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

Andriy Gapon wrote:
> 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))
>
>   

Yes.  Your analysis is correct.

Alan