Date: Fri, 19 Jun 2009 12:01:53 -0500 From: Alan Cox <alc@cs.rice.edu> To: Andriy Gapon <avg@icyb.net.ua> Cc: Alan Cox <alc@freebsd.org>, John Birrell <jb@freebsd.org>, freebsd-current@freebsd.org, Thomas Backman <serenity@exscape.org> Subject: Re: DTrace panic while probing syscall::open (and possibly many others) Message-ID: <4A3BC481.1010600@cs.rice.edu> In-Reply-To: <4A3BBF5A.6060702@icyb.net.ua> References: <949B5884-5303-4EFF-AC7D-293640FFA012@exscape.org> <0C235698-3ED2-4AE9-A7D1-5DC56D8324A4@exscape.org> <200905212129.47892.mel.flynn%2Bfbsd.current@mailing.thruhere.net> <44F486FA-E798-448D-BE31-F7A51EF1F612@exscape.org> <60173AF0-7E54-4BDD-8927-0DADA9DAD1B4@exscape.org> <20090522200306.GE2630@atarininja.org> <20090617225849.GB28509@atarininja.org> <B2F32C8F-810B-4EA5-9E34-39ADD5E5CED4@exscape.org> <4A3A1D27.4010802@icyb.net.ua> <DD13EADF-CE41-465A-8D70-53DCDEFD65A7@exscape.org> <4A3BBF5A.6060702@icyb.net.ua>
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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
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