Force userspace to do post-syscall EINTR check.

src/syscall/mod.rs

@@ -67,7 +67,7 @@ pub fn syscall(
     e: usize,
     f: usize,
     stack: &mut InterruptStack,
-) {
+) -> usize {
     #[inline(always)]
     fn inner(
         a: usize,
@@ -247,13 +247,5 @@ pub fn syscall(
     PercpuBlock::current().inside_syscall.set(false);
 
     // errormux turns Result<usize> into -errno
-    stack.set_syscall_ret_reg(Error::mux(result));
-
-    if result == Err(Error::new(EINTR)) {
-        // Although it would be cleaner to simply run the signal trampoline right after switching
-        // back to any given context, where the signal set/queue is nonempty, syscalls need to
-        // complete *before* any signal is delivered. Otherwise the return value would probably be
-        // overwritten.
-        crate::context::signal::signal_handler();
-    }
+    Error::mux(result)
 }