treat prctl like a variadic function

src/shims/posix/fs.rs

@@ -921,7 +921,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         dirfd_op: &OpTy<'tcx, Tag>,    // Should be an `int`
         pathname_op: &OpTy<'tcx, Tag>, // Should be a `const char *`
         flags_op: &OpTy<'tcx, Tag>,    // Should be an `int`
-        mask_op: &OpTy<'tcx, Tag>,    // Should be an `unsigned int`
+        mask_op: &OpTy<'tcx, Tag>,     // Should be an `unsigned int`
         statxbuf_op: &OpTy<'tcx, Tag>, // Should be a `struct statx *`
     ) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();

src/shims/posix/linux/foreign_items.rs

@@ -106,9 +106,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
             // Threading
             "prctl" => {
-                let &[ref option, ref arg2, ref arg3, ref arg4, ref arg5] =
-                    this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
-                let result = this.prctl(option, arg2, arg3, arg4, arg5)?;
+                // prctl is variadic. (It is not documented like that in the manpage, but defined like that in the libc crate.)
+                this.check_abi_and_shim_symbol_clash(abi, Abi::C { unwind: false }, link_name)?;
+                let result = this.prctl(args)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_condattr_setclock" => {

src/shims/posix/thread.rs

@@ -97,28 +97,42 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         this.write_scalar(Scalar::from_uint(thread_id.to_u32(), dest.layout.size), dest)
     }
 
-    fn prctl(
-        &mut self,
-        option: &OpTy<'tcx, Tag>,
-        arg2: &OpTy<'tcx, Tag>,
-        _arg3: &OpTy<'tcx, Tag>,
-        _arg4: &OpTy<'tcx, Tag>,
-        _arg5: &OpTy<'tcx, Tag>,
-    ) -> InterpResult<'tcx, i32> {
+    fn prctl(&mut self, args: &[OpTy<'tcx, Tag>]) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
         this.assert_target_os("linux", "prctl");
 
-        let option = this.read_scalar(option)?.to_i32()?;
+        if args.len() < 1 {
+            throw_ub_format!(
+                "incorrect number of arguments for `prctl`: got {}, expected at least 1",
+                args.len()
+            );
+        }
+
+        let option = this.read_scalar(&args[0])?.to_i32()?;
         if option == this.eval_libc_i32("PR_SET_NAME")? {
-            let address = this.read_pointer(arg2)?;
+            if args.len() < 2 {
+                throw_ub_format!(
+                    "incorrect number of arguments for `prctl` with `PR_SET_NAME`: got {}, expected at least 2",
+                    args.len()
+                );
+            }
+
+            let address = this.read_pointer(&args[1])?;
             let mut name = this.read_c_str(address)?.to_owned();
             // The name should be no more than 16 bytes, including the null
             // byte. Since `read_c_str` returns the string without the null
             // byte, we need to truncate to 15.
             name.truncate(15);
             this.set_active_thread_name(name);
         } else if option == this.eval_libc_i32("PR_GET_NAME")? {
-            let address = this.read_pointer(arg2)?;
+            if args.len() < 2 {
+                throw_ub_format!(
+                    "incorrect number of arguments for `prctl` with `PR_SET_NAME`: got {}, expected at least 2",
+                    args.len()
+                );
+            }
+
+            let address = this.read_pointer(&args[1])?;
             let mut name = this.get_active_thread_name().to_vec();
             name.push(0u8);
             assert!(name.len() <= 16);

tests/compile-fail/fs/unix_open_missing_required_mode.rs

@@ -12,5 +12,5 @@ fn main() {
 fn test_file_open_missing_needed_mode() {
     let name = b"missing_arg.txt\0";
     let name_ptr = name.as_ptr().cast::<libc::c_char>();
-    let _fd = unsafe { libc::open(name_ptr, libc::O_CREAT) }; //~ ERROR Undefined Behavior: incorrect number of arguments for `open` with `O_CREAT`: got 2, expected 3
+    let _fd = unsafe { libc::open(name_ptr, libc::O_CREAT) }; //~ ERROR Undefined Behavior: incorrect number of arguments for `open` with `O_CREAT`: got 2, expected at least 3
 }