mm/madvise: report SIGBUS as -EFAULT for MADV_POPULATE_(READ|WRITE)

Doing some extended tests and polishing the man page update for
MADV_POPULATE_(READ|WRITE), I realized that we end up converting also
SIGBUS (via -EFAULT) to -EINVAL, making it look like yet another
madvise() user error.

We want to report only problematic mappings and permission problems that
the user could have know as -EINVAL.

Let's not convert -EFAULT arising due to SIGBUS (or SIGSEGV) to -EINVAL,
but instead indicate -EFAULT to user space.  While we could also convert
it to -ENOMEM, using -EFAULT looks more helpful when user space might
want to troubleshoot what's going wrong: MADV_POPULATE_(READ|WRITE) is
not part of an final Linux release and we can still adjust the behavior.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 4ca9b38 ("mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault page tables")
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Michael S. Tsirkin <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: "James E.J. Bottomley" <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Rolf Eike Beer <[email protected]>
Cc: Ram Pai <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: davidhildenbrand

mm/gup.c

@@ -1558,9 +1558,12 @@ long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
 		gup_flags |= FOLL_WRITE;
 
 	/*
-	 * See check_vma_flags(): Will return -EFAULT on incompatible mappings
-	 * or with insufficient permissions.
+	 * We want to report -EINVAL instead of -EFAULT for any permission
+	 * problems or incompatible mappings.
 	 */
+	if (check_vma_flags(vma, gup_flags))
+		return -EINVAL;
+
 	return __get_user_pages(mm, start, nr_pages, gup_flags,
 				NULL, NULL, locked);
 }

mm/madvise.c

@@ -862,10 +862,12 @@ static long madvise_populate(struct vm_area_struct *vma,
 			switch (pages) {
 			case -EINTR:
 				return -EINTR;
-			case -EFAULT: /* Incompatible mappings / permissions. */
+			case -EINVAL: /* Incompatible mappings / permissions. */
 				return -EINVAL;
 			case -EHWPOISON:
 				return -EHWPOISON;
+			case -EFAULT: /* VM_FAULT_SIGBUS or VM_FAULT_SIGSEGV */
+				return -EFAULT;
 			default:
 				pr_warn_once("%s: unhandled return value: %ld\n",
 					     __func__, pages);