This adds a getpid shim

src/shims/env.rs

@@ -465,4 +465,40 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         Ok(())
     }
+
+    fn getpid(&mut self) -> InterpResult<'tcx, i32> {
+        let this = self.eval_context_mut();
+        let target_os = &this.tcx.sess.target.os;
+        assert!(
+            target_os == "linux" || target_os == "macos",
+            "`getpid` is only available for the UNIX target family"
+        );
+
+        if let IsolatedOp::Reject(reject_with) = this.machine.isolated_op {
+            this.reject_in_isolation("getpid", reject_with)?;
+            this.set_last_error_from_io_error(ErrorKind::PermissionDenied)?;
+
+            return Ok(-1);
+        }
+
+        // The reason we need to do this wacky of a conversion is because
+        // `libc::getpid` returns an i32, however, `std::process::id()` return an u32,
+        // So we need to fit that u32 into an i32 and pray
+        Ok(std::process::id() as i32)
+    }
+
+    #[allow(non_snake_case)]
+    fn GetCurrentProcessId(&mut self) -> InterpResult<'tcx, i32> {
+        let this = self.eval_context_mut();
+        this.assert_target_os("windows", "GetProcessId");
+
+        if let IsolatedOp::Reject(reject_with) = this.machine.isolated_op {
+            this.reject_in_isolation("`GetProcessId`", reject_with)?;
+            this.set_last_error_from_io_error(ErrorKind::PermissionDenied)?;
+            return Ok(0);
+        }
+
+        Ok(std::process::id() as i32)
+    }
 }
+

src/shims/unix/foreign_items.rs

@@ -474,6 +474,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.write_null(dest)?;
             }
 
+            "getpid" => {
+                let result = this.getpid()?;
+                this.write_scalar(Scalar::from_i32(result), dest)?;
+            }
+
             // Platform-specific shims
             _ => {
                 match this.tcx.sess.target.os.as_ref() {

src/shims/windows/foreign_items.rs

@@ -419,6 +419,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 let [] = this.check_shim(abi, Abi::System { unwind: false }, link_name, args)?;
                 this.write_scalar(Scalar::from_machine_isize(1, this), dest)?;
             }
+            "GetCurrentProcessId" if this.frame_in_std() => {
+                let result = this.GetCurrentProcessId()?;
+                this.write_scalar(Scalar::from_i32(result), dest)?;
+            }
 
             _ => return Ok(EmulateByNameResult::NotSupported),
         }