uptime: construct a raw buffer for the sysinfo syscall; drop libc::sysinfo

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I63887d9f73e6fed670e497086d30d2a46a6a6964

src/uptime.rs

@@ -17,14 +17,41 @@ fn itoa(mut n: u64, buf: &mut [u8]) -> &str {
   unsafe { std::str::from_utf8_unchecked(&buf[i..]) }
 }
 
-/// Direct `sysinfo` syscall using inline assembly
+/// Raw buffer for the `sysinfo(2)` syscall.
+///
+/// In the Linux ABI `uptime` is a `long` at offset 0. The remaining fields are
+/// not needed, but are declared to give the struct its correct size (112 bytes
+/// on 64-bit Linux).
+///
+/// The layout matches the kernel's `struct sysinfo` *exactly*:
+/// `mem_unit` ends at offset 108, then 4 bytes of implicit padding to 112.
+#[repr(C)]
+struct SysInfo {
+  uptime:    i64,
+  loads:     [u64; 3],
+  totalram:  u64,
+  freeram:   u64,
+  sharedram: u64,
+  bufferram: u64,
+  totalswap: u64,
+  freeswap:  u64,
+  procs:     u16,
+  _pad:      u16,
+  _pad2:     u32, // alignment padding to reach 8-byte boundary for totalhigh
+  totalhigh: u64,
+  freehigh:  u64,
+  mem_unit:  u32,
+  // 4 bytes implicit trailing padding to reach 112 bytes total; no field
+  // needed
+}
+
+/// Direct `sysinfo(2)` syscall using inline assembly
 ///
 /// # Safety
 ///
-/// This function uses inline assembly to make a direct syscall.
 /// The caller must ensure the sysinfo pointer is valid.
 #[inline]
-unsafe fn sys_sysinfo(info: *mut libc::sysinfo) -> i64 {
+unsafe fn sys_sysinfo(info: *mut SysInfo) -> i64 {
   #[cfg(target_arch = "x86_64")]
   {
     let ret: i64;
@@ -59,7 +86,7 @@ unsafe fn sys_sysinfo(info: *mut libc::sysinfo) -> i64 {
 
   #[cfg(not(any(target_arch = "x86_64", target_arch = "aarch64")))]
   {
-    unsafe { libc::sysinfo(info) as i64 }
+    compile_error!("Unsupported architecture for inline assembly syscalls");
   }
 }