runtime: mark findObject nosplit

findObject takes the pointer argument as uintptr. If the pointer is to
the local stack and calling findObject happens to require the stack to
be reallocated, then spanOf is called for the old pointer.

Marking findObject as nosplit fixes the issue.

Fixes #35068

Change-Id: I029d36f9c23f91812f18f98839edf02e0ba4082e
Reviewed-on: https://go-review.googlesource.com/c/go/+/202798
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mbitmap.go

@@ -385,6 +385,10 @@ func badPointer(s *mspan, p, refBase, refOff uintptr) {
 // refBase and refOff optionally give the base address of the object
 // in which the pointer p was found and the byte offset at which it
 // was found. These are used for error reporting.
+//
+// It is nosplit so it is safe for p to be a pointer to the current goroutine's stack.
+// Since p is a uintptr, it would not be adjusted if the stack were to move.
+//go:nosplit
 func findObject(p, refBase, refOff uintptr) (base uintptr, s *mspan, objIndex uintptr) {
 	s = spanOf(p)
 	// If s is nil, the virtual address has never been part of the heap.