[lldb-dev] strange issues on linux

Ewan Crawford ewan at codeplay.com
Mon Jun 1 10:53:37 PDT 2015

Hi Ryan,
  I made changes to StopInfo recently to add the stack, r238323. I'll 
have a closer look at this and see if I can figure out what's going on.
  Thanks for bringing this up.


On 01/06/2015 18:09, Ryan Brown wrote:
> I can't reproduce the server crash at the moment, but it was crashing 
> immediately at process launch, so I don't think attaching to it would 
> work.
> I think I found the other problem.
> ProcessGDBRemote now has a stack of stop infos.
> RefreshStateAfterStop loops through them and calls SetThreadStopInfo, 
> which in turn calls UpdateThreadListIfNecessary.
> But what if these stop infos have different thread lists? Which do you 
> believe? Currently the first update succeeds and then the others seem 
> to be ignored because the stop info hasn't changed.
> So ignoring the threads for the 2nd stop packet is part of the 
> problem. But why are there two in the first place?
> ProcessGDBRemote::DoLaunch calls SetLastStopPacket which pushes the 
> packet onto the stack. But then it directly calls SetThreadStopInfo, 
> bypassing RefreshStateAfterStop. So the packet stays on the stack and 
> gets processed again along with the next stop packet.
> -- Ryan Brown
> On Mon, Jun 1, 2015 at 3:35 AM, Pavel Labath <labath at google.com 
> <mailto:labath at google.com>> wrote:
>     Hello,
>     I'm not really sure what is going on, but I don't think the problem is
>     in the server component. The server reports the correct register
>     values. You say your plugin does not do anything which should change
>     the registers (and the packet log confirms this), so it looks like the
>     corruption happens in LLDB. I'm afraid I don't know enough about the
>     client to know what is the problem, but it seems like you are hitting
>     some kind of a race there. Possibly you are reading the register
>     values before they have been populated (possibly even before the whole
>     thread list is populated, which could explain why you are getting
>     inconsistent thread counts).
>     On 29 May 2015 at 17:15, Ryan Brown <ribrdb at google.com
>     <mailto:ribrdb at google.com>> wrote:
>     > It seemed like this is the way to connect to an already running
>     gdbserver. I
>     > was trying to see the output of gdb server to see why it was
>     crashing.
>     > Although I suppose the "OS or CPU not supported!" message might
>     just mean I
>     > started lldb-server with the wrong args and not the real problem.
>     Yeah, debugging the server is not very easy. I usually just attach to
>     a running instance, or load the resulting core file. As I said, this
>     looks like a client issue, but it should still not crash the server.
>     If you encounter a server crash (without using "process connect"), I
>     would be interested in it and can probably fix it.
>     > Here's a log from one session:
>     >
>     <snip>
>     > < 588> read packet:
>     >
>     $T05thread:5cdf;name:vc.test;threads:5cd5,5cdb,5cdc,5cdd,5cde,5cdf,5ce0,5ce1,5ce2,5ce3;00:883e4c08c2000000;01:0000000000000000;02:401a0008c2000000;03:90d3950000000000;04:0000000000000000;05:683f4c08c2000000;06:78d5ab0000000000;07:083e4c08c2000000;08:2e8d685500000000;09:0700000000000000;0a:403d053600000000;0b:7b00000000000000;0c:1100000000000000;0d:7cc4950000000000;0e:0800000000000000;0f:0000000000000000;10:79f1490000000000;11:4602000000000000;12:3300000000000000;13:0000000000000000;14:0000000000000000;15:2b00000000000000;16:0000000000000000;17:0000000000000000;reason:breakpoint;#14
>     This is the packet notifying the client you've hit a breakpoint. It
>     also provides the values of the general purpose registers. The values
>     seem reasonable here.
>     After this, you are only doing memory read commands, and these should
>     not affect the registers.
>     > (lldb) register read
>     > General Purpose Registers:
>     >        rax = 0x0000000000000000
>     >        rbx = 0x0000000000000000
>     >        rcx = 0x0000000000000000
>     >        rdx = 0x0000000000000000
>     >        rdi = 0x0000000000000000
>     >        rsi = 0x0000000000000000
>     >        rbp = 0x0000000000000000
>     >        rsp = 0x00007fffffffdde0
>     >         r8 = 0x0000000000000000
>     >         r9 = 0x0000000000000000
>     >        r10 = 0x0000000000000000
>     >        r11 = 0x0000000000000000
>     >        r12 = 0x0000000000000000
>     >        r13 = 0x0000000000000000
>     >        r14 = 0x0000000000000000
>     >        r15 = 0x0000000000000000
>     >        rip = 0x00007ffff7ddb2d0
>     >     rflags = 0x0000000000000200
>     >         cs = 0x0000000000000033
>     >         fs = 0x0000000000000000
>     >         gs = 0x0000000000000000
>     >         ss = 0x000000000000002b
>     >         ds = 0x0000000000000000
>     >         es = 0x0000000000000000
>     "register read" does not need to issue any new server commands, since
>     it thinks the register values have already been provided. However, it
>     seems they got corrupted on their way...
>     hope this helps,
>     pl
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Ewan Crawford
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