uefi: fs: Implement exists

Also adds the initial file abstractions.

The file opening algorithm is inspired from UEFI shell. It starts by
classifying if the Path is Shell mapping, text representation of device
path protocol, or a relative path and converts into an absolute text
representation of device path protocol.

After that, it queries all handles supporting
EFI_SIMPLE_FILE_SYSTEM_PROTOCOL and opens the volume that matches the
device path protocol prefix (similar to Windows drive). After that, it
opens the file in the volume using the remaining pat.

It also introduces OwnedDevicePath and BorrowedDevicePath abstractions
to allow working with the base UEFI and Shell device paths efficiently.

DevicePath in UEFI behaves like an a group of nodes laied out in the
memory contiguously and thus can be modeled using iterators.

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

Author: Ayush1325

library/std/src/sys/pal/uefi/fs.rs

@@ -311,8 +311,13 @@ pub fn remove_dir_all(_path: &Path) -> io::Result<()> {
     unsupported()
 }
 
-pub fn exists(_path: &Path) -> io::Result<bool> {
-    unsupported()
+pub fn exists(path: &Path) -> io::Result<bool> {
+    let f = uefi_fs::File::from_path(path, r_efi::protocols::file::MODE_READ, 0);
+    match f {
+        Ok(_) => Ok(true),
+        Err(e) if e.kind() == io::ErrorKind::NotFound => Ok(false),
+        Err(e) => Err(e),
+    }
 }
 
 pub fn readlink(_p: &Path) -> io::Result<PathBuf> {
@@ -342,3 +347,185 @@ pub fn canonicalize(_p: &Path) -> io::Result<PathBuf> {
 pub fn copy(_from: &Path, _to: &Path) -> io::Result<u64> {
     unsupported()
 }
+
+mod uefi_fs {
+    use r_efi::protocols::{device_path, file, simple_file_system};
+
+    use super::super::helpers;
+    use crate::boxed::Box;
+    use crate::io;
+    use crate::mem::MaybeUninit;
+    use crate::path::{Path, PathBuf};
+    use crate::ptr::NonNull;
+    use crate::sys::unsupported_err;
+
+    const COLON: u8 = b':';
+
+    pub(crate) struct File(NonNull<file::Protocol>);
+
+    impl File {
+        pub(crate) fn from_path(path: &Path, open_mode: u64, attr: u64) -> io::Result<Self> {
+            let absoulte = absolute_path(path)?;
+
+            let p = helpers::OwnedDevicePath::from_text(absoulte.as_os_str())?;
+            let (vol, mut path_remaining) = Self::open_volume_from_device_path(p.borrow())?;
+
+            vol.open(&mut path_remaining, open_mode, attr)
+        }
+
+        fn open_volume_from_device_path(
+            path: helpers::BorrowedDevicePath<'_>,
+        ) -> io::Result<(Self, Box<[u16]>)> {
+            let handles = match helpers::locate_handles(simple_file_system::PROTOCOL_GUID) {
+                Ok(x) => x,
+                Err(e) => return Err(e),
+            };
+            for handle in handles {
+                let volume_device_path: NonNull<device_path::Protocol> =
+                    match helpers::open_protocol(handle, device_path::PROTOCOL_GUID) {
+                        Ok(x) => x,
+                        Err(_) => continue,
+                    };
+                let volume_device_path = helpers::BorrowedDevicePath::new(volume_device_path);
+
+                if let Some(left_path) = path_best_match(&volume_device_path, &path) {
+                    return Ok((Self::open_volume(handle)?, left_path));
+                }
+            }
+
+            Err(io::const_error!(io::ErrorKind::NotFound, "Volume Not Found"))
+        }
+
+        // Open volume on device_handle using SIMPLE_FILE_SYSTEM_PROTOCOL
+        fn open_volume(device_handle: NonNull<crate::ffi::c_void>) -> io::Result<Self> {
+            let simple_file_system_protocol = helpers::open_protocol::<simple_file_system::Protocol>(
+                device_handle,
+                simple_file_system::PROTOCOL_GUID,
+            )?;
+
+            let mut file_protocol: MaybeUninit<*mut file::Protocol> = MaybeUninit::uninit();
+            let r = unsafe {
+                ((*simple_file_system_protocol.as_ptr()).open_volume)(
+                    simple_file_system_protocol.as_ptr(),
+                    file_protocol.as_mut_ptr(),
+                )
+            };
+            if r.is_error() {
+                return Err(io::Error::from_raw_os_error(r.as_usize()));
+            }
+
+            // Since no error was returned, file protocol should be non-NULL.
+            let p = NonNull::new(unsafe { file_protocol.assume_init() }).unwrap();
+            Ok(Self(p))
+        }
+
+        fn open(&self, path: &mut [u16], open_mode: u64, attr: u64) -> io::Result<Self> {
+            let file_ptr = self.0.as_ptr();
+            let mut file_opened: MaybeUninit<*mut file::Protocol> = MaybeUninit::uninit();
+
+            let r = unsafe {
+                ((*file_ptr).open)(
+                    file_ptr,
+                    file_opened.as_mut_ptr(),
+                    path.as_mut_ptr(),
+                    open_mode,
+                    attr,
+                )
+            };
+
+            if r.is_error() {
+                return Err(io::Error::from_raw_os_error(r.as_usize()));
+            }
+
+            // Since no error was returned, file protocol should be non-NULL.
+            let p = NonNull::new(unsafe { file_opened.assume_init() }).unwrap();
+            Ok(File(p))
+        }
+    }
+
+    impl Drop for File {
+        fn drop(&mut self) {
+            let file_ptr = self.0.as_ptr();
+            let _ = unsafe { ((*self.0.as_ptr()).close)(file_ptr) };
+        }
+    }
+
+    fn path_best_match<'a>(
+        source: &helpers::BorrowedDevicePath<'a>,
+        target: &helpers::BorrowedDevicePath<'a>,
+    ) -> Option<Box<[u16]>> {
+        let mut source_iter = source.iter().take_while(|x| !x.is_end_instance());
+        let mut target_iter = target.iter().take_while(|x| !x.is_end_instance());
+
+        loop {
+            match (source_iter.next(), target_iter.next()) {
+                (Some(x), Some(y)) if x == y => continue,
+                (None, Some(y)) => {
+                    return y.to_path().to_text_raw().ok();
+                }
+                _ => return None,
+            }
+        }
+    }
+
+    /// Get device path protocol associated with shell mapping.
+    ///
+    /// returns None in case no such mapping is exists
+    fn get_device_path_from_map(map: &Path) -> io::Result<helpers::BorrowedDevicePath<'static>> {
+        let shell = helpers::open_shell()
+            .ok_or(io::const_error!(io::ErrorKind::NotFound, "UEFI Shell not found"))?;
+        let mut path = helpers::os_string_to_raw(map.as_os_str()).ok_or(io::const_error!(
+            io::ErrorKind::InvalidFilename,
+            "Invalid UEFI shell mapping"
+        ))?;
+
+        let protocol = unsafe { ((*shell.as_ptr()).get_device_path_from_map)(path.as_mut_ptr()) };
+        let protocol = NonNull::new(protocol)
+            .ok_or(io::const_error!(io::ErrorKind::NotFound, "UEFI Shell mapping not found"))?;
+
+        Ok(helpers::BorrowedDevicePath::new(protocol))
+    }
+
+    fn absolute_path(path: &Path) -> io::Result<PathBuf> {
+        const FORWARD_SLASH: u8 = b'/';
+
+        // Absoulte Shell Path
+        if path.as_os_str().as_encoded_bytes().contains(&COLON) {
+            let mut path_components = path.components();
+            // Since path is not empty, it has at least one Component
+            let prefix = path_components.next().unwrap();
+
+            let dev_path = get_device_path_from_map(prefix.as_ref())?;
+            let dev_path_text = dev_path.to_text().map_err(|_| unsupported_err())?;
+
+            let mut ans = PathBuf::new();
+            ans.push(&dev_path_text);
+            // UEFI Shell does not seem to end device path with `/`
+            if *dev_path_text.as_encoded_bytes().last().unwrap() != FORWARD_SLASH {
+                ans.push("/");
+            }
+            ans.push(path_components);
+
+            return Ok(ans);
+        }
+
+        // Absoulte Device Path
+        if path.as_os_str().as_encoded_bytes().contains(&FORWARD_SLASH) {
+            return Ok(path.to_path_buf());
+        }
+
+        // cur_dir() always returns something
+        let cur_dir = crate::env::current_dir().unwrap();
+        let mut path_components = path.components();
+
+        // Relative Root
+        if path_components.next().unwrap() == crate::path::Component::RootDir {
+            let mut ans = PathBuf::new();
+            ans.push(cur_dir.components().next().unwrap());
+            ans.push(path_components);
+            return absolute_path(&ans);
+        }
+
+        absolute_path(&cur_dir.join(path))
+    }
+}