(offset, count))