Re: DTrace panic while probing syscall::open (and possibly many others)

on 18/06/2009 14:42 Thomas Backman said the following:
> 
> On Jun 18, 2009, at 12:55 PM, Andriy Gapon wrote:
> 
>> on 18/06/2009 12:43 Thomas Backman said the following:
>>>
>>>    at dtrace_isa.c:527
>>> #14 0xffffffff816b31fc in dtrace_copyinstr (uaddr=34365163021,
>>>    kaddr=18446743524025463312, size=256, flags=0xffffffff8146e0c0)
>>>    at dtrace_isa.c:558
>>
>> kaddr=18446743524025463312 == FFFFFF8004467210
>> I think kernelbase on amd64 is 0xFFFFFFFF80000000.
>> FFFFFF8004467210 kaddr
>> is smaller than
>> FFFFFFFF80000000 kernelbase
>>
>> The numbers do look suspiciously similar, so I am not sure if you are
>> seeing a
>> race or a real bug somewhere.
>> -- 
>> Andriy Gapon
> Hmmm...
> Looking around a bit for these numbers, I found, in
> /sys/amd64/include/vmparam.h:
> 
> /*
>  * Virtual addresses of things.  Derived from the page directory and
>  * page table indexes from pmap.h for precision.
>  *
>  * 0x0000000000000000 - 0x00007fffffffffff   user map
>  * 0x0000800000000000 - 0xffff7fffffffffff   does not exist (hole)
>  * 0xffff800000000000 - 0xffff804020100fff   recursive page table (512GB
> slot)
>  * 0xffff804020101000 - 0xfffffeffffffffff   unused
>  * 0xffffff0000000000 - 0xffffff7fffffffff   512GB direct map mappings
>  * 0xffffff8000000000 - 0xffffffffffffffff   512GB kernel map
>  *
>  * Within the kernel map:
>  *
>  * 0xffffffff80000000                        KERNBASE
>  */
> 
> So, kaddr is inside the "kernel map", but not KERNBASE. What this means,
> I have no clue whatsoever. (I'm not a kernel developer and I don't know
> too much about (virtual) memory either!)

Thomas,

I think that you were correct that one needs to be somewhat of a VM expert here.
It seems that amd64 is the only[?] platform where KERNBASE !=
VM_MIN_KERNEL_ADDRESS (0xffffffff80000000 and 0xffffff8000000000 correspondingly).
That makes the assert in sys/cddl/dev/dtrace/amd64/dtrace_isa.c bogus in my opinion:
static int
dtrace_copycheck(uintptr_t uaddr, uintptr_t kaddr, size_t size)
{
        ASSERT(kaddr >= kernelbase && kaddr + size >= kaddr);

If the purpose of the assert is to ensure that [kaddr:kaddr+size) is within kernel
memory, then it should use VM_MIN_KERNEL_ADDRESS instead of KERNBASE. Or maybe
even use something like the macro in sys/amd64/include/stack.h:
#define INKERNEL(va) (((va) >= DMAP_MIN_ADDRESS && (va) < DMAP_MAX_ADDRESS) \
            || ((va) >= VM_MIN_KERNEL_ADDRESS && (va) < VM_MAX_KERNEL_ADDRESS))

The above is just my understanding, not a fact, so I am CC-ing people that are
really knowledgeable of our VM and the porter/author of our DTrace code too.

-- 
Andriy Gapon