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mm: open-code PageTail in folio_flags() and const_folio_flags()

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It is unsafe to call PageTail() in dump_page() as page_is_fake_head() will
almost certainly return true when called on a head page that is copied to
the stack.  That will cause the VM_BUG_ON_PGFLAGS() in const_folio_flags()
to trigger when it shouldn't.  Fortunately, we don't need to call
PageTail() here; it's fine to have a pointer to a virtual alias of the
page's flag word rather than the real page's flag word.

Link: https://lkml.kernel.org/r/20241125201721.2963278-1-willy@infradead.org
Fixes: fae7d834c4 ("mm: add __dump_folio()")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Kees Cook <kees@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2024-11-25 20:17:18 +00:00 committed by Andrew Morton
parent d699440f58
commit 4de22b2a6a
1 changed files with 2 additions and 2 deletions
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4
include/linux/page-flags.h
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@ -306,7 +306,7 @@ static const unsigned long *const_folio_flags(const struct folio *folio,
{
const struct page *page = &folio->page;
VM_BUG_ON_PGFLAGS(PageTail(page), page);
VM_BUG_ON_PGFLAGS(page->compound_head & 1, page);
VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page);
return &page[n].flags;
}
@ -315,7 +315,7 @@ static unsigned long *folio_flags(struct folio *folio, unsigned n)
{
struct page *page = &folio->page;
VM_BUG_ON_PGFLAGS(PageTail(page), page);
VM_BUG_ON_PGFLAGS(page->compound_head & 1, page);
VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page);
return &page[n].flags;
}