[lldb-dev] Unreliable process attach on Linux

[Florian Weimer via lldb-dev]

* Pavel Labath:

> On 04/01/2019 17:38, Florian Weimer via lldb-dev wrote:
>> Consider this example program:
>>
>> #include <err.h>
>> #include <sys/wait.h>
>> #include <unistd.h>
>>
>> #include <lldb/API/SBDebugger.h>
>> #include <lldb/API/SBProcess.h>
>> #include <lldb/API/SBTarget.h>
>>
>> int
>> main(void)
>> {
>>    // Target process for the debugger.
>>    pid_t pid = fork();
>>    if (pid < 0)
>>      err(1, "fork");
>>    if (pid == 0)
>>      while (true)
>>        pause();
>>
>>    lldb::SBDebugger::Initialize();
>>    {
>>      auto debugger(lldb::SBDebugger::Create());
>>      if (!debugger.IsValid())
>>        errx(1, "SBDebugger::Create failed");
>>
>>      auto target(debugger.CreateTarget(nullptr));
>>      if (!target.IsValid())
>>        errx(1, "SBDebugger::CreateTarget failed");
>>
>>      lldb::SBAttachInfo attachinfo(pid);
>>      lldb::SBError error;
>>      auto process(target.Attach(attachinfo, error));
>>      if (!process.IsValid())
>>        errx(1, "SBTarget::Attach failed: %s", error.GetCString());
>>      error = process.Detach();
>>      if (error.Fail())
>>        errx(1, "SBProcess::Detach failed: %s", error.GetCString());
>>    }
>>    lldb::SBDebugger::Terminate();
>>
>>    if (kill(pid, SIGKILL) != 0)
>>      err(1, "kill");
>>    if (waitpid(pid, NULL, 0) < 0)
>>      err(1, "waitpid");
>>
>>    return 0;
>> }
>>
>> Run it in a loop like this:
>>
>> $ while ./test-attach ; do date; done
>>
>> On Linux x86-64 (Fedora 29), with LLDB 7 (lldb-7.0.0-1.fc29.x86_64) and
>> kernel 4.19.12 (kernel-4.19.12-301.fc29.x86_64), after 100 iterations or
>> so, attaching to the newly created process fails:
>>
>> test-attach: SBTarget::Attach failed: lost connection
>>
>> This also reproduces occasionally with LLDB itself (with “lldb -p PID”).
>>
>> Any suggestions how to get more information about the cause of this
>> error?
>>
>
> I would recommend enabling gdb-remote logging (so something like:
> debugger.HandleCommand("log enable gdb-remote packets")) to see at
> which stage do we actually lose the gdb-server connection.

Thanks.  I enabled logging like this:

    auto debugger(lldb::SBDebugger::Create());
    if (!debugger.IsValid())
      errx(1, "SBDebugger::Create failed");

    debugger.HandleCommand("log enable gdb-remote packets");

    auto target(debugger.CreateTarget(nullptr));
    if (!target.IsValid())
      errx(1, "SBDebugger::CreateTarget failed");

And here's the output I get:

test-attach      <   1> send packet: +
test-attach      history[1] tid=0x1cab <   1> send packet: +
test-attach      <  19> send packet: $QStartNoAckMode#b0
test-attach      <   1> read packet: +
test-attach      <   6> read packet: $OK#9a
test-attach      <   1> send packet: +
test-attach      <  41> send packet: $qSupported:xmlRegisters=i386,arm,mips#12
test-attach      < 124> read packet: $PacketSize=20000;QStartNoAckMode+;QThreadSuffixSupported+;QListThreadsInStopReply+;qEcho+;QPassSignals+;qXfer:auxv:read+#be
test-attach      <  26> send packet: $QThreadSuffixSupported#e4
test-attach      <   6> read packet: $OK#9a
test-attach      <  27> send packet: $QListThreadsInStopReply#21
test-attach      <   6> read packet: $OK#9a
test-attach      <  13> send packet: $qHostInfo#9b
test-attach      <  11> send packet: $qEcho:1#5b
test-attach: SBTarget::Attach failed: lost connection

Florian