Messages in this thread | | | From | Blaisorblade <> | Subject | remap_file_pages - Bug with _PAGE_PROTNONE - is it used in current kernels? | Date | Mon, 20 Feb 2006 23:54:48 +0100 |
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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.
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.
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. -- Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!". Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894) http://www.user-mode-linux.org/~blaisorblade
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