[PATCH 51/80] rust: file: add from_path, from_path_in_root_mnt and read_with_offset methods to File

[Ariel Miculas <amiculas@cisco.com>]

Signed-off-by: Ariel Miculas <amiculas@cisco.com>
---
 rust/kernel/error.rs |  4 +--
 rust/kernel/file.rs  | 74 +++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 74 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 231f7dfd8005..6fff9a4f0672 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -234,9 +234,7 @@ pub fn to_result(err: core::ffi::c_int) -> Result {
 ///     }
 /// }
 /// ```
-// TODO: Remove `dead_code` marker once an in-kernel client is available.
-#[allow(dead_code)]
-pub(crate) fn from_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
+pub fn from_err_ptr<T>(ptr: *mut T) -> Result<*mut T> {
     // CAST: Casting a pointer to `*const core::ffi::c_void` is always valid.
     let const_ptr: *const core::ffi::c_void = ptr.cast();
     // SAFETY: The FFI function does not deref the pointer.
diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs
index 0062d8b17990..a4926b316573 100644
--- a/rust/kernel/file.rs
+++ b/rust/kernel/file.rs
@@ -8,11 +8,13 @@
 use crate::{
     bindings,
     cred::Credential,
-    error::{code::*, from_kernel_result, Error, Result},
+    error::{code::*, from_err_ptr, from_kernel_result, Error, Result},
     fs,
     io_buffer::{IoBufferReader, IoBufferWriter},
     iov_iter::IovIter,
     mm,
+    mount::Vfsmount,
+    str::CStr,
     sync::CondVar,
     types::ARef,
     types::AlwaysRefCounted,
@@ -131,6 +133,76 @@ pub fn from_fd(fd: u32) -> Result<ARef<Self>> {
         Ok(unsafe { ARef::from_raw(ptr.cast()) })
     }
 
+    /// Constructs a new [`struct file`] wrapper from a path.
+    pub fn from_path(filename: &CStr, flags: i32, mode: u16) -> Result<ARef<Self>> {
+        let file_ptr = unsafe {
+            from_err_ptr(bindings::filp_open(
+                filename.as_ptr() as *const i8,
+                flags,
+                mode,
+            ))?
+        };
+        let file_ptr = ptr::NonNull::new(file_ptr).ok_or(ENOENT)?;
+
+        // SAFETY: `filp_open` initializes the refcount with 1
+        Ok(unsafe { ARef::from_raw(file_ptr.cast()) })
+    }
+
+    /// Constructs a new [`struct file`] wrapper from a path and a vfsmount.
+    pub fn from_path_in_root_mnt(
+        mount: &Vfsmount,
+        filename: &CStr,
+        flags: i32,
+        mode: u16,
+    ) -> Result<ARef<Self>> {
+        let file_ptr = unsafe {
+            let mnt = mount.get();
+            let raw_path = bindings::path {
+                mnt,
+                dentry: (*mnt).mnt_root,
+            };
+            from_err_ptr(bindings::file_open_root(
+                &raw_path,
+                filename.as_ptr() as *const i8,
+                flags,
+                mode,
+            ))?
+        };
+        let file_ptr = ptr::NonNull::new(file_ptr).ok_or(ENOENT)?;
+
+        // SAFETY: `file_open_root` increments the refcount before returning. (TODO does it?)
+        Ok(unsafe { ARef::from_raw(file_ptr.cast()) })
+    }
+
+    /// Read from the file into the specified buffer
+    pub fn read_with_offset(&self, buf: &mut [u8], offset: u64) -> Result<usize> {
+        Ok(unsafe {
+            // kernel_read_file expects a pointer to a "void *" buffer
+            let mut ptr_to_buf = buf.as_mut_ptr() as *mut core::ffi::c_void;
+            // Unless we give a non-null pointer to the file size:
+            // 1. we cannot give a non-zero value for the offset
+            // 2. we cannot have offset 0 and buffer_size > file_size
+            let mut file_size = 0;
+
+            // SAFETY: all the pointers to kernel_read_file are valid
+            let result = bindings::kernel_read_file(
+                self.0.get(),
+                offset.try_into()?,
+                &mut ptr_to_buf,
+                buf.len(),
+                &mut file_size,
+                bindings::kernel_read_file_id_READING_UNKNOWN,
+            );
+
+            // kernel_read_file returns the number of bytes read on success or negative on error.
+            if result < 0 {
+                return Err(Error::from_errno(result.try_into()?));
+            }
+
+            result.try_into()?
+        })
+    }
+
     /// Creates a reference to a [`File`] from a valid pointer.
     ///
     /// # Safety
-- 
2.40.1