Messages in this thread | | | From | Blaisorblade <> | Subject | Re: remap_file_pages - Bug with _PAGE_PROTNONE - is it used in current kernels? | Date | Tue, 28 Feb 2006 17:53:44 +0100 |
| |
On Tuesday 21 February 2006 14:09, Hugh Dickins wrote: > 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. English humor :-) ?
> 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.
Ok, thanks for the explaination.
The bug is born from the patched install_file_pte(). Before there was no need to store the protection bits, now it's needed.
So, it sets a pte_file PTE containing no page, and on PROT_NONE it uses _PAGE_PROTNONE|_PAGE_FILE.
Indeed, what I've actually coded and tested was safer, but I wanted to know if it could be simpler (and faster). For i386 it should be (I've re-tested only UML so far):
-#define pte_present(x) ((x).pte_low & (_PAGE_PRESENT | _PAGE_PROTNONE)) +#define pte_present(x) (((x).pte_low & _PAGE_PRESENT) || \ + (((x).pte_low & (_PAGE_PROTNONE|_PAGE_FILE)) == _PAGE_PROTNONE))
--- linux-2.6.git.orig/include/asm-um/pgtable.h +++ linux-2.6.git/include/asm-um/pgtable.h @@
-#define pte_present(x) pte_get_bits(x, (_PAGE_PRESENT | _PAGE_PROTNONE)) +#define pte_present(x) (pte_get_bits(x, (_PAGE_PRESENT)) || (pte_get_bits(x, (_PAGE_PROTNONE)) && !pte_file(x)))
-- 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
___________________________________ Yahoo! Mail: gratis 1GB per i messaggi e allegati da 10MB http://mail.yahoo.it
- 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/
| |