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//! Encapsulation for system call arguments and return values.
//!
//! The inline-asm and outline-asm code paths do some amount of reordering
//! of arguments; to ensure that we don't accidentally misroute an argument
//! or return value, we use distinct types for each argument index and
//! return value.
//!
//! # Safety
//!
//! The `ToAsm` and `FromAsm` traits are unsafe to use; they should only be
//! used by the syscall code which executes actual syscall machine
//! instructions.
#![allow(unsafe_code)]
use super::c;
use super::fd::RawFd;
use core::marker::PhantomData;
pub(super) trait ToAsm: private::Sealed {
/// Convert `self` to a `usize` ready to be passed to a syscall
/// machine instruction.
///
/// # Safety
///
/// This should be used immediately before the syscall instruction, and
/// the returned value shouldn't be used for any other purpose.
#[must_use]
unsafe fn to_asm(self) -> *mut Opaque;
}
pub(super) trait FromAsm: private::Sealed {
/// Convert `raw` from a value produced by a syscall machine instruction
/// into a `Self`.
///
/// # Safety
///
/// This should be used immediately after the syscall instruction, and
/// the operand value shouldn't be used for any other purpose.
#[must_use]
unsafe fn from_asm(raw: *mut Opaque) -> Self;
}
/// To preserve provenance, syscall arguments and return values are passed as
/// pointer types. They need a type to point to, so we define a custom private
/// type, to prevent it from being used for anything else.
#[repr(transparent)]
pub(super) struct Opaque(c::c_void);
// Argument numbers.
pub(super) struct A0(());
pub(super) struct A1(());
pub(super) struct A2(());
pub(super) struct A3(());
pub(super) struct A4(());
pub(super) struct A5(());
#[cfg(target_arch = "mips")]
pub(super) struct A6(());
#[cfg(target_arch = "x86")]
pub(super) struct SocketArg;
pub(super) trait ArgNumber: private::Sealed {}
impl ArgNumber for A0 {}
impl ArgNumber for A1 {}
impl ArgNumber for A2 {}
impl ArgNumber for A3 {}
impl ArgNumber for A4 {}
impl ArgNumber for A5 {}
#[cfg(target_arch = "mips")]
impl ArgNumber for A6 {}
#[cfg(target_arch = "x86")]
impl ArgNumber for SocketArg {}
// Return value numbers.
pub(super) struct R0(());
pub(super) trait RetNumber: private::Sealed {}
impl RetNumber for R0 {}
/// Syscall arguments use register-sized types. We use a newtype to
/// discourage accidental misuse of the raw integer values.
///
/// This type doesn't implement `Clone` or `Copy`; it should be used exactly
/// once. And it has a lifetime to ensure that it doesn't outlive any resources
/// it might be pointing to.
#[repr(transparent)]
#[must_use]
pub(super) struct ArgReg<'a, Num: ArgNumber> {
raw: *mut Opaque,
_phantom: PhantomData<(&'a (), Num)>,
}
impl<'a, Num: ArgNumber> ToAsm for ArgReg<'a, Num> {
#[inline]
unsafe fn to_asm(self) -> *mut Opaque {
self.raw
}
}
/// Syscall return values use register-sized types. We use a newtype to
/// discourage accidental misuse of the raw integer values.
///
/// This type doesn't implement `Clone` or `Copy`; it should be used exactly
/// once.
#[repr(transparent)]
#[must_use]
pub(super) struct RetReg<Num: RetNumber> {
raw: *mut Opaque,
_phantom: PhantomData<Num>,
}
impl<Num: RetNumber> RetReg<Num> {
#[inline]
pub(super) fn decode_usize(self) -> usize {
debug_assert!(!(-4095..0).contains(&(self.raw as isize)));
self.raw as usize
}
#[inline]
pub(super) fn decode_raw_fd(self) -> RawFd {
let bits = self.decode_usize();
let raw_fd = bits as RawFd;
// Converting `raw` to `RawFd` should be lossless.
debug_assert_eq!(raw_fd as usize, bits);
raw_fd
}
#[inline]
pub(super) fn decode_c_int(self) -> c::c_int {
let bits = self.decode_usize();
let c_int_ = bits as c::c_int;
// Converting `raw` to `c_int` should be lossless.
debug_assert_eq!(c_int_ as usize, bits);
c_int_
}
#[inline]
pub(super) fn decode_c_uint(self) -> c::c_uint {
let bits = self.decode_usize();
let c_uint_ = bits as c::c_uint;
// Converting `raw` to `c_uint` should be lossless.
debug_assert_eq!(c_uint_ as usize, bits);
c_uint_
}
#[inline]
pub(super) fn decode_void_star(self) -> *mut c::c_void {
self.raw.cast()
}
#[cfg(target_pointer_width = "64")]
#[inline]
pub(super) fn decode_u64(self) -> u64 {
self.decode_usize() as u64
}
#[inline]
pub(super) fn decode_void(self) {
let ignore = self.decode_usize();
debug_assert_eq!(ignore, 0);
}
#[inline]
pub(super) fn decode_error_code(self) -> u16 {
let bits = self.raw as usize;
// `raw` must be in `-4095..0`. Linux always returns errors in
// `-4095..0`, and we double-check it here.
debug_assert!((-4095..0).contains(&(bits as isize)));
bits as u16
}
#[inline]
pub(super) fn is_nonzero(&self) -> bool {
!self.raw.is_null()
}
#[inline]
pub(super) fn is_negative(&self) -> bool {
(self.raw as isize) < 0
}
#[inline]
pub(super) fn is_in_range(&self, range: core::ops::Range<isize>) -> bool {
range.contains(&(self.raw as isize))
}
}
impl<Num: RetNumber> FromAsm for RetReg<Num> {
#[inline]
unsafe fn from_asm(raw: *mut Opaque) -> Self {
Self {
raw,
_phantom: PhantomData,
}
}
}
#[repr(transparent)]
pub(super) struct SyscallNumber<'a> {
nr: usize,
_phantom: PhantomData<&'a ()>,
}
impl<'a> ToAsm for SyscallNumber<'a> {
#[inline]
unsafe fn to_asm(self) -> *mut Opaque {
self.nr as usize as *mut Opaque
}
}
/// Encode a system call argument as an `ArgReg`.
#[inline]
pub(super) fn raw_arg<'a, Num: ArgNumber>(raw: *mut Opaque) -> ArgReg<'a, Num> {
ArgReg {
raw,
_phantom: PhantomData,
}
}
/// Encode a system call number (a `__NR_*` constant) as a `SyscallNumber`.
#[inline]
pub(super) const fn nr<'a>(nr: u32) -> SyscallNumber<'a> {
SyscallNumber {
nr: nr as usize,
_phantom: PhantomData,
}
}
/// Seal our various traits using the technique documented [here].
///
/// [here]: https://rust-lang.github.io/api-guidelines/future-proofing.html
mod private {
pub trait Sealed {}
// Implement for those same types, but no others.
impl<'a, Num: super::ArgNumber> Sealed for super::ArgReg<'a, Num> {}
impl<Num: super::RetNumber> Sealed for super::RetReg<Num> {}
impl<'a> Sealed for super::SyscallNumber<'a> {}
impl Sealed for super::A0 {}
impl Sealed for super::A1 {}
impl Sealed for super::A2 {}
impl Sealed for super::A3 {}
impl Sealed for super::A4 {}
impl Sealed for super::A5 {}
#[cfg(target_arch = "mips")]
impl Sealed for super::A6 {}
#[cfg(target_arch = "x86")]
impl Sealed for super::SocketArg {}
impl Sealed for super::R0 {}
}