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    SubjectRe: [RFC PATCH] Add CRC checksum for RCU lists


    --
    On Fri, 21 Sep 2007, Paul E. McKenney wrote:

    > On Thu, Sep 20, 2007 at 02:34:11PM -0400, Steven Rostedt wrote:
    > > In recent development of the RT kernel, some of our experimental code
    > > corrupted the rcu header. But the side effect (crashing) didn't rear its
    > > ugly head until way after the fact. Discussing this with Paul, he
    > > suggested that RCU should have a "self checking" mechanism to detect
    > > these kind of issues. He also suggested putting in a CRC into the
    > > rcu_head structure.
    > >
    > > This patch does so.
    >
    > Very cool!!!

    Thanks :-)



    >
    > > This patch also takes care to update the crc when rcu_head items are
    > > moved from list to list and whenever the next pointer is modified due to
    > > addition.
    >
    > We can only be thankful that it is not possible to cancel outstanding
    > RCU callbacks...

    true

    > Looks good -- a few suggestions for better fault coverage interspersed
    > below. But would be useful as is. (And it would be good to apply after
    > the preempt/boost patches, which are currently undergoing integration
    > with the CPU hotplug patches -- the good news is that this integration
    > eliminates the need for patch #4!)
    >
    > Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    Thanks, but this is still going to go through changes, from your comments
    as well as your latest patches.

    >
    > > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    > >
    > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    > > index fe17d7d..baca7e6 100644
    > > --- a/include/linux/rcupdate.h
    > > +++ b/include/linux/rcupdate.h
    > > @@ -50,13 +50,81 @@
    > > struct rcu_head {
    > > struct rcu_head *next;
    > > void (*func)(struct rcu_head *head);
    > > +#ifdef CONFIG_RCU_CRC_HEADER_CHECK
    > > + /*
    > > + * Checks are made in C files knowing that "next" is
    > > + * the first element. So keep it the first element.
    > > + */
    > > + unsigned long crc;
    > > + void *caller;
    > > +#endif
    > > };
    >
    > Looks good, but one question -- why not include the caller in the CRC?
    > Not a big deal either way, but would catch a few more cases of corruption.
    > Also, as things stand, the caller pointer can be silently corrupted,
    > which might causes confusion if someone had to examine the RCU callback
    > lists from a crash dump.

    One reason was that the caller was an addition. I originally didn't have
    it, but during my tests, the output was basically useless. It didn't give
    any hint to where things went wrong, so I added it. The CRC is to check
    the rcu is working, not really the checker itself.

    Note, it helped us out lately with Peter's latest file_table patches in
    -rt. With this patch applied, it triggered corruption. That was due to the
    union in the fs.h between the llist and rcu_head there was a dependency
    in the llist where the rcu_head would not grow. The llist can still do a
    spin lock on the lock _after_ the item was added to the call_rcu. Because
    of that union, the locking of the lock corrupted the crc. Set it to one.

    You'll see patches from Peter soon to get rid of that dependency.

    >
    > Interchanging the order of "crc" and "caller" would change the strategy,
    > if I understand correctly.

    yep.

    >
    > > -#define RCU_HEAD_INIT { .next = NULL, .func = NULL }
    > > +#ifdef CONFIG_RCU_CRC_HEADER_CHECK
    > > +
    > > +#define RCU_CRC_MAGIC 0xC4809168UL
    >
    > Very magic indeed -- Google doesn't find it, other than in your
    > patch. ;-)

    Paul, I'm disappointed in you. That number doesn't ring a bell at all??

    (hint, ignore the 'C' that was added by me).

    >
    > > +static inline unsigned long rcu_crc_calc(struct rcu_head *head)
    > > +{
    > > + unsigned int *p = (unsigned int*)head; /* 32 bit */
    > > + unsigned long crc = RCU_CRC_MAGIC;
    > > +
    > > + for (p=(void *)head; (void*)p < (void*)&head->crc; p++)
    > > + crc ^= *p;
    > > + return crc;
    > > +}
    >
    > Why initialize "p" twice? (Once in the declaration, and once in the
    > "for" loop, but with different casts.)

    Why? probably because I was half asleep when writing it ;-)
    Will fix.

    >
    > > +static inline void rcu_crc_check(struct rcu_head *head)
    > > +{
    > > + static int once;
    > > + if (unlikely(head->crc != rcu_crc_calc(head)) && !once) {
    > > + once++;
    >
    > Do we want exactly one (give or take concurrent checks), or do we want
    > the first ten or so? Sometimes having a modest sample rather than
    > exactly one is helpful.

    I added that because during testing, it would flood the serial console. I
    can modify this to whatever deems fit. Perhaps more hits will give a
    better clue to what went wrong. I could also just add a print_ratelimit to
    it too.


    >
    > And I know that it doesn't matter in this case, but I cannot stop myself
    > from pointing out the possibility of making "once" be an atomic_t
    > that is initialized to (say) -10, then making the !once check be an
    > atomic_add_return(). (Whew! Good to get that off my chest!!!)

    Would you prefer the above or the print_ratelimit? Maybe 10 would be best.
    I really don't have a preference.

    >
    > Now back to real problems. ;-)
    >
    > (Note to self...) The way this is coded could possibly result in false
    > positives. Suppose that the last element in a given callback list has
    > its CRC correctly calculated. Now suppose that a new callback is being
    > added to the end of the list. This addition is non-atomic, so the of
    > the old last element will be momentarily incorrect. So, need to check
    > that all list checks are protected by some lock. (And all the cases
    > I saw below are in fact OK.)

    I believe that I tried to keep all the checks done in the same locations
    that could possible modify the lists. So they should be protect by the
    same mechanism. On the rcupreempt side, I made the checks within the
    holding of the data lock (for when you see that patch ;-)

    >
    > > + printk("BUG: RCU check failed!");
    > > + if (head->caller)
    > > + printk(" (caller=%p)",
    > > + head->caller);
    > > + printk("\n");
    > > + printk(" CRC was %08lx, expected %08lx\n",
    > > + head->crc, rcu_crc_calc(head));
    >
    > I suggest also printing head->crc^rcu_crc_calc(head) to make cases
    > where a single bit is being corrupted more obvious.

    OK, will do.

    >
    > > + printk(" %p %p\n",
    > > + head->next, head->func);
    > > + dump_stack();
    > > + }
    > > +}
    > > +
    > > +static inline void rcu_assign_crc(struct rcu_head *head)
    > > +{
    > > + head->crc = rcu_crc_calc(head);
    > > + head->caller = __builtin_return_address(0);
    > > +}
    >
    > If you do decide to move the caller into the CRC calculation, it
    > will be necessary to reverse the above pair of lines.

    Noted.

    >
    > > +static inline void rcu_check_list(struct rcu_head *head)
    > > +{
    > > + int loop;
    > > + for (loop = 0;
    > > + head != NULL && loop < 100;
    > > + head=head->next, loop++)
    > > + rcu_crc_check(head);
    > > +}
    > > +
    > > +# define RCU_CRC_INIT , .crc = RCU_CRC_MAGIC
    >
    > Would need to initialize caller here if you add it to the CRC.
    >
    > > +# define RCU_CRC_SET(ptr) (ptr)->crc = RCU_CRC_MAGIC
    >
    > And here as well.
    >
    > But this has the effect of causing the CRC to be born correct. Do we
    > really want that? Suppose someone incorrectly re-initialized a callback
    > that was still on a list. Wouldn't it be better to get a CRC warning than
    > a NULL pointer exception? So suggest something like RCU_CRC_MAGIC+1 --
    > perhaps with a RCU_CRC_BAD_MAGIC symbol.

    Yeah, I can scrap those initialization macros. That came about my first
    attempt where I forgot to add the assignment to the call_rcu and it
    obviously failed. So I added these macros, and it stilled failed. Then I
    saw the mistake I made, fixed it, and it worked. But I never removed these
    macros. I think we can just keep the crc as zero. That would also fail
    the test. Hmm, maybe we should add a BAD_CRC number so that it will give
    us a hint that something was initiazed incorrectly.

    >
    > > +#else
    > > +# define rcu_crc_calc(head) 0
    > > +# define rcu_crc_check(head) do { } while(0)
    > > +# define rcu_assign_crc(head) do { } while(0)
    > > +# define rcu_check_list(head) do { } while(0)
    > > +# define RCU_CRC_INIT
    > > +# define RCU_CRC_SET(ptr) do { } while(0)
    > > +#endif /* CONFIG_RCU_CRC_HEADER_CHECK */
    > > +
    > > +#define RCU_HEAD_INIT { .next = NULL, .func = NULL RCU_CRC_INIT }
    > > #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
    > > -#define INIT_RCU_HEAD(ptr) do { \
    > > - (ptr)->next = NULL; (ptr)->func = NULL; \
    > > -} while (0)
    > > +#define INIT_RCU_HEAD(ptr) do { \
    > > + (ptr)->next = NULL; (ptr)->func = NULL; \
    > > + RCU_CRC_SET(ptr); \
    > > + } while (0)
    > >
    > >
    > >
    > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
    > > index 2c2dd84..4c3cc9c 100644
    > > --- a/kernel/rcupdate.c
    > > +++ b/kernel/rcupdate.c
    > > @@ -76,6 +76,23 @@ static atomic_t rcu_barrier_cpu_count;
    > > static DEFINE_MUTEX(rcu_barrier_mutex);
    > > static struct completion rcu_barrier_completion;
    > >
    > > +#ifdef CONFIG_RCU_CRC_HEADER_CHECK
    > > +#define rcu_check_rdp(rdp) \
    > > + do { \
    > > + rcu_check_list(rdp->nxtlist); \
    > > + rcu_check_list(rdp->curlist); \
    > > + rcu_check_list(rdp->donelist); \
    > > + } while(0)
    > > +#define rcu_crc_update_tail(rdp, tail, list) \
    > > + do { \
    > > + if ((rdp)->tail != &(rdp)->list) \
    > > + rcu_assign_crc((struct rcu_head*)(rdp)->tail); \
    > > + } while(0)
    > > +#else
    > > +# define rcu_check_rdp(rdp) do { } while(0)
    > > +# define rcu_crc_update_tail(rdp, tail, list) do { } while(0)
    > > +#endif
    > > +
    > > #ifdef CONFIG_SMP
    > > static void force_quiescent_state(struct rcu_data *rdp,
    > > struct rcu_ctrlblk *rcp)
    > > @@ -122,14 +139,19 @@ void fastcall call_rcu(struct rcu_head *head,
    > >
    > > head->func = func;
    > > head->next = NULL;
    > > + rcu_assign_crc(head);
    > > local_irq_save(flags);
    > > rdp = &__get_cpu_var(rcu_data);
    > > *rdp->nxttail = head;
    >
    > The CRC of the tail element is incorrect at this point, but that is OK
    > because we have interrupts disabled and no other CPU can access our list
    > in the meantime.

    Yep, that was planned.

    >
    > > + rcu_crc_update_tail(rdp, nxttail, nxtlist);
    > > rdp->nxttail = &head->next;
    > > if (unlikely(++rdp->qlen > qhimark)) {
    > > rdp->blimit = INT_MAX;
    > > force_quiescent_state(rdp, &rcu_ctrlblk);
    > > }
    > > +
    > > + rcu_check_rdp(rdp);
    >
    > This check is OK -- no other CPU should be able to manipulate our
    > rdp, and we have interrupts disabled. Same situation for call_rcu_bh()
    > below.
    >
    > > local_irq_restore(flags);
    > > }
    > >
    > > @@ -157,9 +179,11 @@ void fastcall call_rcu_bh(struct rcu_head *head,
    > >
    > > head->func = func;
    > > head->next = NULL;
    > > + rcu_assign_crc(head);
    > > local_irq_save(flags);
    > > rdp = &__get_cpu_var(rcu_bh_data);
    > > *rdp->nxttail = head;
    > > + rcu_crc_update_tail(rdp, nxttail, nxtlist);
    > > rdp->nxttail = &head->next;
    > >
    > > if (unlikely(++rdp->qlen > qhimark)) {
    > > @@ -167,6 +191,8 @@ void fastcall call_rcu_bh(struct rcu_head *head,
    > > force_quiescent_state(rdp, &rcu_bh_ctrlblk);
    > > }
    > >
    > > + rcu_check_rdp(rdp);
    > > +
    > > local_irq_restore(flags);
    > > }
    > >
    > > @@ -233,6 +259,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
    > > struct rcu_head *next, *list;
    > > int count = 0;
    > >
    > > + rcu_check_rdp(rdp);
    >
    > OK, interrupts disabled here.
    >
    > > list = rdp->donelist;
    > > while (list) {
    > > next = list->next;
    >
    > Why not invalidate the CRC of the element that we just removed? This
    > would catch some cases of list mangling.

    Good idea, will add.

    >
    > > @@ -373,6 +401,7 @@ static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
    > > {
    > > local_irq_disable();
    > > *this_rdp->nxttail = list;
    >
    > Momentarily wrong CRC OK, interrupts disabled here. Ditto for
    > __rcu_process_callbacks() below.

    yep

    >
    > > + rcu_crc_update_tail(this_rdp, nxttail, nxtlist);
    > > if (list)
    > > this_rdp->nxttail = tail;
    > > local_irq_enable();
    > > @@ -424,6 +453,7 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
    > > {
    > > if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) {
    > > *rdp->donetail = rdp->curlist;
    > > + rcu_crc_update_tail(rdp, donetail, donelist);
    > > rdp->donetail = rdp->curtail;
    > > rdp->curlist = NULL;
    > > rdp->curtail = &rdp->curlist;
    > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
    > > index 50a94ee..981fc93 100644
    > > --- a/lib/Kconfig.debug
    > > +++ b/lib/Kconfig.debug
    > > @@ -424,6 +424,17 @@ config RCU_TORTURE_TEST
    > > Say M if you want the RCU torture tests to build as a module.
    > > Say N if you are unsure.
    > >
    > > +config RCU_CRC_HEADER_CHECK
    > > + bool "RCU header self check"
    > > + depends on DEBUG_KERNEL
    > > + help
    > > + This option enables CRC verification of RCU lists to catch
    > > + possible corruption to the RCU list by improper application
    > > + of RCU callbacks. This adds overhead to the running system
    > > + so only enable it if you suspect RCU corruption is occurring.
    > > +
    > > + If unsure, say N.
    > > +
    > > config LKDTM
    > > tristate "Linux Kernel Dump Test Tool Module"
    > > depends on DEBUG_KERNEL
    > >
    > >
    >

    Paul, thanks for all the comments. I'll put out a new round of patches
    after yours becomes offical (no "not for inclussion" statements).

    -- Steve

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