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 +67,45 @@ 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 {
+    // Based on https://man7.org/linux/man-pages/man2/getrandom.2.html
+    // Technically we only have one source (no true /dev/random), but the
+    // behavior should be more expected this way.
+    let maxlen = if flags & libc::GRND_RANDOM != 0 {
+        512
+    } else {
+        0x1FFFFFF
+    };
+    buflen = buflen.min(maxlen);
+
+    let ret = ctru_sys::PS_GenerateRandomBytes(buf, buflen as libc::c_uint);
+
+    // avoid conflicting a real POSIX errno by using a value < 0
+    // should we define this in ctru-sys somewhere or something?
+    const ECTRU: libc::c_int = -1;
+
+    if ctru_sys::R_SUCCEEDED(ret) {
+        // safe because above ensures buflen < isize::MAX
+        buflen as libc::ssize_t
+    } else {
+        // best-effort attempt at translating return codes
+        *__errno() = match ctru_sys::R_SUMMARY(ret) as libc::c_uint {
+            ctru_sys::RS_WOULDBLOCK => libc::EAGAIN,
+            ctru_sys::RS_INVALIDARG | ctru_sys::RS_WRONGARG => {
+                match ctru_sys::R_DESCRIPTION(ret) as libc::c_uint {
+                    // most likely user error, forgot to initialize PS module
+                    ctru_sys::RD_INVALID_HANDLE => ECTRU,
+                    _ => libc::EINVAL,
+                }
+            }
+            _ => ECTRU,
+        };
+        -1
+    }
+}