Date: Fri, 19 Jun 2009 19:41:29 +0200 From: Thomas Backman <serenity@exscape.org> To: FreeBSD current <freebsd-current@freebsd.org> Cc: Alan Cox <alc@freebsd.org>, John Birrell <jb@freebsd.org>, Alan Cox <alc@cs.rice.edu>, Andriy Gapon <avg@icyb.net.ua> Subject: Re: DTrace panic while probing syscall::open (and possibly many others) Message-ID: <F55615D8-8AE3-41C6-BD2F-0DB911918465@exscape.org> In-Reply-To: <F72EDBE7-9A0A-44AB-AF58-23CE34CC93A0@exscape.org> 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> <4A3BC481.1010600@cs.rice.edu> <F72EDBE7-9A0A-44AB-AF58-23CE34CC93A0@exscape.org>
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On Jun 19, 2009, at 07:32 PM, Thomas Backman wrote:
>
> On Jun 19, 2009, at 07:01 PM, Alan Cox wrote:
>
>> 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
> Very interesting.
> I replaced the ASSERT line temporarily:
>
> --- ../src_r194478-UNTOUCHED/sys/cddl/dev/dtrace/amd64/
> dtrace_isa.c 2009-06-19 13:10:05.661079736 +0200
> +++ sys/cddl/dev/dtrace/amd64/dtrace_isa.c 2009-06-19
> 19:24:42.362125129 +0200
> @@ -524,7 +524,7 @@
> static int
> dtrace_copycheck(uintptr_t uaddr, uintptr_t kaddr, size_t size)
> {
> - ASSERT(kaddr >= kernelbase && kaddr + size >= kaddr);
> + ASSERT(kaddr >= 0xffffff8000000000 && kaddr + size >= kaddr);
>
> if (uaddr + size >= kernelbase || uaddr + size < uaddr) {
> DTRACE_CPUFLAG_SET(CPU_DTRACE_BADADDR);
>
> ... and it works! I obviously haven't tried it for extended periods
> or anything, but at least it's working so far.
> Should the ASSERT simply use this (as a #define somewhere) or the
> INKERNEL macro, though?
BTW... Should "kernelbase" in the line following the ASSERT also be
replaced, or not? As far as I can understand (not too far in these
contexts ;) it (should) check/s to see whether the userspace data, to
be copied, is inside the kernel *map*(?)... which at the moment, I
guess it doesn't. Correct?
Regards,
Thomas
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