Re: [PATCH v9 28/42] x86/shstk: Add user-mode shadow stack support

On 6/27/23 10:20, Mark Brown wrote:

On Mon, Jun 12, 2023 at 05:10:54PM -0700, Rick Edgecombe wrote:

quoted

+static void unmap_shadow_stack(u64 base, u64 size)
+{
+	while (1) {
+		int r;
+
+		r = vm_munmap(base, size);
+
+		/*
+		 * vm_munmap() returns -EINTR when mmap_lock is held by
+		 * something else, and that lock should not be held for a
+		 * long time.  Retry it for the case.
+		 */
+		if (r == -EINTR) {
+			cond_resched();
+			continue;
+		}

This looks generic, not even shadow stack specific - was there any
discussion of making it a vm_munmap_retry() (that's not a great name...)
or similar?  I didn't see any in old versions of the thread but I
might've missed something.

Yeah, that looks odd.  Also odd is that none of the other users of
vm_munmap() bother to check the return value (except for one passthrough
in the nommu code).

I don't think the EINTR happens during contention, though.  It's really
there to be able break out in the face of SIGKILL.  I think that's why
nobody handles it: the task is dying anyway and nobody cares.

Rick, was this hunk here for a specific reason or were you just trying
to be diligent in handling errors?