[lldb-dev] How does attach work on non-Windows?

Ted Woodward via lldb-dev lldb-dev at lists.llvm.org
Wed Aug 26 16:16:56 PDT 2015

On Linux, according to the proc(5) man page, you look at /proc/<pid>/status.

There's a field "TracerPid" that gives the pid of the tracing process. The value is 0 if not being traced.

TracerPid:      13849

TracerPid:      0

/proc/pid/stat has a field called "flags" that contains kernel flags, but they don't have anything useful to determine if a process is being traced.

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-----Original Message-----
From: lldb-dev [mailto:lldb-dev-bounces at lists.llvm.org] On Behalf Of Jim Ingham via lldb-dev
Sent: Wednesday, August 26, 2015 5:26 PM
To: Zachary Turner
Subject: Re: [lldb-dev] How does attach work on non-Windows?

There's even a Tech Note for how to do this:


I think you should be able to see that w/o a developer account, but if you can't, here's the code:

#include <assert.h>
#include <stdbool.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/sysctl.h>

static bool AmIBeingDebugged(void)
    // Returns true if the current process is being debugged (either 
    // running under the debugger or has a debugger attached post facto).
    int                 junk;
    int                 mib[4];
    struct kinfo_proc   info;
    size_t              size;

    // Initialize the flags so that, if sysctl fails for some bizarre 
    // reason, we get a predictable result.

    info.kp_proc.p_flag = 0;

    // Initialize mib, which tells sysctl the info we want, in this case
    // we're looking for information about a specific process ID.

    mib[0] = CTL_KERN;
    mib[1] = KERN_PROC;
    mib[2] = KERN_PROC_PID;
    mib[3] = getpid();

    // Call sysctl.

    size = sizeof(info);
    junk = sysctl(mib, sizeof(mib) / sizeof(*mib), &info, &size, NULL, 0);
    assert(junk == 0);

    // We're being debugged if the P_TRACED flag is set.

    return ( (info.kp_proc.p_flag & P_TRACED) != 0 ); }

The document warns that this is an "unstable API" OTOH it's been there since 2004 at least so...

If most platforms have ways to tell if you have been attached to, I have no objections to using that directly.


> On Aug 26, 2015, at 3:09 PM, Zachary Turner <zturner at google.com> wrote:
> Assuming we can find a reasonable way to detect this on all platforms, can I replace current wait-for-debugger-attach code in the test inferiors to use this method?  It's all very racy right now, and there are combinations of sleeps and loops in the executables sometimes working together with sleeps in the test cases to synchronize the test and the executable.  If we had a common method that allowed inferiors to just say "wait until some debugger is attached to me" I think some of our problems would go away.
> Would you mind posting a code snippet for how to do this on OS X so someone familiar with FreeBSD and/or Linux can comment on if there's a similar one for their platform?
> On Wed, Aug 26, 2015 at 3:04 PM Jim Ingham <jingham at apple.com> wrote:
> There is a way on OS X.  There is a sysctl that will give you information on the current process state, and one of the bits you get back says whether the process is being traced.  sysctl's are a generic UNIX thing, but I don't know if the bit OS X uses is shared with other Unix's.
> Jim
> > On Aug 26, 2015, at 2:20 PM, Zachary Turner via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> >
> > Slightly related, but do other platforms have a way to check from an inferior if a debugger is present?
> >
> > We need to do this frequently from the test inferiors, and I see lots of different approaches used in the test programs, none of which work correctly on Windows.
> >
> > On Wed, Aug 26, 2015 at 2:09 PM Zachary Turner <zturner at google.com> wrote:
> > On Windows, when we attach to process, we basically invoke a system call which tells the kernel to kick off the process necessary for a debugger to be able to communicate with the process.
> >
> > The end result of all this is that eventually the OS itself will generate a breakpoint in the inferior by injecting an additional thread into the inferior and breaking on that thread.
> >
> > LLDB picks this up, and the result is that LLDB stops and waits for the user to continue the inferior just as it would with any other breakpoint, and if you were to get a backtrace you might see something like this:
> >
> > looking at: Stack traces for SBProcess: pid = 12588, state = 
> > stopped, threads = 2, executable = 
> > test_with_dwarf_and_attach_to_process_with_id_api
> > Stack trace for thread id=0x3428 name=None queue=None stop reason=none
> >   frame #0: 0xffffffffffffffff ntdll.dll`DbgBreakPoint + 1
> >
> > Stack trace for thread id=0x4314 name=None queue=None stop reason=none
> >   frame #0: 0x00000077908c2c None`None + -18446744071703589843
> >
> >
> > My question is: Do other platforms have this concept of an OS-generated breakpoint?  Or when you attach to the process, would the first breakpoint opcode encountered by the inferior be one which was created by the user through some command from the debugger?
> >
> > This creates a problem for some of our tests, because we have this extra breakpoint that is messing up the stack frame expectations unless we issue a manual continue first to get past the initial breakpoitn and to the first user breakpoint.
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