Message-ID: <CALNs47svt4481_3Te9FzskAJ29ur6h115NdLfej3Ejfcd5tO7w@mail.gmail.com>
Date:   Wed, 26 Jul 2023 19:52:13 -0400
From:   Trevor Gross <tmgross@...ch.edu>
To:     Ariel Miculas <amiculas@...co.com>
Cc:     rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tycho@...ho.pizza,
        brauner@...nel.org, viro@...iv.linux.org.uk, ojeda@...nel.org,
        alex.gaynor@...il.com, wedsonaf@...il.com
Subject: Re: [RFC PATCH v2 04/10] rust: file: Add a new RegularFile newtype
 useful for reading files

On Wed, Jul 26, 2023 at 12:54 PM Ariel Miculas <amiculas@...co.com> wrote:
>
> Implement from_path, from_path_in_root_mnt, read_with_offset,
> read_to_end and get_file_size methods for a RegularFile newtype.
>
> Signed-off-by: Ariel Miculas <amiculas@...co.com>
> ---
>  rust/helpers.c       |   6 ++
>  rust/kernel/error.rs |   4 +-
>  rust/kernel/file.rs  | 129 ++++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 135 insertions(+), 4 deletions(-)
>
> diff --git a/rust/helpers.c b/rust/helpers.c
> index eed8ace52fb5..9e860a554cda 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -213,6 +213,12 @@ void *rust_helper_alloc_inode_sb(struct super_block *sb,
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_alloc_inode_sb);
>
> +loff_t rust_helper_i_size_read(const struct inode *inode)
> +{
> +       return i_size_read(inode);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_i_size_read);
> +
>  /*
>   * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
>   * as the Rust `usize` type, so we can use it in contexts where Rust
> diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
> index 05fcab6abfe6..e061c83f806a 100644
> --- a/rust/kernel/error.rs
> +++ b/rust/kernel/error.rs
> @@ -273,9 +273,7 @@ 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 494939ba74df..a3002c416dbb 100644
> --- a/rust/kernel/file.rs
> +++ b/rust/kernel/file.rs
> @@ -8,11 +8,13 @@
>  use crate::{
>      bindings,
>      cred::Credential,
> -    error::{code::*, from_result, Error, Result},
> +    error::{code::*, from_err_ptr, from_result, Error, Result},
>      fs,
>      io_buffer::{IoBufferReader, IoBufferWriter},
>      iov_iter::IovIter,
>      mm,
> +    mount::Vfsmount,
> +    str::CStr,
>      sync::CondVar,
>      types::ARef,
>      types::AlwaysRefCounted,
> @@ -20,6 +22,7 @@
>      types::Opaque,
>      user_ptr::{UserSlicePtr, UserSlicePtrReader, UserSlicePtrWriter},
>  };
> +use alloc::vec::Vec;
>  use core::convert::{TryFrom, TryInto};
>  use core::{marker, mem, ptr};
>  use macros::vtable;
> @@ -201,6 +204,130 @@ 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
> +    fn create_if_regular(file_ptr: ptr::NonNull<bindings::file>) -> Result<RegularFile> {

This function should be unsafe, correct? You instead take a
`&bindings::file` instead of `NonNull` but still keep it unsafe, so
the "valid pointer" invariant is always reached.

Or, could this take `&kernel::file::File` instead to reduce some duplication?

> +        // 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
> +        unsafe {
> +            if bindings::S_IFMT & ((*inode).i_mode) as u32 != bindings::S_IFREG {
> +                return Err(EINVAL);
> +            }
> +        }

Nit: factor `unsafe { ((*inode).i_mode) }` out so it doesn't look like
the whole statement is unsafe

> +        // 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> {
> +        let file_ptr = unsafe {
> +            // SAFETY: filename is a reference, so it's a valid pointer
> +            from_err_ptr(bindings::filp_open(
> +                filename.as_ptr() as *const i8,
> +                flags,
> +                mode,
> +            ))?
> +        };

I mentioned in another email that `.cast::<i8>()` can be more
idiomatic and a bit safer than `as`, it's a stylistic choice but there
are a few places it could be changed here if desired

Also, the `// SAFETY` comments need to go outside the unsafe block

> +        let file_ptr = ptr::NonNull::new(file_ptr).ok_or(ENOENT)?;
> +
> +        // SAFETY: `filp_open` initializes the refcount with 1
> +        Self::create_if_regular(file_ptr)
> +    }

Will need unsafe block if `create_if_regular` becomes unsafe

> +
> +    /// 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 file_ptr = {
> +            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 },
> +            };
> +            unsafe {
> +                // SAFETY: raw_path and filename are both references
> +                from_err_ptr(bindings::file_open_root(
> +                    &raw_path,
> +                    filename.as_ptr() as *const i8,
> +                    flags,
> +                    mode,
> +                ))?
> +            }
> +        };

Is there a reason to use the larger scope block, rather than moving
`mnt` and `raw_path` out and doing `let file_ptr = unsafe { ... }`? If
so, a comment would be good.

> +        let file_ptr = ptr::NonNull::new(file_ptr).ok_or(ENOENT)?;
> +
> +        // SAFETY: `file_open_root` initializes the refcount with 1
> +        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> {
> +        Ok({
> +            // 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(),

Is this spacing intentional? If so, `(self.0).0.get()` may be cleaner

> +                    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()?
> +        })
> +    }

I think you could remove the block here and just return `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::try_with_capacity(file_size)?;
> +        buffer.try_resize(file_size, 0)?;
> +        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()?)
> +    }
> +}
> +
>  /// A file descriptor reservation.
>  ///
>  /// This allows the creation of a file descriptor in two steps: first, we reserve a slot for it,
> --
> 2.41.0
>
>

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