syscall: Implement exec()

Implement the EXEC syscall interface in user space.

Co-developed-by: Vijay Dhanraj <[email protected]>
Signed-off-by: Peter Fang <[email protected]>

syscall/src/class0.rs

@@ -4,8 +4,9 @@
 //
 // Author: Chuanxiao Dong <[email protected]>
 
-use super::call::syscall1;
-use super::SYS_EXIT;
+use super::call::{syscall1, syscall3, SysCallError};
+use super::{SYS_EXEC, SYS_EXIT};
+use core::ffi::CStr;
 
 pub fn exit(code: u32) -> ! {
     // SAFETY: SYS_EXIT is supported syscall number by the svsm kernel.
@@ -14,3 +15,19 @@ pub fn exit(code: u32) -> ! {
     }
     unreachable!("Should never return from SYS_EXIT syscall");
 }
+
+#[allow(dead_code)]
+#[derive(Debug)]
+pub struct Tid(u32);
+
+pub fn exec(file: &CStr, root: &CStr, flags: u32) -> Result<Tid, SysCallError> {
+    unsafe {
+        syscall3(
+            SYS_EXEC,
+            file.as_ptr() as u64,
+            root.as_ptr() as u64,
+            u64::from(flags),
+        )
+        .map(|ret| Tid(ret as u32))
+    }
+}