Fix errors with char signedness. (#984)

When rustix is compiled with default-features = false, and linux-raw-sys
is compiled with no_std, there is a difference in the signedness of
`c_char`; fix rustix's code to handle this.

Fixes #983.

src/backend/linux_raw/net/netdevice.rs

@@ -1,20 +1,31 @@
 #![allow(unsafe_code)]
 
-#[cfg(feature = "alloc")]
-use crate::alloc::string::String;
 use crate::backend::io::syscalls::ioctl;
 use crate::fd::AsFd;
 use crate::io;
+use core::slice;
+use core::str;
+use linux_raw_sys::ctypes::c_char;
 use linux_raw_sys::ioctl::SIOCGIFINDEX;
 #[cfg(feature = "alloc")]
 use linux_raw_sys::ioctl::SIOCGIFNAME;
 use linux_raw_sys::net::{ifreq, ifreq__bindgen_ty_1, ifreq__bindgen_ty_2, IFNAMSIZ};
+#[cfg(feature = "alloc")]
+use {alloc::borrow::ToOwned, alloc::string::String};
 
 pub(crate) fn name_to_index(fd: impl AsFd, if_name: &str) -> io::Result<u32> {
     let if_name_bytes = if_name.as_bytes();
     if if_name_bytes.len() >= IFNAMSIZ as usize {
         return Err(io::Errno::NODEV);
     }
+    if if_name_bytes.contains(&0) {
+        return Err(io::Errno::NODEV);
+    }
+
+    // SAFETY: Convert `&[u8]` to `&[c_char]`.
+    let if_name_bytes = unsafe {
+        slice::from_raw_parts(if_name_bytes.as_ptr().cast::<c_char>(), if_name_bytes.len())
+    };
 
     let mut ifreq = ifreq {
         ifr_ifrn: ifreq__bindgen_ty_1 { ifrn_name: [0; 16] },
@@ -40,14 +51,17 @@ pub(crate) fn index_to_name(fd: impl AsFd, index: u32) -> io::Result<String> {
 
     if let Some(nul_byte) = unsafe { ifreq.ifr_ifrn.ifrn_name }
         .iter()
-        .position(|char| *char == 0)
+        .position(|ch| *ch == 0)
     {
-        let name = unsafe { ifreq.ifr_ifrn.ifrn_name }[..nul_byte]
-            .iter()
-            .map(|v| *v as char)
-            .collect();
+        let ifrn_name = unsafe { &ifreq.ifr_ifrn.ifrn_name[..nul_byte] };
+
+        // SAFETY: Convert `&[c_char]` to `&[u8]`.
+        let ifrn_name =
+            unsafe { slice::from_raw_parts(ifrn_name.as_ptr().cast::<u8>(), ifrn_name.len()) };
 
-        Ok(name)
+        str::from_utf8(ifrn_name)
+            .map_err(|_| io::Errno::ILSEQ)
+            .map(ToOwned::to_owned)
     } else {
         Err(io::Errno::INVAL)
     }