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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1s6t69-00000009A31-36xb; Tue, 14 May 2024 14:25:45 +0000 Date: Tue, 14 May 2024 15:25:45 +0100 From: Matthew Wilcox To: Kees Cook Cc: linux-mm@kvack.org Subject: Re: [PATCH 4/8] mm: Add __dump_folio() Message-ID: References: <20240227192337.757313-5-willy@infradead.org> <202405132126.E26FE7B9@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202405132126.E26FE7B9@keescook> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 92C63100019 X-Stat-Signature: xkjdbuqsaw3cabutbb4gyq4jo3dxk68r X-Rspam-User: X-HE-Tag: 1715696752-713968 X-HE-Meta: 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 X4EjbUBf Sk9pVhlpLdgqp5Rq0fj2nHeIyj9GF8myHswe1pznC04VgrD/FEZ+lzXE7MroS/zGyaKit1ENEAYPOql23kfTJBuWWySouMjjP7kXB2pd6PQVYRSRW4secxZ4obRJidoc49uKi3GyECDfN2wHwRlGjgKvlhosyybjxC5LdiEQ/NNNUg16qd+RegOeJ5g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, May 13, 2024 at 09:33:57PM -0700, Kees Cook wrote: > In function 'page_fixed_fake_head', > inlined from '_compound_head' at ../include/linux/page-flags.h:251:24, > inlined from '__dump_page' at ../mm/debug.c:123:11: > ../include/asm-generic/rwonce.h:44:26: warning: array subscript 9 is outside array bounds of 'struct page[1]' [-Warray-bounds=] > > (Not noted in this warning is that the code passes through page_folio() > _Generic macro.) > > It doesn't like that it can see that "precise" is exactly one page, so > looking at page[1] later is going to freak out. I suspect this may be > "impossible" at run-time, but I'm not 100% sure. Regardless, the compiler > can't tell. Actually, I'm not sure that I can tell that it's impossible. I think we just need to open-code page_folio() here so that we don't get into the fixed fake head palaver. Something like this, although it's only compile-tested. diff --git a/mm/debug.c b/mm/debug.c index e3ff3ac19fa1..47ba8b0a4872 100644 --- a/mm/debug.c +++ b/mm/debug.c @@ -110,19 +110,22 @@ static void __dump_page(const struct page *page) { struct folio *foliop, folio; struct page precise; + unsigned long head; unsigned long pfn = page_to_pfn(page); unsigned long idx, nr_pages = 1; int loops = 5; again: memcpy(&precise, page, sizeof(*page)); - foliop = page_folio(&precise); - if (foliop == (struct folio *)&precise) { + head = precise.compound_head; + if ((head & 1) == 0) { + foliop = (struct folio *)&precise; idx = 0; if (!folio_test_large(foliop)) goto dump; foliop = (struct folio *)page; } else { + foliop = (struct folio *)(head - 1); idx = folio_page_idx(foliop, page); }