From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <42EE0021.3010208@yahoo.com.au> Date: Mon, 01 Aug 2005 20:57:37 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [patch 2.6.13-rc4] fix get_user_pages bug References: <20050801032258.A465C180EC0@magilla.sf.frob.com> <42EDDB82.1040900@yahoo.com.au> <20050801091956.GA3950@elte.hu> <42EDEAFE.1090600@yahoo.com.au> <20050801101547.GA5016@elte.hu> In-Reply-To: <20050801101547.GA5016@elte.hu> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Ingo Molnar Cc: Robin Holt , Andrew Morton , Linus Torvalds , Roland McGrath , Hugh Dickins , linux-mm@kvack.org, linux-kernel List-ID: Ingo Molnar wrote: > > Hugh's posting said: > > "it's trying to avoid an endless loop of finding the pte not writable > when ptrace is modifying a page which the user is currently protected > against writing to (setting a breakpoint in readonly text, perhaps?)" > > i'm wondering, why should that case generate an infinite fault? The > first write access should copy the shared-library page into a private > page and map it into the task's MM, writable. If this make-writable It will be mapped readonly. > operation races with a read access then we return a minor fault and the > page is still readonly, but retrying the write should then break up the > COW protection and generate a writable page, and a subsequent > follow_page() success. If the page cannot be made writable, shouldnt the > vma flags reflect this fact by not having the VM_MAYWRITE flag, and > hence get_user_pages() should have returned with -EFAULT earlier? > If it cannot be written to, then yes. If it can be written to but is mapped readonly then you have the problem. Aside, that brings up an interesting question - why should readonly mappings of writeable files (with VM_MAYWRITE set) disallow ptrace write access while readonly mappings of readonly files not? Or am I horribly confused? -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com -- 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