fix: fix undefined reference to `memset'

Cargo.toml

@@ -8,9 +8,3 @@ members = [
 
 [profile.release]
 strip = true
-
-[profile.dev.package.loader-shim]
-opt-level = 1
-
-[profile.release.package.loader-shim]
-opt-level = "s"

loader-shim/src/main.rs

@@ -17,7 +17,10 @@
 // system standard libraries.
 // See https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html
 //
-// Since _start is defined in the system startup files, with this option
+// The `-nostdlib` flag is much like a combination of `-nostartfiles` and
+// `-nodefaultlibs`.
+//
+// Since `_start` is defined in the system startup files, with this option
 // we can use our own `_start` function to override the program entry point.
 #[link_args = "-nostdlib"]
 #[link_args = "-ffreestanding"]
@@ -28,6 +31,18 @@
 #[cfg_attr(target_arch = "aarch64", link_args = "-Wl,-Ttext=0x2000000000")]
 extern "C" {}
 
+// The compiler may emit a call to the `memset()` function even if there is
+// no such call in our code. However, since we use `-nostdlib` or
+// `-nodefaultlibs`, this means we will not link to libc, which provides the
+// implementation of `memset()`.
+//
+// In this case, we will get an `undefined reference to \`memset'` error.
+// Fortunately, the crate `rlibc` provides an unoptimized implementation of
+// `memset()`.
+//
+// See `-nodefaultlibs` at https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html
+extern crate rlibc;
+
 mod script;
 
 use core::{fmt::Write, panic::PanicInfo};