Messages in this thread | | | Date | Tue, 21 Feb 2006 13:09:26 +0000 (GMT) | From | Hugh Dickins <> | Subject | Re: remap_file_pages - Bug with _PAGE_PROTNONE - is it used in current kernels? |
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On Mon, 20 Feb 2006, Blaisorblade wrote:
> I've been hitting a bug on a patch I'm working on and have considered (and > more or less tested with good results) doing this change: > > -#define pte_present(x) ((x).pte_low & (_PAGE_PRESENT | _PAGE_PROTNONE)) > +#define pte_present(x) ((x).pte_low & (_PAGE_PRESENT)) > > (and the corresponding thing on other architecture). > > In general, the question is whether __P000 and __S000 in protection_map are > ever used except for MAP_POPULATE, and even then if they work well. > > I'm seeking for objections to this change and/or anything I'm missing.
Objection, your honor.
> This bug showed up while porting remap_file_pages protection support to > 2.6.16-rc3. It always existed but couldn't trigger before the PageReserved > changes. > > Consider a _PAGE_PROTNONE pte, which has then pte_pfn(pte) == 0 (with > remap_file_pages you need them to exist). Obviously pte_pfn(pte) on such a PTE > doesn't make sense, but since pte_present(pte) gives true the code doesn't > know that.
I didn't fully understand you there, but I think you've got it the wrong way round: _PAGE_PROTNONE is included in the pte_present() test precisely because there is a valid page there, pfn is set (it might be pfn 0, yes, but much more likely to be pfn non-0).
I've never used PROT_NONE myself (beyond testing), but I think the traditional way it's used is this: mmap(,,PROT_READ|PROT_WRITE,,,), initialize the pages of that mapping, then mprotect(,,PROT_NONE) - which retains all those pages but make them generate SIGSEGVs - so the app can detect accesses and decide if it wants to do something special with them, other than the obvious mprotect(,,PROT_READ) or whatever.
PROT_NONE gives you a way of holding the page present (unlike munmap), yet failing access. And since those pages remain present, they do need to be freed later when you get to zap_pte_range. They are normal pages, but user access to them has been restricted.
Hugh
> Consider a call to munmap on this range. We get to zap_pte_range() which (in > condensed source code): > > zap_pte_range() > ... > if (pte_present(ptent)) { > //This test is passed > struct page *page = vm_normal_page(vma, addr, ptent); > //Now page points to page 0 - which is wrong, page should be NULL > page_remove_rmap(page); > //Which doesn't make any sense. > //If mem_map[0] wasn't mapped we hit a BUG now, if it was we'll hit it later - > //i.e. negative page_mapcount(). > > Now, since this code doesn't work in this situation, I wonder whether PROTNONE > is indeed used anywhere in the code *at the moment*, since faults on pages > mapped as such are handled with SIGSEGV. > > The only possible application, which is only possible in 2.6 and not in 2.4 > where _PAGE_PROTNONE still exists, is mmap(MAP_POPULATE) with prot == > PROT_NONE. > > Instead I need to make use of PROTNONE, so the handling of it may need > changes. In particular, I wonder about why: > > #define pte_present(x) ((x).pte_low & (_PAGE_PRESENT | _PAGE_PROTNONE)) > > I see why that _PAGE_PROTNONE can make sense, but in the above code it > doesn't. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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