Messages in this thread | | | From | Vince Weaver <> | Date | Sat, 20 Aug 2016 00:44:54 -0400 (EDT) | Subject | Re: perf: fuzzer crashes immediately on AMD system |
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On Fri, 19 Aug 2016, Peter Zijlstra wrote:
> On Thu, Aug 18, 2016 at 10:46:31AM -0400, Vince Weaver wrote: > > On Thu, 18 Aug 2016, Vince Weaver wrote: > > > > > Tried the perf_fuzzer on my A10 fam15h/model13h system with 4.8-rc2 and it > > > falls over more or less immediately. > > > > > > This maps to variable_test_bit() > > > called by ctx = find_get_context(pmu, task, event); > > > in kernel/events/core.c:9467 > > > > > > It happens quickly enough I can probably track down the exact event that > > > causes this, if needed. > > > > I have a one line reproducer: > > > > perf stat -a -e amd_nb/config=0x37,config1=0x20/ /bin/ls > > OK, cannot reproduce on my fam15h/model1h. I'll go dig through the > various manuals to see if I can spot the fail. > > Huang could you either prod someone at AMD or do yourself, audit the AMD > perf code for all the various new models?
This is bizzarre, I can't make any sense of the crash.
To recap, the crash looks like this: BUG: unable to handle kernel paging request at ffffffff85e67600 IP: [<ffffffff810e4cb1>] find_get_context.isra.75+0x28/0x20f
The code in question is this code:
if (!cpu_online(cpu))
which maps to test_bit(cpumask_check(cpu), cpumask_bits((cpumask)));
which assembles to
ffffffff810e4ca9: 41 89 cc mov %ecx,%r12d ffffffff810e4cac: 7f 1e jg ffffffff810e4ccc <find_get_context.isra.75+0x43> ffffffff810e4cae: 44 89 e0 mov %r12d,%eax * ffffffff810e4cb1: 48 0f a3 05 87 0f 7f bt %rax,0x7f0f87(%rip) # ffffffff818d5c40 <__cpu_online_mask> ffffffff810e4cb8: 00 ffffffff810e4cb9: 0f 92 c0 setb %al ffffffff810e4cbc: 84 c0 test %al,%al
There is no way that 0x7f0f87(%rip) should ever possibly be the ffffffff85e67600 value that causes the fault.
Though oddly rax when the call happens (according to the oops message) is RAX: 0000000022c8ce30 which seems nonsensical for a CPU number, but shouldn't cause an invalid memory address. Also oddly RDI matches RAX but RCX doesn't which I think should be true with that assembly.
So very weird. I even wrote a kernel module and dumped the raw kernel memory to make sure the instruction stream didn't get overwritten somehow, but as far as I can tell the code in memory matches the disassembly.
anyway I am out of time to look at this for now.
Vince
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