Add getrandom impl using ctru_sys bindings

Cargo.toml

@@ -6,4 +6,5 @@ license = "MIT/Apache 2.0"
 edition = "2018"
 
 [dependencies]
+ctru-sys = { git = "https://github.com/Meziu/ctru-rs.git" }
 libc = "0.2.116"

src/lib.rs

@@ -67,3 +67,33 @@ unsafe extern "C" fn clock_gettime(
 
     retval
 }
+
+#[no_mangle]
+unsafe extern "C" fn getrandom(
+    buf: *mut libc::c_void,
+    mut buflen: libc::size_t,
+    flags: libc::c_uint,
+) -> libc::ssize_t {
+    // TODO: is this needed? Maybe just `buflen = buflen.min(libc::ssize_t::MAX)` ?
+    buflen = buflen.min(0x1FFFFFF);
+
+    if flags != 0 {
+        // no flags are supported on 3DS
+        *__errno() = libc::EINVAL;
+        return -1;
+    }
+
+    let ret = ctru_sys::PS_GenerateRandomBytes(buf, buflen as libc::c_uint) as libc::ssize_t;
+    if ret < 0 {
+        // this is kind of a hack, but at least gives some visibility to the
+        // error code returned by PS_GenerateRandomBytes I guess? Another option
+        // might be to panic, which could use a payload of a specific error type
+        // that the ctru panic handler could decode into 3DS-specific human-readable
+        // errors.
+        *__errno() = ret as libc::c_int;
+        -1
+    } else {
+        // safe because above ensures buflen < isize::MAX
+        buflen as libc::ssize_t
+    }
+}