[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] xen: Fix possible page fault in fifo events
On Tue, Jul 15, 2014 at 03:32:58PM +0100, Andrew Cooper wrote: > On 15/07/14 14:48, Frediano Ziglio wrote: > > sync_test_bit function require a long* read access to pointer. > > This is a problem if the you are using last entry in the page causing > > an access to next page. If this page is not readable you get a memory > > access failure (page fault). > > All other x64 bit functions access memory using 32 bit operations. > > For processors different than x64 long aligned operations are used. > > > > Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx> > > The core issue is that the Linux bitops primitives are inconsistent. > They all use unsigned long pointers to refer to memory; the purely C > primitives then make memory accesses at the native width of an unsigned > long, while the assembly optimised primitives use 32bit accesses (either > explicitly with an 'l' asm suffix, or implicitly as the default operand > width is 32bit without a REX prefix in x86_64). > > Xen suffers from a similar mess of primitives, but all its C primitives > use unsigned int pointers rather than unsigned long, meaning that they > still generate 32bit memory accesses when compiled as 64bit. This means > the Xen side of the event fifo code is safe, but by luck rather than > good guidance. > > In this case, an event_word_t is strictly a 32bit quantity, and should > never be accessed with a 64bit memory access. This in turn would fix > the alignment issues which affected arm64, and this pagefault because > the 4 bytes we didn't care about were in a non-present page. > > However, there doesn't appear to be a systematic way of enforcing a > specific memory access width given the existing primitives. I like your explanation and I think it should be part of the commit description. This is David's baby so I would like his opinion on this - however he is enjoying his vacation so will have to wait. Once he is back I think we can squeeze it in 3.16 or worst case in 3.17 with stable tree backport. > > ~Andrew > > > --- > > drivers/xen/events/events_fifo.c | 9 +++++++-- > > 1 file changed, 7 insertions(+), 2 deletions(-) > > > > diff --git a/drivers/xen/events/events_fifo.c > > b/drivers/xen/events/events_fifo.c > > index d302639..af4672d 100644 > > --- a/drivers/xen/events/events_fifo.c > > +++ b/drivers/xen/events/events_fifo.c > > @@ -168,6 +168,11 @@ static int evtchn_fifo_setup(struct irq_info *info) > > return ret; > > } > > > > +static __always_inline int test_fifo_bit(int nr, event_word_t *word) > > +{ > > + return (ACCESS_ONCE(*word) & (((event_word_t) 1) << nr)) != 0; > > +} > > + > > static void evtchn_fifo_bind_to_cpu(struct irq_info *info, unsigned cpu) > > { > > /* no-op */ > > @@ -188,7 +193,7 @@ static void evtchn_fifo_set_pending(unsigned port) > > static bool evtchn_fifo_is_pending(unsigned port) > > { > > event_word_t *word = event_word_from_port(port); > > - return sync_test_bit(EVTCHN_FIFO_BIT(PENDING, word), BM(word)); > > + return test_fifo_bit(EVTCHN_FIFO_PENDING, word); > > } > > > > static bool evtchn_fifo_test_and_set_mask(unsigned port) > > @@ -206,7 +211,7 @@ static void evtchn_fifo_mask(unsigned port) > > static bool evtchn_fifo_is_masked(unsigned port) > > { > > event_word_t *word = event_word_from_port(port); > > - return sync_test_bit(EVTCHN_FIFO_BIT(MASKED, word), BM(word)); > > + return test_fifo_bit(EVTCHN_FIFO_MASKED, word); > > } > > /* > > * Clear MASKED, spinning if BUSY is set. > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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