On Mon, 1 Aug 2005, Nick Piggin wrote: > 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.
Yes. Sorry to leave you so long pondering over my words! > > 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. Yes. The problem case is the one where "maybe_mkwrite" finds !VM_WRITE and so doesn't set writable (but as Linus has observed, dirty is set). I'm no expert on that case, was just guessing its use, I think Robin determined that Roland is the expert on it; but what's very clear is that it's intentional, allowing strace to write where the user cannot currently write. > 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? Either you or I. You'll have to spell that out to me in more detail, I don't see it that way. What I do see and am slightly disturbed by, is that do_wp_page on a shared maywrite but not currently writable area, will go the break cow route in mainline, and has done so forever; whereas my page_mkwrite fixes in -mm inadvertently change that to go the the reuse route. I think my inadvertent change is correct, and the current behaviour (putting anonymous pages into a shared vma) is surprising, and may have bad consequences (would at least get the overcommit accounting wrong). Hugh - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
