[Vm-dev] Fixing crashes on delivery of SIGIO etc [Was Re: [Pharo-dev] Seg Fault Pharo 7.0.3, Was Difficult to debug VM crash with full blocks and Sista V1]

Eliot Miranda eliot.miranda at gmail.com
Wed Oct 9 20:10:25 UTC 2019

Hi Nicolas, Clément, et al,

On Tue, Oct 8, 2019 at 1:40 PM Nicolas Cellier <
nicolas.cellier.aka.nice at gmail.com> wrote:

> More on the problem that Eliot is speaking about: it can happen in these
> conditions:
> if is SIGIO is delivered while executing any trampoline transiting from
> SmalltalkToCStackSwitch.
> What happens here?
> We must load 64bits contents of memory (cStackPointerAddress) to the stack
> pointer register (SPReg = $rsp) - See genLoadCStackPointer.
> This is done by using the CogRTLOpcode abstract instruction MoveAwR
> But we have no matching instruction in IA-32 X64... We can only load a
> 64bits memory content in $rax
> So the idea is to generate this sequence
> (CogX64Compiler>>concretizeMoveAwR):
>     xchgq  %rsp, %rax
>     movabsq 0x10027c338, %rax ; cStackPointerAddress
>     xchgq  %rsp, %rax
> That's clever because it preserves $rax which could be in use when we want
> to MoveAwR.

Clever, but in the case of %rsp and %rbp, dead wrong :blush:.  The
constraint is that x86_64 only provides a 64-bit load from an absolute
address into %rax; no other register can be used.

> But it has an unfortunate side effect: the stack pointer temporarily gets
> the contents of $rax and can thus temporarily point anywhere.

Right; which is unacceptable.  It's a tiny window, but one we hit all the

> What happens when performing Squeak SocketTest is that we use some
> trampolines (I guess for invoking primitives for example) and we generate
> SIGIO (for some reasons, there are a lot of SIGIO generated on my
> particular macos machine, so I can trigger the bug more easily than Eliot).
> We previously installed a handler for SIGIO via signal(). When we use
> signal(), the handler shares the stack pointer with user program.
> If the event is delivered in between the two xchgq instructions above, the
> signal handler will then use a corrupted stack pointer pointing anywhere
> (depending no contents of $rax) when the VM enter the signal handler
> function, it uses stack pointer to save some states, and corrupt a memory
> zone, segfault or whatever.
> In the case described by the opensmalltalk vm-dev thread, $rax was
> pointing to the generated code zone (jitted methodZone), so we corrupted
> the generated code and soon get punished for that. But it's probable that
> there might be other (rare) occurrences of this bug.
> Not sure if it is causing the bugs described by Sean, but it's important
> to use the fix from Eliot ASAP and retry.
> There might be other occurrence of signal(SIGIO,forceInterruptCheck) in
> minheadless flavour, I did not check if Eliot also corrected it, if not it
> should also be corrected ASAP, as should every usage of signal() be
> replaced by ussage of sigaction() with appropriate flags to use
> sigaltstack() - see Eliot's commit details.

I want to discuss potential fixes with Clément and Nicolas, and anyone else
interested. So...

1. the straight forward fix is to generate different code for setting %rsp
and %rbp.  I shall do this very soon.  On x86_64 we dedicate a register for
code generation purposes, this is called RISCTempReg, and is either %r8
(SysV ABI (unix)) or %r11 (Windows).  RISCTempReg is never assumed to be
live except within a single instruction sequence.  We can swap with this
before assigning to %rsp, e.g.

    xchgq  %r8, %rax
    movabsq 0x10027c338, %rax ; cStackPointerAddress
    xchgq  %r8, %rax
    movq %r8, %rsp

this adds a couple of bytes, but is reliable.

2. one could imagine exchanging TempReg and RISCTempReg, i.e. TempReg would
be either %r8 or %r11, and RISCTemptReg would be %rax.  That would allow

    movabsq 0x10027c338, %rax ; cStackPointerAddress
    movq %rax, %rsp

this at least worked on Ryan's MIPS32 back end when it was in use (no one
has tested it in as while AFAIA) where TempReg is S5 (r21), RISCTempReg is
AT (r1) and CResultRegister is V0 (r2).  So this is worth investigating.
 [I had tried something similar with HPS and it completely broke the code
generator so my gut is trying to tell me this will never work; Ryan has
proved otherwise].

3. I would much rather implement some form of tracking whether a particular
register is live or not.  A trivial implementation would only track TempReg
and only track being not live up until the first assignment to TempReg.  A
more sophisticated approach could deal with control flow branching and
merging of the liveness info.  I like the trivial approach for now.  That
would simply track TempReg and only up until the first explicit assignment
to TempReg.

4. we could simply treat assignments to SPReg and FPReg specially, knowing
that these are done only in trampolines and enilopmarts (these are the
pieces of code that sit b between JITted code and the rest of the run-time
(interpreter, memory manager, primitives, etc), either to call the runtime
from JITted code or, as in this case, to enter machine code from the
run-time.  I suppose this is OK as long as we document it.  This would
allow us to generate what we would generate for #2 above, i.e.

    movabsq 0x10027c338, %rax ; cStackPointerAddress
    movq %rax, %rsp

N. are there better ways?

Clément, Nicolas (et al), what do you think?

Le dim. 6 oct. 2019 à 12:36, Eliot Miranda <eliot.miranda at gmail.com> a
écrit :

> Hi Sean, Hi All,
>>     this may be because of the issue described here:
>> http://forum.world.st/Difficult-to-debug-VM-crash-with-full-blocks-and-Sista-V1-tt5103810.html
>> This issue is characterized by the system crashing soon after start up
>> when some significant i/o is done, typically either to files or sockets.
>> It affects macOS only and may indeed affect only 64-bits.  We have strong
>> evidence that it is caused by the dynamic linker being invoked in the
>> signal handler for SIGIO when the signal is delivered while the VM is
>> executing JITted code.  The symptom that causes the crash is corruption of
>> a particular jitted method’s machine code, eg Delay class>>#startEventLoop,
>> and we believe that the corruption is caused by the linker when it
>> misinterprets a jitted Smalltalk stack frame as an ABI-compliant stack
>> frame and attempts to scan code to link it.
>> Our diagnosis is speculative; this is extremely hard to reproduce.
>> Typically in repeating a crashing run SIGIO may no longer be delivered at
>> the same point because any remote server has now woken up and delivers
>> results sooner, etc.  However, Nicolas Cellier and I are both confident
>> that we have correctly identified the bug.
>> The fix is simple; SIGIO should be delivered on a dedicated signal stack
>> (see sigaltstack(2)).  I committed a fix yesterday evening and we should
>> see within a week or so if these crashes have disappeared.
>> I encourage the Pharo vm maintainers to build and release vms that
>> include
>> https://github.com/OpenSmalltalk/opensmalltalk-vm/commit/c24970eb2859a474065c6f69060c0324aef2b211
>>  asap.
>> Cheers,
>> Eliot
>> _,,,^..^,,,_ (phone)
>> On Oct 3, 2019, at 1:24 PM, Sean P. DeNigris <sean at clipperadams.com>
>> wrote:
>> Segmentation fault Thu Oct  3 15:52:33 2019
>> VM: 201901051900 https://github.com/OpenSmalltalk/opensmalltalk-vm.git
>> Date: Sat Jan 5 20:00:11 2019 CommitHash: 7a3c6b6
>> Plugins: 201901051900
>> https://github.com/OpenSmalltalk/opensmalltalk-vm.git
>> C stack backtrace & registers:
>>    rax 0x0000000124380000 rbx 0x00007ffeebd00050 rcx 0x0000000000468260
>> rdx
>> 0x0000000000dd6800
>>    rdi 0x0000000124cee5a0 rsi 0x0000000124cee5a0 rbp 0x00007ffeebcffe50
>> rsp
>> 0x00007ffeebcffe50
>>    r8  0x00007fff3f2cefe5 r9  0x0000000000000b00 r10 0x0000000000006000
>> r11
>> 0xfffffffffcd8d5a0
>>    r12 0x0000000000000002 r13 0x0000000035800000 r14 0x00007ffeebd00064
>> r15
>> 0x0000000000002800
>>    rip 0x00007fff630f7d09
>> 0   libsystem_platform.dylib            0x00007fff630f7d09
>> _platform_memmove$VARIANT$Haswell + 41
>> 1   Pharo                               0x0000000103f52642
>> reportStackState
>> + 952
>> 2   Pharo                               0x0000000103f52987 sigsegv + 174
>> 3   libsystem_platform.dylib            0x00007fff630fab3d _sigtramp + 29
>> 4   ???                                 0x0000058900000a00 0x0 +
>> 6085968660992
>> 5   libGLImage.dylib                    0x00007fff3f2ce29e
>> glgProcessPixelsWithProcessor + 2149
>> 6   AMDRadeonX5000GLDriver              0x000000010db16db1
>> glrATIStoreLevels
>> + 1600
>> 7   AMDRadeonX5000GLDriver              0x000000010db52c83
>> glrAMD_GFX9_LoadSysTextureStandard + 45
>> 8   AMDRadeonX5000GLDriver              0x000000010db519bb
>> glrUpdateTexture
>> + 1346
>> 9   libGPUSupportMercury.dylib          0x00007fff5181279d
>> gpusLoadCurrentTextures + 591
>> 10  AMDRadeonX5000GLDriver              0x000000010db5a099
>> gldUpdateDispatch
>> + 397
>> 11  GLEngine                            0x00007fff3ff72078
>> gleDoDrawDispatchCore + 629
>> 12  GLEngine                            0x00007fff3ff16369
>> glDrawArraysInstanced_STD_Exec + 264
>> 13  GLEngine                            0x00007fff3ff1625a
>> glDrawArrays_UnpackThread + 40
>> 14  GLEngine                            0x00007fff3ff6dce1
>> gleCmdProcessor +
>> 77
>> 15  libdispatch.dylib                   0x00007fff62ec2dcf
>> _dispatch_client_callout + 8
>> 16  libdispatch.dylib                   0x00007fff62ecea2c
>> _dispatch_lane_barrier_sync_invoke_and_complete + 60
>> 17  GLEngine                            0x00007fff3fec4b85
>> glFlush_ExecThread + 15
>> 18  Pharo                               0x0000000103f4cc62
>> -[sqSqueakOSXOpenGLView drawRect:flush:] + 314
>> 19  Pharo                               0x0000000103f4cb22 -
>> ...
>> Smalltalk stack dump:
>>    0x7ffeebd14238 M DelaySemaphoreScheduler>unscheduleAtTimingPriority
>> 0x10fab3ad0: a(n) DelaySemaphoreScheduler
>>    0x7ffeebd14270 M [] in
>> DelaySemaphoreScheduler(DelayBasicScheduler)>runBackendLoopAtTimingPriority
>> 0x10fab3ad0: a(n) DelaySemaphoreScheduler
>>       0x1125923f8 s BlockClosure>ensure:
>>       0x111e88d30 s
>> DelaySemaphoreScheduler(DelayBasicScheduler)>runBackendLoopAtTimingPriority
>>       0x112590a50 s [] in
>> DelaySemaphoreScheduler(DelayBasicScheduler)>startTimerEventLoopPriority:
>>       0x111e88e08 s [] in BlockClosure>newProcess
>> Most recent primitives
>> @
>> actualScreenSize
>> millisecondClockValue
>> tempAt:
>> -----
>> Cheers,
>> Sean
>> --
>> Sent from: http://forum.world.st/Pharo-Smalltalk-Developers-f1294837.html

best, Eliot
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