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src/shims/fs.rs

@@ -140,6 +140,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.handle_not_found()
             }
         } else if cmd == this.eval_libc_i32("F_DUPFD")? || cmd == this.eval_libc_i32("F_DUPFD_CLOEXEC")? {
+            // Note that we always assume the FD_CLOEXEC flag is set for every open file, in part
+            // because exec() isn't supported. The F_DUPFD and F_DUPFD_CLOEXEC commands only
+            // differ in whether the FD_CLOEXEC flag is pre-set on the duplicated file descriptor,
+            // thus they can share the same implementation here.
             let arg = match arg_op {
                 Some(arg_op) => this.read_scalar(arg_op)?.to_i32()?,
                 None => throw_unsup_format!("fcntl with command F_DUPFD or F_DUPFD_CLOEXEC requires a third argument"),