Fix Linux custom ioctl opcode computation (#861)

* Fix Linux ioctl opcode computation, add test

* Document the corresponding C macros for ioctl opcode computation

* Rename shift_and_mask -> mask_and_shift in ioctl opcode computation

Author: kevinmehall

src/ioctl/linux.rs

@@ -10,9 +10,9 @@ pub(super) const fn compose_opcode(
     num: RawOpcode,
     size: RawOpcode,
 ) -> RawOpcode {
-    macro_rules! shift_and_mask {
+    macro_rules! mask_and_shift {
         ($val:expr, $shift:expr, $mask:expr) => {{
-            ($val << $shift) & $mask
+            ($val & $mask) << $shift
         }};
     }
 
@@ -23,10 +23,10 @@ pub(super) const fn compose_opcode(
         Direction::ReadWrite => READ | WRITE,
     };
 
-    shift_and_mask!(group, GROUP_SHIFT, GROUP_MASK)
-        | shift_and_mask!(num, NUM_SHIFT, NUM_MASK)
-        | shift_and_mask!(size, SIZE_SHIFT, SIZE_MASK)
-        | shift_and_mask!(dir, DIR_SHIFT, DIR_MASK)
+    mask_and_shift!(group, GROUP_SHIFT, GROUP_MASK)
+        | mask_and_shift!(num, NUM_SHIFT, NUM_MASK)
+        | mask_and_shift!(size, SIZE_SHIFT, SIZE_MASK)
+        | mask_and_shift!(dir, DIR_SHIFT, DIR_MASK)
 }
 
 const NUM_BITS: RawOpcode = 8;
@@ -81,3 +81,37 @@ mod consts {
     pub(super) const SIZE_BITS: RawOpcode = 13;
     pub(super) const DIR_BITS: RawOpcode = 3;
 }
+
+#[cfg(not(any(
+    // These have no ioctl opcodes defined in linux_raw_sys
+    // so can't use that as a known-good value for this test.
+    target_arch = "sparc",
+    target_arch = "sparc64"
+)))]
+#[test]
+fn check_known_opcodes() {
+    use crate::backend::c::{c_long, c_uint};
+    use core::mem::size_of;
+
+    // _IOR('U', 15, unsigned int)
+    assert_eq!(
+        compose_opcode(
+            Direction::Read,
+            b'U' as RawOpcode,
+            15,
+            size_of::<c_uint>() as RawOpcode
+        ),
+        linux_raw_sys::ioctl::USBDEVFS_CLAIMINTERFACE as RawOpcode
+    );
+
+    // _IOW('v', 2, long)
+    assert_eq!(
+        compose_opcode(
+            Direction::Write,
+            b'v' as RawOpcode,
+            2,
+            size_of::<c_long>() as RawOpcode
+        ),
+        linux_raw_sys::ioctl::FS_IOC_SETVERSION as RawOpcode
+    );
+}

src/ioctl/mod.rs

@@ -205,6 +205,8 @@ impl Opcode {
     }
 
     /// Create a new opcode from a direction, group, number and size.
+    ///
+    /// This corresponds to the C macro `_IOC(direction, group, number, size)`
     #[cfg(any(linux_kernel, bsd))]
     #[inline]
     pub const fn from_components(
@@ -227,6 +229,8 @@ impl Opcode {
 
     /// Create a new non-mutating opcode from a group, a number and the type of
     /// data.
+    ///
+    /// This corresponds to the C macro `_IO(group, number)` when `T` is zero sized.
     #[cfg(any(linux_kernel, bsd))]
     #[inline]
     pub const fn none<T>(group: u8, number: u8) -> Self {
@@ -235,6 +239,8 @@ impl Opcode {
 
     /// Create a new reading opcode from a group, a number and the type of
     /// data.
+    ///
+    /// This corresponds to the C macro `_IOR(group, number, T)`.
     #[cfg(any(linux_kernel, bsd))]
     #[inline]
     pub const fn read<T>(group: u8, number: u8) -> Self {
@@ -243,6 +249,8 @@ impl Opcode {
 
     /// Create a new writing opcode from a group, a number and the type of
     /// data.
+    ///
+    /// This corresponds to the C macro `_IOW(group, number, T)`.
     #[cfg(any(linux_kernel, bsd))]
     #[inline]
     pub const fn write<T>(group: u8, number: u8) -> Self {
@@ -251,6 +259,8 @@ impl Opcode {
 
     /// Create a new reading and writing opcode from a group, a number and the
     /// type of data.
+    ///
+    /// This corresponds to the C macro `_IOWR(group, number, T)`.
     #[cfg(any(linux_kernel, bsd))]
     #[inline]
     pub const fn read_write<T>(group: u8, number: u8) -> Self {

src/ioctl/patterns.rs

@@ -166,6 +166,8 @@ impl<const OPCODE: RawOpcode> CompileTimeOpcode for BadOpcode<OPCODE> {
 }
 
 /// Provides a read code at compile time.
+///
+/// This corresponds to the C macro `_IOR(GROUP, NUM, Data)`.
 #[cfg(any(linux_kernel, bsd))]
 pub struct ReadOpcode<const GROUP: u8, const NUM: u8, Data>(Data);
 
@@ -175,6 +177,8 @@ impl<const GROUP: u8, const NUM: u8, Data> CompileTimeOpcode for ReadOpcode<GROU
 }
 
 /// Provides a write code at compile time.
+///
+/// This corresponds to the C macro `_IOW(GROUP, NUM, Data)`.
 #[cfg(any(linux_kernel, bsd))]
 pub struct WriteOpcode<const GROUP: u8, const NUM: u8, Data>(Data);
 
@@ -184,6 +188,8 @@ impl<const GROUP: u8, const NUM: u8, Data> CompileTimeOpcode for WriteOpcode<GRO
 }
 
 /// Provides a read/write code at compile time.
+///
+/// This corresponds to the C macro `_IOWR(GROUP, NUM, Data)`.
 #[cfg(any(linux_kernel, bsd))]
 pub struct ReadWriteOpcode<const GROUP: u8, const NUM: u8, Data>(Data);
 
@@ -193,6 +199,8 @@ impl<const GROUP: u8, const NUM: u8, Data> CompileTimeOpcode for ReadWriteOpcode
 }
 
 /// Provides a `None` code at compile time.
+///
+/// This corresponds to the C macro `_IO(GROUP, NUM)` when `Data` is zero sized.
 #[cfg(any(linux_kernel, bsd))]
 pub struct NoneOpcode<const GROUP: u8, const NUM: u8, Data>(Data);