[RFC PATCH v3 12/22] rust: file: Add support for reading files using their path

[Ariel Miculas <amiculas@cisco.com>]

Implement from_path, from_path_in_root_mnt, read_with_offset,
read_to_end and get_file_size methods for a RegularFile newtype.
`kernel_read_file` is used under the hood for reading files.

These functions will be used for reading the PuzzleFS metadata files and
the chunks in the data store.

Signed-off-by: Ariel Miculas <amiculas@cisco.com>
---
 rust/kernel/error.rs |   2 +-
 rust/kernel/file.rs  | 124 ++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 124 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index edada157879a..3cf916bc884f 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -263,7 +263,7 @@ pub fn to_result(err: core::ffi::c_int) -> Result {
 ///     from_err_ptr(unsafe { bindings::devm_platform_ioremap_resource(pdev.to_ptr(), index) })
 /// }
 /// ```
-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 99657adf2472..c381cc297f3a 100644
--- a/rust/kernel/file.rs
+++ b/rust/kernel/file.rs
@@ -5,11 +5,16 @@
 //! C headers: [`include/linux/fs.h`](../../../../include/linux/fs.h) and
 //! [`include/linux/file.h`](../../../../include/linux/file.h)
 
+use crate::alloc::flags::GFP_KERNEL;
+use crate::alloc::vec_ext::VecExt;
 use crate::{
     bindings,
-    error::{code::*, Error, Result},
+    error::{code::*, from_err_ptr, Error, Result},
+    mount::Vfsmount,
+    str::CStr,
     types::{ARef, AlwaysRefCounted, Opaque},
 };
+use alloc::vec::Vec;
 use core::ptr;
 
 /// Flags associated with a [`File`].
@@ -164,6 +169,123 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
     }
 }
 
+/// A newtype over file, specific to regular files
+pub struct RegularFile(ARef<File>);
+impl RegularFile {
+    /// Creates a new instance of Self if the file is a regular file
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure file_ptr.f_inode is initialized to a valid pointer (e.g. file_ptr is
+    /// a pointer returned by path_openat); It must also ensure that file_ptr's reference count was
+    /// incremented at least once
+    unsafe fn create_if_regular(file_ptr: *mut bindings::file) -> Result<RegularFile> {
+        let file_ptr = ptr::NonNull::new(file_ptr).ok_or(ENOENT)?;
+        // SAFETY: file_ptr is a NonNull pointer
+        let inode = unsafe { core::ptr::addr_of!((*file_ptr.as_ptr()).f_inode).read() };
+        // SAFETY: the caller must ensure f_inode is initialized to a valid pointer
+        let inode_mode = unsafe { (*inode).i_mode as u32 };
+        if bindings::S_IFMT & inode_mode != bindings::S_IFREG {
+            return Err(EINVAL);
+        }
+        // SAFETY: the safety requirements state that file_ptr's reference count was incremented at
+        // least once
+        Ok(RegularFile(unsafe { ARef::from_raw(file_ptr.cast()) }))
+    }
+    /// Constructs a new [`struct file`] wrapper from a path.
+    pub fn from_path(filename: &CStr, flags: i32, mode: u16) -> Result<Self> {
+        // SAFETY: filename is a reference, so it's a valid pointer
+        let file_ptr = unsafe {
+            from_err_ptr(bindings::filp_open(
+                filename.as_ptr().cast::<i8>(),
+                flags,
+                mode,
+            ))?
+        };
+
+        // SAFETY: `filp_open` initializes the refcount with 1
+        unsafe { Self::create_if_regular(file_ptr) }
+    }
+
+    /// 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<Self> {
+        let mnt = mount.get();
+        // construct a path from vfsmount, see file_open_root_mnt
+        let raw_path = bindings::path {
+            mnt,
+            // SAFETY: Vfsmount structure stores a valid vfsmount object
+            dentry: unsafe { (*mnt).mnt_root },
+        };
+        let file_ptr = unsafe {
+            // SAFETY: raw_path and filename are both references
+            from_err_ptr(bindings::file_open_root(
+                &raw_path,
+                filename.as_ptr().cast::<i8>(),
+                flags,
+                mode,
+            ))?
+        };
+        // SAFETY: `file_open_root` initializes the refcount with 1
+        unsafe { Self::create_if_regular(file_ptr) }
+    }
+
+    /// Read from the file into the specified buffer
+    pub fn read_with_offset(&self, buf: &mut [u8], offset: u64) -> Result<usize> {
+        // 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: 'file' is valid because it's taken from Self, 'buf' and 'file_size` are
+        // references to the stack variables 'ptr_to_buf' and 'file_size'; ptr_to_buf is also
+        // a pointer to a valid buffer that was obtained from a reference
+        let result = unsafe {
+            bindings::kernel_read_file(
+                self.0 .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()?));
+        }
+
+        Ok(result.try_into()?)
+    }
+
+    /// Allocate and return a vector containing the contents of the entire file
+    pub fn read_to_end(&self) -> Result<Vec<u8>> {
+        let file_size = self.get_file_size()?;
+        let mut buffer = Vec::with_capacity(file_size, GFP_KERNEL)?;
+        buffer.resize(file_size, 0, GFP_KERNEL)?;
+        self.read_with_offset(&mut buffer, 0)?;
+        Ok(buffer)
+    }
+
+    fn get_file_size(&self) -> Result<usize> {
+        // SAFETY: 'file' is valid because it's taken from Self
+        let file_size = unsafe { bindings::i_size_read((*self.0 .0.get()).f_inode) };
+
+        if file_size < 0 {
+            return Err(EINVAL);
+        }
+
+        Ok(file_size.try_into()?)
+    }
+}
+
 /// Represents the EBADF error code.
 ///
 /// Used for methods that can only fail with EBADF.
-- 
2.34.1