kcov: don't generate a warning on vm_insert_page()'s failure

vm_insert_page()'s failure is not an unexpected condition, so don't do
WARN_ONCE() in such a case.

Instead, print a kernel message and just return an error code.

This flaw has been reported under an OOM condition by sysbot [1].

The message is mainly for the benefit of the test log, in this case the
fuzzer's log so that humans inspecting the log can figure out what was
going on.  KCOV is a testing tool, so I think being a little more chatty
when KCOV unexpectedly is about to fail will save someone debugging
time.

We don't want the WARN, because it's not a kernel bug that syzbot should
report, and failure can happen if the fuzzer tries hard enough (as
above).

Link: https://lkml.kernel.org/r/Ylkr2xrVbhQYwNLf@elver.google.com [1]
Link: https://lkml.kernel.org/r/20220401182512.249282-1-nogikh@google.com
Fixes: b3d7fe86 ("kcov: properly handle subsequent mmap calls"),
Signed-off-by: NAleksandr Nogikh <nogikh@google.com>
Acked-by: NMarco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Taras Madan <tarasmadan@google.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/kcov.c

@@ -475,8 +475,11 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
 	vma->vm_flags |= VM_DONTEXPAND;
 	for (off = 0; off < size; off += PAGE_SIZE) {
 		page = vmalloc_to_page(kcov->area + off);
-		if (vm_insert_page(vma, vma->vm_start + off, page))
-			WARN_ONCE(1, "vm_insert_page() failed");
+		res = vm_insert_page(vma, vma->vm_start + off, page);
+		if (res) {
+			pr_warn_once("kcov: vm_insert_page() failed\n");
+			return res;
+		}
 	}
 	return 0;
 exit: