Fix c_char vs u8 errors in the linux_raw backend. (#971)

* Fix `c_char` vs `u8` errors in the linux_raw backend.

Fix the linux_raw backend to handle the case where linux-raw-sys defines
`c_char` as `i8`.

linux-raw-sys used to always define `c_char` as `u8` because it usually
isn't important to match the platform `char` type, but that makes it
inconvenient to work with Rust's `CStr`/`CString` types which use `c_char`,
so linux-raw-sys has started defining `c_char` as `i8` on platforms
which define it that way.

* Fix redundant `unsafe` blocks.

src/backend/linux_raw/net/addr.rs

@@ -12,6 +12,7 @@ use crate::{io, path};
 use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{Hash, Hasher};
+use core::slice;
 
 /// `struct sockaddr_un`
 #[derive(Clone)]
@@ -36,7 +37,7 @@ impl SocketAddrUnix {
             return Err(io::Errno::NAMETOOLONG);
         }
         for (i, b) in bytes.iter().enumerate() {
-            unix.sun_path[i] = *b;
+            unix.sun_path[i] = *b as _;
         }
         let len = offsetof_sun_path() + bytes.len();
         let len = len.try_into().unwrap();
@@ -48,6 +49,10 @@ impl SocketAddrUnix {
     pub fn new_abstract_name(name: &[u8]) -> io::Result<Self> {
         let mut unix = Self::init();
         let id = &mut unix.sun_path[1..];
+
+        // SAFETY: Convert `&mut [c_char]` to `&mut [u8]`.
+        let id = unsafe { slice::from_raw_parts_mut(id.as_mut_ptr().cast::<u8>(), id.len()) };
+
         if let Some(id) = id.get_mut(..name.len()) {
             id.copy_from_slice(name);
             let len = offsetof_sun_path() + 1 + name.len();
@@ -69,9 +74,13 @@ impl SocketAddrUnix {
     #[inline]
     pub fn path(&self) -> Option<&CStr> {
         let len = self.len();
-        if len != 0 && self.unix.sun_path[0] != b'\0' {
+        if len != 0 && self.unix.sun_path[0] as u8 != b'\0' {
             let end = len as usize - offsetof_sun_path();
             let bytes = &self.unix.sun_path[..end];
+
+            // SAFETY: Convert `&[c_char]` to `&[u8]`.
+            let bytes = unsafe { slice::from_raw_parts(bytes.as_ptr().cast::<u8>(), bytes.len()) };
+
             // SAFETY: `from_bytes_with_nul_unchecked` since the string is
             // NUL-terminated.
             unsafe { Some(CStr::from_bytes_with_nul_unchecked(bytes)) }
@@ -84,9 +93,14 @@ impl SocketAddrUnix {
     #[inline]
     pub fn abstract_name(&self) -> Option<&[u8]> {
         let len = self.len();
-        if len != 0 && self.unix.sun_path[0] == b'\0' {
+        if len != 0 && self.unix.sun_path[0] as u8 == b'\0' {
             let end = len as usize - offsetof_sun_path();
-            Some(&self.unix.sun_path[1..end])
+            let bytes = &self.unix.sun_path[1..end];
+
+            // SAFETY: Convert `&[c_char]` to `&[u8]`.
+            let bytes = unsafe { slice::from_raw_parts(bytes.as_ptr().cast::<u8>(), bytes.len()) };
+
+            Some(bytes)
         } else {
             None
         }

src/backend/linux_raw/net/read_sockaddr.rs

@@ -6,6 +6,7 @@ use crate::backend::c;
 use crate::io;
 use crate::net::{Ipv4Addr, Ipv6Addr, SocketAddrAny, SocketAddrUnix, SocketAddrV4, SocketAddrV6};
 use core::mem::size_of;
+use core::slice;
 
 // This must match the header of `sockaddr`.
 #[repr(C)]
@@ -93,17 +94,22 @@ pub(crate) unsafe fn read_sockaddr(
                 //
                 // [abstract namespace]: https://man7.org/linux/man-pages/man7/unix.7.html
                 if decode.sun_path[0] == 0 {
-                    return SocketAddrUnix::new_abstract_name(
-                        &decode.sun_path[1..len - offsetof_sun_path],
-                    )
-                    .map(SocketAddrAny::Unix);
+                    let bytes = &decode.sun_path[1..len - offsetof_sun_path];
+
+                    // SAFETY: Convert `&[c_char]` to `&[u8]`.
+                    let bytes = slice::from_raw_parts(bytes.as_ptr().cast::<u8>(), bytes.len());
+
+                    return SocketAddrUnix::new_abstract_name(bytes).map(SocketAddrAny::Unix);
                 }
 
                 // Otherwise we expect a NUL-terminated filesystem path.
+                let bytes = &decode.sun_path[..len - 1 - offsetof_sun_path];
+
+                // SAFETY: Convert `&[c_char]` to `&[u8]`.
+                let bytes = slice::from_raw_parts(bytes.as_ptr().cast::<u8>(), bytes.len());
+
                 assert_eq!(decode.sun_path[len - 1 - offsetof_sun_path], 0);
-                Ok(SocketAddrAny::Unix(SocketAddrUnix::new(
-                    &decode.sun_path[..len - 1 - offsetof_sun_path],
-                )?))
+                Ok(SocketAddrAny::Unix(SocketAddrUnix::new(bytes)?))
             }
         }
         _ => Err(io::Errno::NOTSUP),
@@ -165,19 +171,23 @@ pub(crate) unsafe fn read_sockaddr_os(storage: *const c::sockaddr, len: usize) -
                 //
                 // [abstract namespace]: https://man7.org/linux/man-pages/man7/unix.7.html
                 if decode.sun_path[0] == 0 {
-                    return SocketAddrAny::Unix(
-                        SocketAddrUnix::new_abstract_name(
-                            &decode.sun_path[1..len - offsetof_sun_path],
-                        )
-                        .unwrap(),
-                    );
+                    let bytes = &decode.sun_path[1..len - offsetof_sun_path];
+
+                    // SAFETY: Convert `&[c_char]` to `&[u8]`.
+                    let bytes = slice::from_raw_parts(bytes.as_ptr().cast::<u8>(), bytes.len());
+
+                    return SocketAddrAny::Unix(SocketAddrUnix::new_abstract_name(bytes).unwrap());
                 }
 
                 // Otherwise we expect a NUL-terminated filesystem path.
                 assert_eq!(decode.sun_path[len - 1 - offsetof_sun_path], 0);
-                SocketAddrAny::Unix(
-                    SocketAddrUnix::new(&decode.sun_path[..len - 1 - offsetof_sun_path]).unwrap(),
-                )
+
+                let bytes = &decode.sun_path[..len - 1 - offsetof_sun_path];
+
+                // SAFETY: Convert `&[c_char]` to `&[u8]`.
+                let bytes = slice::from_raw_parts(bytes.as_ptr().cast::<u8>(), bytes.len());
+
+                SocketAddrAny::Unix(SocketAddrUnix::new(bytes).unwrap())
             }
         }
         other => unimplemented!("{:?}", other),