Add Padding type for uninitialized padding representation

.github/workflows/main.yml

@@ -54,6 +54,7 @@ jobs:
         cargo update --package=half --precise=2.2.1
         cargo update --package=flate2 --precise=1.0.35
         cargo update --package=serde --precise=1.0.228
+        cargo update --package=web-sys --precise=0.3.104
         cargo update --package=syn --precise=2.0.106
         cargo update --package=textwrap --precise=0.16.1
         cargo update --package=parking_lot --precise=0.12.4
@@ -596,6 +597,7 @@ jobs:
         cargo update --package=half --precise=2.2.1
         cargo update --package=flate2 --precise=1.0.35
         cargo update --package=serde --precise=1.0.228
+        cargo update --package=web-sys --precise=0.3.104
         cargo update --package=syn --precise=2.0.106
         cargo update --package=textwrap --precise=0.16.1
         cargo update --package=parking_lot --precise=0.12.4

src/backend/libc/fs/dir.rs

@@ -30,6 +30,8 @@ use crate::io;
 #[cfg(not(any(target_os = "fuchsia", target_os = "vita", target_os = "wasi")))]
 #[cfg(feature = "process")]
 use crate::process::fchdir;
+#[cfg(target_os = "openbsd")]
+use crate::types::Padding;
 use alloc::borrow::ToOwned as _;
 #[cfg(not(any(linux_like, target_os = "hurd")))]
 use c::readdir as libc_readdir;
@@ -408,7 +410,7 @@ struct libc_dirent {
     d_reclen: u16,
     d_type: u8,
     d_namlen: u8,
-    __d_padding: [u8; 4],
+    __d_padding: Padding<[u8; 4]>,
     d_name: [c::c_char; 256],
 }
 
@@ -434,7 +436,7 @@ fn check_dirent_layout(dirent: &c::dirent) {
                 d_reclen: 0_u16,
                 d_type: 0_u8,
                 d_namlen: 0_u8,
-                __d_padding: [0_u8; 4],
+                __d_padding: Default::default(),
                 d_name: [0 as c::c_char; 256],
             };
             let base = as_ptr(&z) as usize;

src/backend/libc/fs/types.rs

@@ -1,5 +1,7 @@
 use crate::backend::c;
 use crate::ffi;
+#[cfg(target_os = "netbsd")]
+use crate::types::Padding;
 use bitflags::bitflags;
 
 #[cfg(not(any(target_os = "espidf", target_os = "horizon", target_os = "vita")))]
@@ -1054,7 +1056,7 @@ pub struct Stat {
     pub st_blksize: c::blksize_t,
     pub st_flags: u32,
     pub st_gen: u32,
-    pub st_spare: [u32; 2],
+    pub(crate) st_spare: Padding<[u32; 2]>,
 }
 
 /// `struct statfs` for use with [`statfs`] and [`fstatfs`].

src/backend/linux_raw/fs/types.rs

@@ -1,4 +1,6 @@
 use crate::ffi;
+#[allow(unused_imports)]
+use crate::types::Padding;
 use bitflags::bitflags;
 
 bitflags! {
@@ -640,7 +642,7 @@ pub struct Stat {
     pub st_mode: ffi::c_uint,
     pub st_uid: ffi::c_uint,
     pub st_gid: ffi::c_uint,
-    pub(crate) __pad0: ffi::c_uint,
+    pub(crate) __pad0: Padding<ffi::c_uint>,
     pub st_rdev: ffi::c_ulong,
     pub st_size: ffi::c_long,
     pub st_blksize: ffi::c_long,
@@ -651,7 +653,7 @@ pub struct Stat {
     pub st_mtime_nsec: ffi::c_ulong,
     pub st_ctime: ffi::c_long,
     pub st_ctime_nsec: ffi::c_ulong,
-    pub(crate) __unused: [ffi::c_long; 3],
+    pub(crate) __unused: Padding<[ffi::c_long; 3]>,
 }
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
@@ -666,19 +668,19 @@ pub struct Stat {
     pub st_uid: ffi::c_uint,
     pub st_gid: ffi::c_uint,
     pub st_rdev: ffi::c_ulong,
-    pub(crate) __pad1: ffi::c_ulong,
+    pub(crate) __pad1: Padding<ffi::c_ulong>,
     pub st_size: ffi::c_long,
     pub st_blksize: ffi::c_int,
-    pub(crate) __pad2: ffi::c_int,
+    pub(crate) __pad2: Padding<ffi::c_int>,
     pub st_blocks: ffi::c_long,
     pub st_atime: ffi::c_long,
     pub st_atime_nsec: ffi::c_ulong,
     pub st_mtime: ffi::c_long,
     pub st_mtime_nsec: ffi::c_ulong,
     pub st_ctime: ffi::c_long,
     pub st_ctime_nsec: ffi::c_ulong,
-    pub(crate) __unused4: ffi::c_uint,
-    pub(crate) __unused5: ffi::c_uint,
+    pub(crate) __unused4: Padding<ffi::c_uint>,
+    pub(crate) __unused5: Padding<ffi::c_uint>,
 }
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
@@ -693,19 +695,19 @@ pub struct Stat {
     pub st_uid: ffi::c_uint,
     pub st_gid: ffi::c_uint,
     pub st_rdev: ffi::c_ulong,
-    pub(crate) __pad1: ffi::c_ulong,
+    pub(crate) __pad1: Padding<ffi::c_ulong>,
     pub st_size: ffi::c_long,
     pub st_blksize: ffi::c_int,
-    pub(crate) __pad2: ffi::c_int,
+    pub(crate) __pad2: Padding<ffi::c_int>,
     pub st_blocks: ffi::c_long,
     pub st_atime: ffi::c_long,
     pub st_atime_nsec: ffi::c_ulong,
     pub st_mtime: ffi::c_long,
     pub st_mtime_nsec: ffi::c_ulong,
     pub st_ctime: ffi::c_long,
     pub st_ctime_nsec: ffi::c_ulong,
-    pub(crate) __unused4: ffi::c_uint,
-    pub(crate) __unused5: ffi::c_uint,
+    pub(crate) __unused4: Padding<ffi::c_uint>,
+    pub(crate) __unused5: Padding<ffi::c_uint>,
 }
 // This follows `stat`. powerpc64 defines a `stat64` but it's not used.
 #[repr(C)]
@@ -730,9 +732,9 @@ pub struct Stat {
     pub st_mtime_nsec: ffi::c_ulong,
     pub st_ctime: ffi::c_long,
     pub st_ctime_nsec: ffi::c_ulong,
-    pub(crate) __unused4: ffi::c_ulong,
-    pub(crate) __unused5: ffi::c_ulong,
-    pub(crate) __unused6: ffi::c_ulong,
+    pub(crate) __unused4: Padding<ffi::c_ulong>,
+    pub(crate) __unused5: Padding<ffi::c_ulong>,
+    pub(crate) __unused6: Padding<ffi::c_ulong>,
 }
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
@@ -746,7 +748,7 @@ pub struct Stat {
     pub st_mode: ffi::c_uint,
     pub st_uid: ffi::c_uint,
     pub st_gid: ffi::c_uint,
-    pub(crate) __pad1: ffi::c_uint,
+    pub(crate) __pad1: Padding<ffi::c_uint>,
     pub st_rdev: ffi::c_ulong,
     pub st_size: ffi::c_long, // Linux has `c_ulong` but we make it signed.
     pub st_atime: ffi::c_long,
@@ -757,7 +759,7 @@ pub struct Stat {
     pub st_ctime_nsec: ffi::c_ulong,
     pub st_blksize: ffi::c_ulong,
     pub st_blocks: ffi::c_long,
-    pub(crate) __unused: [ffi::c_ulong; 3],
+    pub(crate) __unused: Padding<[ffi::c_ulong; 3]>,
 }
 
 /// `struct statfs` for use with [`statfs`] and [`fstatfs`].
@@ -795,9 +797,9 @@ pub struct StatFs {
     #[cfg(any(target_arch = "arm", target_arch = "s390x"))]
     pub f_flags: ffi::c_uint,
     #[cfg(not(target_arch = "s390x"))]
-    pub(crate) f_spare: [ffi::c_long; 4],
+    pub(crate) f_spare: Padding<[ffi::c_long; 4]>,
     #[cfg(target_arch = "s390x")]
-    pub(crate) f_spare: [ffi::c_uint; 5],
+    pub(crate) f_spare: Padding<[ffi::c_uint; 5]>,
 }
 
 /// `fsid_t` for use with [`StatFs`].

src/event/epoll.rs

@@ -80,6 +80,8 @@ use crate::buffer::Buffer;
 use crate::fd::{AsFd, OwnedFd};
 use crate::io;
 use crate::timespec::Timespec;
+#[cfg(target_pointer_width = "32")]
+use crate::types::Padding;
 use core::ffi::c_void;
 use core::hash::{Hash, Hasher};
 
@@ -276,7 +278,7 @@ impl EventData {
             sixty_four_bit_pointer: SixtyFourBitPointer {
                 pointer: value,
                 #[cfg(target_pointer_width = "32")]
-                _padding: 0,
+                _padding: Padding::new(0),
             },
         }
     }
@@ -321,13 +323,13 @@ impl Hash for EventData {
 struct SixtyFourBitPointer {
     #[cfg(target_endian = "big")]
     #[cfg(target_pointer_width = "32")]
-    _padding: u32,
+    _padding: Padding<u32>,
 
     pointer: *mut c_void,
 
     #[cfg(target_endian = "little")]
     #[cfg(target_pointer_width = "32")]
-    _padding: u32,
+    _padding: Padding<u32>,
 }
 
 #[cfg(test)]

src/fs/statx.rs

@@ -2,6 +2,7 @@
 
 use crate::fd::AsFd;
 use crate::fs::AtFlags;
+use crate::types::Padding;
 use crate::{backend, io, path};
 use backend::c;
 use bitflags::bitflags;
@@ -24,7 +25,7 @@ pub struct Statx {
     pub stx_uid: u32,
     pub stx_gid: u32,
     pub stx_mode: u16,
-    pub(crate) __spare0: [u16; 1],
+    pub(crate) __spare0: Padding<[u16; 1]>,
     pub stx_ino: u64,
     pub stx_size: u64,
     pub stx_blocks: u64,
@@ -46,8 +47,8 @@ pub struct Statx {
     pub stx_atomic_write_segments_max: u32,
     pub stx_dio_read_offset_align: u32,
     pub stx_atomic_write_unit_max_opt: u32,
-    pub __spare2: [u32; 1usize],
-    pub __spare3: [u64; 8usize],
+    pub(crate) __spare2: Padding<[u32; 1usize]>,
+    pub(crate) __spare3: Padding<[u64; 8usize]>,
 }
 
 /// `struct statx_timestamp` for use with [`Statx`].
@@ -61,7 +62,7 @@ pub struct StatxTimestamp {
     /// Nanoseconds. Must be less than 1_000_000_000.
     pub tv_nsec: u32,
 
-    pub(crate) __reserved: i32,
+    pub(crate) __reserved: Padding<i32>,
 }
 
 bitflags! {

src/lib.rs

@@ -178,6 +178,8 @@ pub(crate) mod bitcast;
 #[cfg(sanitize_memory)]
 pub(crate) mod msan;
 
+mod types;
+
 // linux_raw: Weak symbols are used by the use-libc-auxv feature for
 // glibc 2.15 support.
 //

src/termios/types.rs

@@ -2,6 +2,8 @@ use crate::backend::c;
 use crate::backend::termios::types;
 #[cfg(target_os = "nto")]
 use crate::ffi;
+#[cfg(target_os = "nto")]
+use crate::types::Padding;
 use crate::{backend, io};
 use bitflags::bitflags;
 
@@ -46,7 +48,7 @@ pub struct Termios {
     pub special_codes: SpecialCodes,
 
     #[cfg(target_os = "nto")]
-    pub(crate) __reserved: [ffi::c_uint; 3],
+    pub(crate) __reserved: Padding<[ffi::c_uint; 3]>,
 
     /// Line discipline.
     // On PowerPC, this field comes after `c_cc`.

src/thread/futex.rs

@@ -34,6 +34,7 @@ use core::sync::atomic::AtomicU32;
 use crate::backend::thread::futex::Operation;
 use crate::backend::thread::syscalls::{futex_timeout, futex_val2};
 use crate::fd::{FromRawFd as _, OwnedFd, RawFd};
+use crate::types::Padding;
 use crate::{backend, io};
 
 pub use crate::clockid::ClockId;
@@ -532,7 +533,7 @@ pub struct Wait {
     pub flags: WaitFlags,
 
     /// Reserved for future use.
-    pub(crate) __reserved: u32,
+    pub(crate) __reserved: Padding<u32>,
 }
 
 impl Wait {
@@ -543,7 +544,7 @@ impl Wait {
             val: 0,
             uaddr: WaitPtr::new(ptr::null_mut()),
             flags: WaitFlags::empty(),
-            __reserved: 0,
+            __reserved: Padding::new(0),
         }
     }
 }

src/types.rs

@@ -0,0 +1,40 @@
+//! Platform-agnostic support types.
+//!
+//! Copied from https://github.com/rust-lang/libc/blob/008f905256d27eb5dd416d0976678ee6c2855554/src/types.rs
+
+use core::fmt;
+use core::mem::MaybeUninit;
+
+/// A transparent wrapper over `MaybeUninit<T>` to represent uninitialized padding
+/// while providing `Default`.
+// This is restricted to `Copy` types since that's a loose indicator that zeros is actually
+// a valid bitpattern. There is no technical reason this is required, though, so it could be
+// lifted in the future if it becomes a problem.
+#[allow(dead_code)]
+#[repr(transparent)]
+#[derive(Clone, Copy)]
+pub(crate) struct Padding<T: Copy>(MaybeUninit<T>);
+
+impl<T: Copy> Default for Padding<T> {
+    fn default() -> Self {
+        Self(MaybeUninit::zeroed())
+    }
+}
+
+impl<T: Copy> Padding<T> {
+    /// Create a `Padding` initialized with the given value.
+    #[allow(dead_code)]
+    pub(crate) const fn new(val: T) -> Self {
+        Self(MaybeUninit::new(val))
+    }
+}
+
+impl<T: Copy> fmt::Debug for Padding<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Taken from `MaybeUninit`'s debug implementation
+        // NB: there is no `.pad_fmt` so we can't use a simpler `format_args!("Padding<{..}>").
+        let full_name = core::any::type_name::<Self>();
+        let prefix_len = full_name.find("Padding").unwrap();
+        f.pad(&full_name[prefix_len..])
+    }
+}