uefi: Implement getcwd and chdir

- Using EFI Shell Protocol. These functions do not make much sense
  unless a shell is present.
- Return the exe dir in case shell protocol is missing.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

std/src/sys/pal/uefi/helpers.rs

@@ -177,16 +177,8 @@ pub(crate) fn device_path_to_text(path: NonNull<device_path::Protocol>) -> io::R
             )
         };
 
-        // SAFETY: `convert_device_path_to_text` returns a pointer to a null-terminated UTF-16
-        // string, and that string cannot be deallocated prior to dropping the `WStrUnits`, so
-        // it's safe for `WStrUnits` to use.
-        let path_len = unsafe {
-            WStrUnits::new(path_ptr)
-                .ok_or(io::const_io_error!(io::ErrorKind::InvalidData, "Invalid path"))?
-                .count()
-        };
-
-        let path = OsString::from_wide(unsafe { slice::from_raw_parts(path_ptr.cast(), path_len) });
+        let path = os_string_from_raw(path_ptr)
+            .ok_or(io::const_io_error!(io::ErrorKind::InvalidData, "Invalid path"))?;
 
         if let Some(boot_services) = crate::os::uefi::env::boot_services() {
             let boot_services: NonNull<r_efi::efi::BootServices> = boot_services.cast();
@@ -420,3 +412,15 @@ impl<T> Drop for OwnedTable<T> {
         unsafe { crate::alloc::dealloc(self.ptr as *mut u8, self.layout) };
     }
 }
+
+/// Create OsString from a pointer to NULL terminated UTF-16 string
+pub(crate) fn os_string_from_raw(ptr: *mut r_efi::efi::Char16) -> Option<OsString> {
+    let path_len = unsafe { WStrUnits::new(ptr)?.count() };
+    Some(OsString::from_wide(unsafe { slice::from_raw_parts(ptr.cast(), path_len) }))
+}
+
+/// Create NULL terminated UTF-16 string
+pub(crate) fn os_string_to_raw(s: &OsStr) -> Option<Box<[r_efi::efi::Char16]>> {
+    let temp = s.encode_wide().chain(Some(0)).collect::<Box<[r_efi::efi::Char16]>>();
+    if temp[..temp.len() - 1].contains(&0) { None } else { Some(temp) }
+}

std/src/sys/pal/uefi/os.rs

@@ -1,7 +1,7 @@
 use r_efi::efi::Status;
 use r_efi::efi::protocols::{device_path, loaded_image_device_path};
 
-use super::{RawOsError, helpers, unsupported};
+use super::{RawOsError, helpers, unsupported_err};
 use crate::error::Error as StdError;
 use crate::ffi::{OsStr, OsString};
 use crate::marker::PhantomData;
@@ -125,11 +125,32 @@ pub fn error_string(errno: RawOsError) -> String {
 }
 
 pub fn getcwd() -> io::Result<PathBuf> {
-    unsupported()
+    match uefi_shell::open_shell() {
+        Some(shell) => {
+            // SAFETY: path_ptr is managed by UEFI shell and should not be deallocated
+            let path_ptr = unsafe { ((*shell.as_ptr()).get_cur_dir)(crate::ptr::null_mut()) };
+            helpers::os_string_from_raw(path_ptr)
+                .map(PathBuf::from)
+                .ok_or(io::const_io_error!(io::ErrorKind::InvalidData, "Invalid path"))
+        }
+        None => {
+            let mut t = current_exe()?;
+            // SAFETY: This should never fail since the disk prefix will be present even for root
+            // executables
+            assert!(t.pop());
+            Ok(t)
+        }
+    }
 }
 
-pub fn chdir(_: &path::Path) -> io::Result<()> {
-    unsupported()
+pub fn chdir(p: &path::Path) -> io::Result<()> {
+    let shell = uefi_shell::open_shell().ok_or(unsupported_err())?;
+
+    let mut p = helpers::os_string_to_raw(p.as_os_str())
+        .ok_or(io::const_io_error!(io::ErrorKind::InvalidData, "Invalid path"))?;
+
+    let r = unsafe { ((*shell.as_ptr()).set_cur_dir)(crate::ptr::null_mut(), p.as_mut_ptr()) };
+    if r.is_error() { Err(io::Error::from_raw_os_error(r.as_usize())) } else { Ok(()) }
 }
 
 pub struct SplitPaths<'a>(!, PhantomData<&'a ()>);
@@ -239,3 +260,37 @@ pub fn exit(code: i32) -> ! {
 pub fn getpid() -> u32 {
     panic!("no pids on this platform")
 }
+
+mod uefi_shell {
+    use r_efi::protocols::shell;
+
+    use super::super::helpers;
+    use crate::ptr::NonNull;
+    use crate::sync::atomic::{AtomicPtr, Ordering};
+
+    pub fn open_shell() -> Option<NonNull<shell::Protocol>> {
+        static LAST_VALID_HANDLE: AtomicPtr<crate::ffi::c_void> =
+            AtomicPtr::new(crate::ptr::null_mut());
+
+        if let Some(handle) = NonNull::new(LAST_VALID_HANDLE.load(Ordering::Acquire)) {
+            if let Ok(protocol) = helpers::open_protocol::<shell::Protocol>(
+                handle,
+                r_efi::protocols::shell::PROTOCOL_GUID,
+            ) {
+                return Some(protocol);
+            }
+        }
+
+        let handles = helpers::locate_handles(shell::PROTOCOL_GUID).ok()?;
+        for handle in handles {
+            if let Ok(protocol) =
+                helpers::open_protocol::<shell::Protocol>(handle, shell::PROTOCOL_GUID)
+            {
+                LAST_VALID_HANDLE.store(handle.as_ptr(), Ordering::Release);
+                return Some(protocol);
+            }
+        }
+
+        None
+    }
+}