check that all syscall arguments are scalars

src/bin/miri.rs

@@ -221,7 +221,7 @@ fn main() {
                             ),
                             FromHexError::OddLength =>
                                 panic!("-Zmiri-seed should have an even number of digits"),
-                            err => panic!("Unknown error decoding -Zmiri-seed as hex: {:?}", err),
+                            err => panic!("unknown error decoding -Zmiri-seed as hex: {:?}", err),
                         });
                     if seed_raw.len() > 8 {
                         panic!(format!(

src/shims/posix/linux/foreign_items.rs

@@ -113,16 +113,14 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
             // Dynamically invoked syscalls
             "syscall" => {
-                // FIXME: The libc syscall() function is a variadic function.
-                // It's valid to call it with more arguments than a syscall
-                // needs, so none of these syscalls should use check_arg_count.
-                // It's even valid to call it with the wrong type of arguments,
-                // as long as they'd end up in the same place with the calling
-                // convention used. (E.g. using a `usize` instead of a pointer.)
-                // It's not directly clear which number, size, and type of arguments
-                // are acceptable in which cases and which aren't. (E.g. some
-                // types might take up the space of two registers.)
-                // So this needs to be researched first.
+                // The syscall variadic function is legal to call with more arguments than needed,
+                // extra arguments are simply ignored. However, all arguments need to be scalars;
+                // other types might be treated differently by the calling convention.
+                for arg in args {
+                    if !matches!(arg.layout.abi, rustc_target::abi::Abi::Scalar(_)) {
+                        throw_ub_format!("`syscall` arguments must all have scalar layout, but {} does not", arg.layout.ty);
+                    }
+                }
 
                 let sys_getrandom = this
                     .eval_libc("SYS_getrandom")?
@@ -144,22 +142,26 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     // is called if a `HashMap` is created the regular way (e.g. HashMap<K, V>).
                     id if id == sys_getrandom => {
                         // The first argument is the syscall id, so skip over it.
-                        let &[_, ptr, len, flags] = check_arg_count(args)?;
-                        getrandom(this, ptr, len, flags, dest)?;
+                        if args.len() < 4 {
+                            throw_ub_format!("incorrect number of arguments for `getrandom` syscall: got {}, expected at least 4", args.len());
+                        }
+                        getrandom(this, args[1], args[2], args[3], dest)?;
                     }
                     // `statx` is used by `libstd` to retrieve metadata information on `linux`
                     // instead of using `stat`,`lstat` or `fstat` as on `macos`.
                     id if id == sys_statx => {
                         // The first argument is the syscall id, so skip over it.
-                        let &[_, dirfd, pathname, flags, mask, statxbuf] = check_arg_count(args)?;
-                        let result = this.linux_statx(dirfd, pathname, flags, mask, statxbuf)?;
+                        if args.len() < 6 {
+                            throw_ub_format!("incorrect number of arguments for `statx` syscall: got {}, expected at least 6", args.len());
+                        }
+                        let result = this.linux_statx(args[1], args[2], args[3], args[4], args[5])?;
                         this.write_scalar(Scalar::from_machine_isize(result.into(), this), dest)?;
                     }
                     // `futex` is used by some synchonization primitives.
                     id if id == sys_futex => {
                         futex(this, args, dest)?;
                     }
-                    id => throw_unsup_format!("miri does not support syscall ID {}", id),
+                    id => throw_unsup_format!("Miri does not support syscall ID {}", id),
                 }
             }
 

src/shims/posix/linux/sync.rs

@@ -17,8 +17,8 @@ pub fn futex<'tcx>(
     // may or may not be left out from the `syscall()` call.
     // Therefore we don't use `check_arg_count` here, but only check for the
     // number of arguments to fall within a range.
-    if !(4..=7).contains(&args.len()) {
-        throw_ub_format!("incorrect number of arguments for futex syscall: got {}, expected between 4 and 7 (inclusive)", args.len());
+    if args.len() < 4 {
+        throw_ub_format!("incorrect number of arguments for `futex` syscall: got {}, expected at least 4", args.len());
     }
 
     // The first three arguments (after the syscall number itself) are the same to all futex operations:
@@ -49,13 +49,13 @@ pub fn futex<'tcx>(
         // or *timeout expires. `timeout == null` for an infinite timeout.
         op if op & !futex_realtime == futex_wait => {
             if args.len() < 5 {
-                throw_ub_format!("incorrect number of arguments for FUTEX_WAIT syscall: got {}, expected at least 5", args.len());
+                throw_ub_format!("incorrect number of arguments for `futex` syscall with `op=FUTEX_WAIT`: got {}, expected at least 5", args.len());
             }
             let timeout = args[4];
             let timeout_time = if this.is_null(this.read_scalar(timeout)?.check_init()?)? {
                 None
             } else {
-                this.check_no_isolation("`syscall(SYS_FUTEX, op=FUTEX_WAIT)` with timeout")?;
+                this.check_no_isolation("`futex` syscall with `op=FUTEX_WAIT` and non-null timeout")?;
                 let duration = match this.read_timespec(timeout)? {
                     Some(duration) => duration,
                     None => {
@@ -126,7 +126,7 @@ pub fn futex<'tcx>(
             }
             this.write_scalar(Scalar::from_machine_isize(n, this), dest)?;
         }
-        op => throw_unsup_format!("miri does not support SYS_futex operation {}", op),
+        op => throw_unsup_format!("Miri does not support `futex` syscall with op={}", op),
     }
 
     Ok(())