| Message ID | 20240418-alice-mm-v6-3-cb8f3e5d688f@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Memory management patches needed by Rust Binder | expand |
On 18.04.24 10:59, Alice Ryhl wrote: > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > index 8fad61268465..9c57c6c75553 100644 > --- a/rust/kernel/types.rs > +++ b/rust/kernel/types.rs > @@ -409,3 +409,67 @@ pub enum Either<L, R> { > /// Constructs an instance of [`Either`] containing a value of type `R`. > Right(R), > } > + > +/// Types for which any bit pattern is valid. > +/// > +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so > +/// reading arbitrary bytes into something that contains a `bool` is not okay. > +/// > +/// It's okay for the type to have padding, as initializing those bytes has no effect. > +/// > +/// # Safety > +/// > +/// All bit-patterns must be valid for this type. This type must not have interior mutability. What is the reason for disallowing interior mutability here? I agree that it is necessary for `AsBytes`, but I don't think we need it here. For example it is fine to convert `u8` to `UnsafeCell<u8>`. Niches also should not be a problem, since eg `Option<UnsafeCell<NonNull<u8>>>` already fails the "All bit-patterns must be valid for this type".
On Thu, Apr 18, 2024 at 3:02 PM Benno Lossin <benno.lossin@proton.me> wrote: > > On 18.04.24 10:59, Alice Ryhl wrote: > > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > > index 8fad61268465..9c57c6c75553 100644 > > --- a/rust/kernel/types.rs > > +++ b/rust/kernel/types.rs > > @@ -409,3 +409,67 @@ pub enum Either<L, R> { > > /// Constructs an instance of [`Either`] containing a value of type `R`. > > Right(R), > > } > > + > > +/// Types for which any bit pattern is valid. > > +/// > > +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so > > +/// reading arbitrary bytes into something that contains a `bool` is not okay. > > +/// > > +/// It's okay for the type to have padding, as initializing those bytes has no effect. > > +/// > > +/// # Safety > > +/// > > +/// All bit-patterns must be valid for this type. This type must not have interior mutability. > > What is the reason for disallowing interior mutability here? I agree > that it is necessary for `AsBytes`, but I don't think we need it here. > For example it is fine to convert `u8` to `UnsafeCell<u8>`. Niches also > should not be a problem, since eg `Option<UnsafeCell<NonNull<u8>>>` > already fails the "All bit-patterns must be valid for this type". If T: FromBytes allows transmuting &[u8; size_of::<T>] into &T, then it would be a problem as you could then use it to modify the original &[u8]. Alice
On 18.04.24 15:17, Alice Ryhl wrote: > On Thu, Apr 18, 2024 at 3:02 PM Benno Lossin <benno.lossin@proton.me> wrote: >> >> On 18.04.24 10:59, Alice Ryhl wrote: >>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs >>> index 8fad61268465..9c57c6c75553 100644 >>> --- a/rust/kernel/types.rs >>> +++ b/rust/kernel/types.rs >>> @@ -409,3 +409,67 @@ pub enum Either<L, R> { >>> /// Constructs an instance of [`Either`] containing a value of type `R`. >>> Right(R), >>> } >>> + >>> +/// Types for which any bit pattern is valid. >>> +/// >>> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so >>> +/// reading arbitrary bytes into something that contains a `bool` is not okay. >>> +/// >>> +/// It's okay for the type to have padding, as initializing those bytes has no effect. >>> +/// >>> +/// # Safety >>> +/// >>> +/// All bit-patterns must be valid for this type. This type must not have interior mutability. >> >> What is the reason for disallowing interior mutability here? I agree >> that it is necessary for `AsBytes`, but I don't think we need it here. >> For example it is fine to convert `u8` to `UnsafeCell<u8>`. Niches also >> should not be a problem, since eg `Option<UnsafeCell<NonNull<u8>>>` >> already fails the "All bit-patterns must be valid for this type". > > If T: FromBytes allows transmuting &[u8; size_of::<T>] into &T, then > it would be a problem as you could then use it to modify the original > &[u8]. Ahh that makes a lot of sense.
On Thu, Apr 18, 2024 at 04:23:06PM +0000, Benno Lossin wrote: > On 18.04.24 15:17, Alice Ryhl wrote: > > On Thu, Apr 18, 2024 at 3:02 PM Benno Lossin <benno.lossin@proton.me> wrote: > >> > >> On 18.04.24 10:59, Alice Ryhl wrote: > >>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > >>> index 8fad61268465..9c57c6c75553 100644 > >>> --- a/rust/kernel/types.rs > >>> +++ b/rust/kernel/types.rs > >>> @@ -409,3 +409,67 @@ pub enum Either<L, R> { > >>> /// Constructs an instance of [`Either`] containing a value of type `R`. > >>> Right(R), > >>> } > >>> + > >>> +/// Types for which any bit pattern is valid. > >>> +/// > >>> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so > >>> +/// reading arbitrary bytes into something that contains a `bool` is not okay. > >>> +/// > >>> +/// It's okay for the type to have padding, as initializing those bytes has no effect. > >>> +/// > >>> +/// # Safety > >>> +/// > >>> +/// All bit-patterns must be valid for this type. This type must not have interior mutability. > >> > >> What is the reason for disallowing interior mutability here? I agree > >> that it is necessary for `AsBytes`, but I don't think we need it here. Hmm.. technically, if the interior mutability behaves in a way that each byte is still initialized during the modification, then it should be fine for `AsBytes`, for example and `AtomicI32` (implemented by asm blocks)? Not making any change suggestion, just checking my understand. Regards, Boqun > >> For example it is fine to convert `u8` to `UnsafeCell<u8>`. Niches also > >> should not be a problem, since eg `Option<UnsafeCell<NonNull<u8>>>` > >> already fails the "All bit-patterns must be valid for this type". > > > > If T: FromBytes allows transmuting &[u8; size_of::<T>] into &T, then > > it would be a problem as you could then use it to modify the original > > &[u8]. > > Ahh that makes a lot of sense. > > -- > Cheers, > Benno > >
On Thu, Apr 18, 2024 at 9:03 AM Benno Lossin <benno.lossin@proton.me> wrote: > > On 18.04.24 10:59, Alice Ryhl wrote: > > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > > index 8fad61268465..9c57c6c75553 100644 > > --- a/rust/kernel/types.rs > > +++ b/rust/kernel/types.rs > > @@ -409,3 +409,67 @@ pub enum Either<L, R> { > > /// Constructs an instance of [`Either`] containing a value of type `R`. > > Right(R), > > } > > + > > +/// Types for which any bit pattern is valid. > > +/// > > +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so > > +/// reading arbitrary bytes into something that contains a `bool` is not okay. > > +/// > > +/// It's okay for the type to have padding, as initializing those bytes has no effect. > > +/// > > +/// # Safety > > +/// > > +/// All bit-patterns must be valid for this type. This type must not have interior mutability. > > What is the reason for disallowing interior mutability here? I agree > that it is necessary for `AsBytes`, but I don't think we need it here. > For example it is fine to convert `u8` to `UnsafeCell<u8>`. Niches also > should not be a problem, since eg `Option<UnsafeCell<NonNull<u8>>>` > already fails the "All bit-patterns must be valid for this type". > > -- > Cheers, > Benno > > > +pub unsafe trait FromBytes {} See also the reasoning in zerocopy [1] and bytemuck [2] [1]: https://docs.rs/zerocopy/latest/zerocopy/derive.FromBytes.html#analysis [2]: https://docs.rs/bytemuck/latest/bytemuck/trait.AnyBitPattern.html#safety
On Thu, Apr 18, 2024 at 7:27 PM Boqun Feng <boqun.feng@gmail.com> wrote: > > On Thu, Apr 18, 2024 at 04:23:06PM +0000, Benno Lossin wrote: > > On 18.04.24 15:17, Alice Ryhl wrote: > > > On Thu, Apr 18, 2024 at 3:02 PM Benno Lossin <benno.lossin@proton.me> wrote: > > >> > > >> On 18.04.24 10:59, Alice Ryhl wrote: > > >>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > > >>> index 8fad61268465..9c57c6c75553 100644 > > >>> --- a/rust/kernel/types.rs > > >>> +++ b/rust/kernel/types.rs > > >>> @@ -409,3 +409,67 @@ pub enum Either<L, R> { > > >>> /// Constructs an instance of [`Either`] containing a value of type `R`. > > >>> Right(R), > > >>> } > > >>> + > > >>> +/// Types for which any bit pattern is valid. > > >>> +/// > > >>> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so > > >>> +/// reading arbitrary bytes into something that contains a `bool` is not okay. > > >>> +/// > > >>> +/// It's okay for the type to have padding, as initializing those bytes has no effect. > > >>> +/// > > >>> +/// # Safety > > >>> +/// > > >>> +/// All bit-patterns must be valid for this type. This type must not have interior mutability. > > >> > > >> What is the reason for disallowing interior mutability here? I agree > > >> that it is necessary for `AsBytes`, but I don't think we need it here. > > Hmm.. technically, if the interior mutability behaves in a way that each > byte is still initialized during the modification, then it should be > fine for `AsBytes`, for example and `AtomicI32` (implemented by asm > blocks)? Not making any change suggestion, just checking my understand. No, that's UB. When the type is not interior mutable, then any two loads from the same immutable reference may be assumed to return the same value. Changing it with an atomic would violate that since the value changes. Alice
On Thu, Apr 18, 2024 at 09:35:53PM +0200, Alice Ryhl wrote: > On Thu, Apr 18, 2024 at 7:27 PM Boqun Feng <boqun.feng@gmail.com> wrote: > > > > On Thu, Apr 18, 2024 at 04:23:06PM +0000, Benno Lossin wrote: > > > On 18.04.24 15:17, Alice Ryhl wrote: > > > > On Thu, Apr 18, 2024 at 3:02 PM Benno Lossin <benno.lossin@proton.me> wrote: > > > >> > > > >> On 18.04.24 10:59, Alice Ryhl wrote: > > > >>> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > > > >>> index 8fad61268465..9c57c6c75553 100644 > > > >>> --- a/rust/kernel/types.rs > > > >>> +++ b/rust/kernel/types.rs > > > >>> @@ -409,3 +409,67 @@ pub enum Either<L, R> { > > > >>> /// Constructs an instance of [`Either`] containing a value of type `R`. > > > >>> Right(R), > > > >>> } > > > >>> + > > > >>> +/// Types for which any bit pattern is valid. > > > >>> +/// > > > >>> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so > > > >>> +/// reading arbitrary bytes into something that contains a `bool` is not okay. > > > >>> +/// > > > >>> +/// It's okay for the type to have padding, as initializing those bytes has no effect. > > > >>> +/// > > > >>> +/// # Safety > > > >>> +/// > > > >>> +/// All bit-patterns must be valid for this type. This type must not have interior mutability. > > > >> > > > >> What is the reason for disallowing interior mutability here? I agree > > > >> that it is necessary for `AsBytes`, but I don't think we need it here. > > > > Hmm.. technically, if the interior mutability behaves in a way that each > > byte is still initialized during the modification, then it should be > > fine for `AsBytes`, for example and `AtomicI32` (implemented by asm > > blocks)? Not making any change suggestion, just checking my understand. > > No, that's UB. When the type is not interior mutable, then any two > loads from the same immutable reference may be assumed to return the > same value. Changing it with an atomic would violate that since the > value changes. > Oh, that makes sense! Thanks! Regards, Boqun > Alice
On Thu, 18 Apr 2024 08:59:19 +0000 Alice Ryhl <aliceryhl@google.com> wrote: > Add safe methods for reading and writing Rust values to and from > userspace pointers. > > The C methods for copying to/from userspace use a function called > `check_object_size` to verify that the kernel pointer is not dangling. > However, this check is skipped when the length is a compile-time > constant, with the assumption that such cases trivially have a correct > kernel pointer. > > In this patch, we apply the same optimization to the typed accessors. > For both methods, the size of the operation is known at compile time to > be size_of of the type being read or written. Since the C side doesn't > provide a variant that skips only this check, we create custom helpers > for this purpose. > > The majority of reads and writes to userspace pointers in the Rust > Binder driver uses these accessor methods. Benchmarking has found that > skipping the `check_object_size` check makes a big difference for the > cases being skipped here. (And that the check doesn't make a difference > for the cases that use the raw read/write methods.) > > This code is based on something that was originally written by Wedson on > the old rust branch. It was modified by Alice to skip the > `check_object_size` check, and to update various comments, including the > notes about kernel pointers in `WritableToBytes`. > > Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com> > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com> > Reviewed-by: Benno Lossin <benno.lossin@proton.me> > Reviewed-by: Boqun Feng <boqun.feng@gmail.com> > Reviewed-by: Trevor Gross <tmgross@umich.edu> > Signed-off-by: Alice Ryhl <aliceryhl@google.com> Reviewed-by: Gary Guo <gary@garyguo.net> > --- > rust/kernel/types.rs | 64 ++++++++++++++++++++++++++++++++++++++++ > rust/kernel/uaccess.rs | 79 ++++++++++++++++++++++++++++++++++++++++++++++++-- > 2 files changed, 141 insertions(+), 2 deletions(-) > > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > index 8fad61268465..9c57c6c75553 100644 > --- a/rust/kernel/types.rs > +++ b/rust/kernel/types.rs > > +/// Types that can be viewed as an immutable slice of initialized bytes. > +/// > +/// If a struct implements this trait, then it is okay to copy it byte-for-byte to userspace. This > +/// means that it should not have any padding, as padding bytes are uninitialized. Reading > +/// uninitialized memory is not just undefined behavior, it may even lead to leaking sensitive > +/// information on the stack to userspace. > +/// > +/// The struct should also not hold kernel pointers, as kernel pointer addresses are also considered > +/// sensitive. However, leaking kernel pointers is not considered undefined behavior by Rust, so > +/// this is a correctness requirement, but not a safety requirement. > +/// > +/// # Safety > +/// > +/// Values of this type may not contain any uninitialized bytes. This type must not have interior > +/// mutability. > +pub unsafe trait AsBytes {} > + > +// SAFETY: Instances of the following types have no uninitialized portions. > +unsafe impl AsBytes for u8 {} > +unsafe impl AsBytes for u16 {} > +unsafe impl AsBytes for u32 {} > +unsafe impl AsBytes for u64 {} > +unsafe impl AsBytes for usize {} > +unsafe impl AsBytes for i8 {} > +unsafe impl AsBytes for i16 {} > +unsafe impl AsBytes for i32 {} > +unsafe impl AsBytes for i64 {} > +unsafe impl AsBytes for isize {} > +unsafe impl AsBytes for bool {} > +unsafe impl AsBytes for char {} > +unsafe impl AsBytes for str {} > +// SAFETY: If individual values in an array have no uninitialized portions, then the array itself > +// does not have any uninitialized portions either. > +unsafe impl<T: AsBytes> AsBytes for [T] {} nit: I would move `str` to here, since `str` is essentially `[u8]` with UTF-8 guarantee. > +unsafe impl<T: AsBytes, const N: usize> AsBytes for [T; N] {}
On Thu, Apr 25, 2024 at 6:14 PM Gary Guo <gary@garyguo.net> wrote: > > On Thu, 18 Apr 2024 08:59:19 +0000 > Alice Ryhl <aliceryhl@google.com> wrote: > > > Add safe methods for reading and writing Rust values to and from > > userspace pointers. > > > > The C methods for copying to/from userspace use a function called > > `check_object_size` to verify that the kernel pointer is not dangling. > > However, this check is skipped when the length is a compile-time > > constant, with the assumption that such cases trivially have a correct > > kernel pointer. > > > > In this patch, we apply the same optimization to the typed accessors. > > For both methods, the size of the operation is known at compile time to > > be size_of of the type being read or written. Since the C side doesn't > > provide a variant that skips only this check, we create custom helpers > > for this purpose. > > > > The majority of reads and writes to userspace pointers in the Rust > > Binder driver uses these accessor methods. Benchmarking has found that > > skipping the `check_object_size` check makes a big difference for the > > cases being skipped here. (And that the check doesn't make a difference > > for the cases that use the raw read/write methods.) > > > > This code is based on something that was originally written by Wedson on > > the old rust branch. It was modified by Alice to skip the > > `check_object_size` check, and to update various comments, including the > > notes about kernel pointers in `WritableToBytes`. > > > > Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com> > > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com> > > Reviewed-by: Benno Lossin <benno.lossin@proton.me> > > Reviewed-by: Boqun Feng <boqun.feng@gmail.com> > > Reviewed-by: Trevor Gross <tmgross@umich.edu> > > Signed-off-by: Alice Ryhl <aliceryhl@google.com> > > Reviewed-by: Gary Guo <gary@garyguo.net> > > > --- > > rust/kernel/types.rs | 64 ++++++++++++++++++++++++++++++++++++++++ > > rust/kernel/uaccess.rs | 79 ++++++++++++++++++++++++++++++++++++++++++++++++-- > > 2 files changed, 141 insertions(+), 2 deletions(-) > > > > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs > > index 8fad61268465..9c57c6c75553 100644 > > --- a/rust/kernel/types.rs > > +++ b/rust/kernel/types.rs > > > > +/// Types that can be viewed as an immutable slice of initialized bytes. > > +/// > > +/// If a struct implements this trait, then it is okay to copy it byte-for-byte to userspace. This > > +/// means that it should not have any padding, as padding bytes are uninitialized. Reading > > +/// uninitialized memory is not just undefined behavior, it may even lead to leaking sensitive > > +/// information on the stack to userspace. > > +/// > > +/// The struct should also not hold kernel pointers, as kernel pointer addresses are also considered > > +/// sensitive. However, leaking kernel pointers is not considered undefined behavior by Rust, so > > +/// this is a correctness requirement, but not a safety requirement. > > +/// > > +/// # Safety > > +/// > > +/// Values of this type may not contain any uninitialized bytes. This type must not have interior > > +/// mutability. > > +pub unsafe trait AsBytes {} > > + > > +// SAFETY: Instances of the following types have no uninitialized portions. > > +unsafe impl AsBytes for u8 {} > > +unsafe impl AsBytes for u16 {} > > +unsafe impl AsBytes for u32 {} > > +unsafe impl AsBytes for u64 {} > > +unsafe impl AsBytes for usize {} > > +unsafe impl AsBytes for i8 {} > > +unsafe impl AsBytes for i16 {} > > +unsafe impl AsBytes for i32 {} > > +unsafe impl AsBytes for i64 {} > > +unsafe impl AsBytes for isize {} > > +unsafe impl AsBytes for bool {} > > +unsafe impl AsBytes for char {} > > +unsafe impl AsBytes for str {} > > +// SAFETY: If individual values in an array have no uninitialized portions, then the array itself > > +// does not have any uninitialized portions either. > > +unsafe impl<T: AsBytes> AsBytes for [T] {} > > nit: I would move `str` to here, since `str` is essentially `[u8]` with > UTF-8 guarantee. > > > +unsafe impl<T: AsBytes, const N: usize> AsBytes for [T; N] {} Yes ... but the safety comment here talks about arrays and their individual values. I don't think it transfers cleanly to str, and that the other safety comment fits str better. Alice
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs index 8fad61268465..9c57c6c75553 100644 --- a/rust/kernel/types.rs +++ b/rust/kernel/types.rs @@ -409,3 +409,67 @@ pub enum Either<L, R> { /// Constructs an instance of [`Either`] containing a value of type `R`. Right(R), } + +/// Types for which any bit pattern is valid. +/// +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so +/// reading arbitrary bytes into something that contains a `bool` is not okay. +/// +/// It's okay for the type to have padding, as initializing those bytes has no effect. +/// +/// # Safety +/// +/// All bit-patterns must be valid for this type. This type must not have interior mutability. +pub unsafe trait FromBytes {} + +// SAFETY: All bit patterns are acceptable values of the types below. +unsafe impl FromBytes for u8 {} +unsafe impl FromBytes for u16 {} +unsafe impl FromBytes for u32 {} +unsafe impl FromBytes for u64 {} +unsafe impl FromBytes for usize {} +unsafe impl FromBytes for i8 {} +unsafe impl FromBytes for i16 {} +unsafe impl FromBytes for i32 {} +unsafe impl FromBytes for i64 {} +unsafe impl FromBytes for isize {} +// SAFETY: If all bit patterns are acceptable for individual values in an array, then all bit +// patterns are also acceptable for arrays of that type. +unsafe impl<T: FromBytes> FromBytes for [T] {} +unsafe impl<T: FromBytes, const N: usize> FromBytes for [T; N] {} + +/// Types that can be viewed as an immutable slice of initialized bytes. +/// +/// If a struct implements this trait, then it is okay to copy it byte-for-byte to userspace. This +/// means that it should not have any padding, as padding bytes are uninitialized. Reading +/// uninitialized memory is not just undefined behavior, it may even lead to leaking sensitive +/// information on the stack to userspace. +/// +/// The struct should also not hold kernel pointers, as kernel pointer addresses are also considered +/// sensitive. However, leaking kernel pointers is not considered undefined behavior by Rust, so +/// this is a correctness requirement, but not a safety requirement. +/// +/// # Safety +/// +/// Values of this type may not contain any uninitialized bytes. This type must not have interior +/// mutability. +pub unsafe trait AsBytes {} + +// SAFETY: Instances of the following types have no uninitialized portions. +unsafe impl AsBytes for u8 {} +unsafe impl AsBytes for u16 {} +unsafe impl AsBytes for u32 {} +unsafe impl AsBytes for u64 {} +unsafe impl AsBytes for usize {} +unsafe impl AsBytes for i8 {} +unsafe impl AsBytes for i16 {} +unsafe impl AsBytes for i32 {} +unsafe impl AsBytes for i64 {} +unsafe impl AsBytes for isize {} +unsafe impl AsBytes for bool {} +unsafe impl AsBytes for char {} +unsafe impl AsBytes for str {} +// SAFETY: If individual values in an array have no uninitialized portions, then the array itself +// does not have any uninitialized portions either. +unsafe impl<T: AsBytes> AsBytes for [T] {} +unsafe impl<T: AsBytes, const N: usize> AsBytes for [T; N] {} diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs index ee5623d7b98f..39481e374c40 100644 --- a/rust/kernel/uaccess.rs +++ b/rust/kernel/uaccess.rs @@ -4,10 +4,16 @@ //! //! C header: [`include/linux/uaccess.h`](srctree/include/linux/uaccess.h) -use crate::{alloc::Flags, bindings, error::Result, prelude::*}; +use crate::{ + alloc::Flags, + bindings, + error::Result, + prelude::*, + types::{AsBytes, FromBytes}, +}; use alloc::vec::Vec; use core::ffi::{c_ulong, c_void}; -use core::mem::MaybeUninit; +use core::mem::{size_of, MaybeUninit}; /// The type used for userspace addresses. pub type UserPtr = usize; @@ -247,6 +253,41 @@ pub fn read_slice(&mut self, out: &mut [u8]) -> Result { self.read_raw(out) } + /// Reads a value of the specified type. + /// + /// Fails with `EFAULT` if the read happens on a bad address, or if the read goes out of bounds + /// of this [`UserSliceReader`]. + pub fn read<T: FromBytes>(&mut self) -> Result<T> { + let len = size_of::<T>(); + if len > self.length { + return Err(EFAULT); + } + let Ok(len_ulong) = c_ulong::try_from(len) else { + return Err(EFAULT); + }; + let mut out: MaybeUninit<T> = MaybeUninit::uninit(); + // SAFETY: The local variable `out` is valid for writing `size_of::<T>()` bytes. + // + // By using the _copy_from_user variant, we skip the check_object_size check that verifies + // the kernel pointer. This mirrors the logic on the C side that skips the check when the + // length is a compile-time constant. + let res = unsafe { + bindings::_copy_from_user( + out.as_mut_ptr().cast::<c_void>(), + self.ptr as *const c_void, + len_ulong, + ) + }; + if res != 0 { + return Err(EFAULT); + } + self.ptr = self.ptr.wrapping_add(len); + self.length -= len; + // SAFETY: The read above has initialized all bytes in `out`, and since `T` implements + // `FromBytes`, any bit-pattern is a valid value for this type. + Ok(unsafe { out.assume_init() }) + } + /// Reads the entirety of the user slice, appending it to the end of the provided buffer. /// /// Fails with `EFAULT` if the read happens on a bad address. @@ -310,4 +351,38 @@ pub fn write_slice(&mut self, data: &[u8]) -> Result { self.length -= len; Ok(()) } + + /// Writes the provided Rust value to this userspace pointer. + /// + /// Fails with `EFAULT` if the write happens on a bad address, or if the write goes out of bounds + /// of this [`UserSliceWriter`]. This call may modify the associated userspace slice even if it + /// returns an error. + pub fn write<T: AsBytes>(&mut self, value: &T) -> Result { + let len = size_of::<T>(); + if len > self.length { + return Err(EFAULT); + } + let Ok(len_ulong) = c_ulong::try_from(len) else { + return Err(EFAULT); + }; + // SAFETY: The reference points to a value of type `T`, so it is valid for reading + // `size_of::<T>()` bytes. + // + // By using the _copy_to_user variant, we skip the check_object_size check that verifies the + // kernel pointer. This mirrors the logic on the C side that skips the check when the length + // is a compile-time constant. + let res = unsafe { + bindings::_copy_to_user( + self.ptr as *mut c_void, + (value as *const T).cast::<c_void>(), + len_ulong, + ) + }; + if res != 0 { + return Err(EFAULT); + } + self.ptr = self.ptr.wrapping_add(len); + self.length -= len; + Ok(()) + } }