From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 2 Aug 2005 13:26:09 +0100 (BST) From: Hugh Dickins Subject: Re: [patch 2.6.13-rc4] fix get_user_pages bug In-Reply-To: Message-ID: References: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Martin Schwidefsky Cc: Linus Torvalds , Andrew Morton , Robin Holt , linux-kernel , linux-mm@kvack.org, Ingo Molnar , Nick Piggin , Roland McGrath List-ID: On Tue, 2 Aug 2005, Martin Schwidefsky wrote: > > Why do we require the !pte_dirty(pte) check? I don't get it. If a writeable > clean pte is just fine then why do we check the dirty bit at all? Doesn't > pte_dirty() imply pte_write()? Not quite. This is all about the peculiar ptrace case, which sets "force" to get_user_pages, and ends up handled by the little maybe_mkwrite function: we sometimes allow ptrace to modify the page while the user does not have have write access to it via the pte. Robin discovered a race which proves it's unsafe for get_user_pages to reset its lookup_write flag (another stage in this peculiar path) after a single try, Nick proposed a patch which adds another VM_ return code which each arch would need to handle, Linus looked for something simpler and hit upon checking pte_dirty rather than pte_write (and removing the then unnecessary lookup_write flag). Linus' changes are in the 2.6.13-rc5 mm/memory.c, but that leaves s390 broken at present. > With the additional !pte_write(pte) check (and if I haven't overlooked > something which is not unlikely) s390 should work fine even without the > software-dirty bit hack. I agree the pte_write check ought to go back in next to the pte_dirty check, and that will leave s390 handling most uses of get_user_pages correctly, but still failing to handle the peculiar case of strace modifying a page to which the user does not currently have write access (e.g. setting a breakpoint in readonly text). Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org