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.

Fixes: 4ca9b3859dac ("mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault page tables")
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Andrea Arcangeli <redacted>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Dave Hansen <redacted>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Kirill A. Shutemov <redacted>
Cc: Vlastimil Babka <redacted>
Cc: Richard Henderson <redacted>
Cc: Ivan Kokshaysky <redacted>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Mike Kravetz <redacted>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rolf Eike Beer <redacted>
Cc: Ram Pai <redacted>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: David Hildenbrand <redacted>
---
  mm/gup.c     | 7 +++++--
  mm/madvise.c | 4 +++-
  2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 42b8b1fa6521..b94717977d17 100644
--- a/mm/gup.c
+++ b/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);
  }

diff --git a/mm/madvise.c b/mm/madvise.c
index 6d3d348b17f4..5c065bc8b5f6 100644
--- a/mm/madvise.c
+++ b/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);