Re: [GIT PULL] hotfixes for 6.3-rc1

[Andrew Morton]

On Sat, 4 Mar 2023 14:35:00 -0800 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Sat, Mar 4, 2023 at 1:15 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > 17 hotfixes.  Eight are for MM and seven are for other parts of the
> > kernel.  Seven are cc:stable and eight address post-6.3 issues or were
> > judged unsuitable for -stable backporting.
> 
> Hmm. Since this pull didn't fix the gcc note about playing pointer
> games that I get for my allmodconfig test build, and since I _really_
> don't want to have an rc1 release tomorrow with that (valid) warning,
> I fixed it up myself.

Ah. Ying did it this way:


From: Huang Ying <ying.huang@xxxxxxxxx>
Subject: migrate_pages: silence gcc notes for mis-casting
Date: Thu, 2 Mar 2023 09:26:10 +0800

The following GCC notes was reported for commit 64c8902ed441
("migrate_pages: split unmap_and_move() to _unmap() and _move()").

    mm/migrate.c: In function `__migrate_folio_extract':
    mm/migrate.c:1050:20: note: randstruct: casting between randomized
structure pointer types (ssa): `struct anon_vma' and `struct
address_space'

     1050 |         *anon_vmap = (void *)dst->mapping;
          |         ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

The casting itself is safe.  Because we only use dst->mapping to store the
pointer itself temporarily and dst is a newly allocated folio and not used
by anyone else during that.  But the notes should be silenced and some
comments are deserved.  So, we do that in this patch.

Link: https://lkml.kernel.org/r/20230302012610.17055-1-ying.huang@xxxxxxxxx
Fixes: 64c8902ed441 ("migrate_pages: split unmap_and_move() to _unmap() and _move()")
Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Bharata B Rao <bharata@xxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---


--- a/mm/migrate.c~migrate_pages-silence-gcc-notes-for-mis-casting
+++ a/mm/migrate.c
@@ -1047,7 +1047,16 @@ static void __migrate_folio_extract(stru
 				   int *page_was_mappedp,
 				   struct anon_vma **anon_vmap)
 {
-	*anon_vmap = (void *)dst->mapping;
+	struct anon_vma *anon_vma;
+
+	/*
+	 * 2 steps assignment to silence gcc notes for mis-casting. The
+	 * casting is safe.  Because we only use dst->mapping to store
+	 * the pointer itself temporarily and dst is a newly allocated
+	 * folio and not used by anyone else during that.
+	 */
+	anon_vma = (void *)dst->mapping;
+	*anon_vmap = anon_vma;
 	*page_was_mappedp = (unsigned long)dst->private;
 	dst->mapping = NULL;
 	dst->private = NULL;
_