uefi: helpers: Introduce OwnedDevicePath

This PR is split off from #135368 to reduce noise.

No real functionality changes, just some quality of life improvements.

Introduce `device_path_to_text_raw` which creates a Box<[u16]> (UTF-16
string) from path instead of creating OsString. The UTF-16 returned by
`EFI_DEVICE_PATH_TO_TEXT` protocol is owned by the caller, so we are
just moving the memory management to box instead of freeing it in the
function itself.

OsString internally is stored as WTF-8, which means converting OsString
to Box<[u16]> requires allocation. This is not ideal for std::fs APIs
where we need to perform Device Path Protocol matching while opening a
volume, and create a UEFI UTF-16 string from the remaining path (which
represents file path inside a volume). This remaining path is never used
on the Rust side, and thus does not need to be converted to WTF-8 to be
used. By introducing direct conversion to Box<[u16]>, we shorten the
conversions from `EFI_DEVICE_PATH_PROTOCOL` -> WTF-8 -> UTF-16 to
`EFI_DEVICE_PATH_PROTOCOL` -> UTF-16 which is required in every file
open operation. That is, we remove 2 intermediate allocation and 1
UTF-16 validation.

Also implement Debug for OwnedDevicePath for some quality of life
improvements.

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

Author: Ayush1325

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

@@ -14,6 +14,7 @@ use r_efi::protocols::{device_path, device_path_to_text, shell};
 
 use crate::ffi::{OsStr, OsString};
 use crate::io::{self, const_error};
+use crate::iter::Iterator;
 use crate::mem::{MaybeUninit, size_of};
 use crate::os::uefi::env::boot_services;
 use crate::os::uefi::ffi::{OsStrExt, OsStringExt};
@@ -160,11 +161,11 @@ pub(crate) fn image_handle_protocol<T>(protocol_guid: Guid) -> io::Result<NonNul
     open_protocol(system_handle, protocol_guid)
 }
 
-pub(crate) fn device_path_to_text(path: NonNull<device_path::Protocol>) -> io::Result<OsString> {
+fn device_path_to_text_raw(path: NonNull<device_path::Protocol>) -> io::Result<Box<[u16]>> {
     fn path_to_text(
         protocol: NonNull<device_path_to_text::Protocol>,
         path: NonNull<device_path::Protocol>,
-    ) -> io::Result<OsString> {
+    ) -> io::Result<Box<[u16]>> {
         let path_ptr: *mut r_efi::efi::Char16 = unsafe {
             ((*protocol.as_ptr()).convert_device_path_to_text)(
                 path.as_ptr(),
@@ -175,17 +176,8 @@ pub(crate) fn device_path_to_text(path: NonNull<device_path::Protocol>) -> io::R
             )
         };
 
-        let path = os_string_from_raw(path_ptr)
-            .ok_or(io::const_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();
-            unsafe {
-                ((*boot_services.as_ptr()).free_pool)(path_ptr.cast());
-            }
-        }
-
-        Ok(path)
+        owned_uefi_string_from_raw(path_ptr)
+            .ok_or(io::const_error!(io::ErrorKind::InvalidData, "Invalid path"))
     }
 
     static LAST_VALID_HANDLE: AtomicPtr<crate::ffi::c_void> =
@@ -214,6 +206,11 @@ pub(crate) fn device_path_to_text(path: NonNull<device_path::Protocol>) -> io::R
     Err(io::const_error!(io::ErrorKind::NotFound, "No device path to text protocol found"))
 }
 
+pub(crate) fn device_path_to_text(path: NonNull<device_path::Protocol>) -> io::Result<OsString> {
+    let p = device_path_to_text_raw(path)?;
+    Ok(OsString::from_wide(&p))
+}
+
 /// Gets RuntimeServices.
 pub(crate) fn runtime_services() -> Option<NonNull<r_efi::efi::RuntimeServices>> {
     let system_table: NonNull<r_efi::efi::SystemTable> =
@@ -222,14 +219,14 @@ pub(crate) fn runtime_services() -> Option<NonNull<r_efi::efi::RuntimeServices>>
     NonNull::new(runtime_services)
 }
 
-pub(crate) struct DevicePath(NonNull<r_efi::protocols::device_path::Protocol>);
+pub(crate) struct OwnedDevicePath(pub(crate) NonNull<r_efi::protocols::device_path::Protocol>);
 
-impl DevicePath {
+impl OwnedDevicePath {
     pub(crate) fn from_text(p: &OsStr) -> io::Result<Self> {
         fn inner(
             p: &OsStr,
             protocol: NonNull<r_efi::protocols::device_path_from_text::Protocol>,
-        ) -> io::Result<DevicePath> {
+        ) -> io::Result<OwnedDevicePath> {
             let path_vec = p.encode_wide().chain(Some(0)).collect::<Vec<u16>>();
             if path_vec[..path_vec.len() - 1].contains(&0) {
                 return Err(const_error!(
@@ -242,7 +239,7 @@ impl DevicePath {
                 unsafe { ((*protocol.as_ptr()).convert_text_to_device_path)(path_vec.as_ptr()) };
 
             NonNull::new(path)
-                .map(DevicePath)
+                .map(OwnedDevicePath)
                 .ok_or_else(|| const_error!(io::ErrorKind::InvalidFilename, "Invalid Device Path"))
         }
 
@@ -275,12 +272,12 @@ impl DevicePath {
         ))
     }
 
-    pub(crate) fn as_ptr(&self) -> *mut r_efi::protocols::device_path::Protocol {
+    pub(crate) const fn as_ptr(&self) -> *mut r_efi::protocols::device_path::Protocol {
         self.0.as_ptr()
     }
 }
 
-impl Drop for DevicePath {
+impl Drop for OwnedDevicePath {
     fn drop(&mut self) {
         if let Some(bt) = boot_services() {
             let bt: NonNull<r_efi::efi::BootServices> = bt.cast();
@@ -291,6 +288,13 @@ impl Drop for DevicePath {
     }
 }
 
+impl crate::fmt::Debug for OwnedDevicePath {
+    fn fmt(&self, f: &mut crate::fmt::Formatter<'_>) -> crate::fmt::Result {
+        let p = device_path_to_text(self.0).unwrap();
+        p.fmt(f)
+    }
+}
+
 pub(crate) struct OwnedProtocol<T> {
     guid: r_efi::efi::Guid,
     handle: NonNull<crate::ffi::c_void>,
@@ -411,6 +415,16 @@ impl<T> Drop for OwnedTable<T> {
     }
 }
 
+/// Create an Owned UEFI string from pointer to NULL terminated UTF-16 string.
+/// Allows string allocations and conversions.
+///
+/// SAFETY: This function assumes that Rust has ownership over this string
+fn owned_uefi_string_from_raw(ptr: *mut r_efi::efi::Char16) -> Option<Box<[r_efi::efi::Char16]>> {
+    let str_len = unsafe { WStrUnits::new(ptr)?.count() };
+    let str_slice = crate::ptr::slice_from_raw_parts_mut(ptr.cast(), str_len);
+    Some(unsafe { Box::from_raw(str_slice) })
+}
+
 /// 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() };

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

@@ -326,7 +326,7 @@ mod uefi_command_internal {
 
     impl Image {
         pub fn load_image(p: &OsStr) -> io::Result<Self> {
-            let path = helpers::DevicePath::from_text(p)?;
+            let path = helpers::OwnedDevicePath::from_text(p)?;
             let boot_services: NonNull<r_efi::efi::BootServices> = boot_services()
                 .ok_or_else(|| const_error!(io::ErrorKind::NotFound, "Boot Services not found"))?
                 .cast();