Re: Non disruptive application core dump infrastructure

["Suzuki K. Poulose" <suzuki@in.ibm.com>]

On 05/29/2014 05:16 PM, Ondrej Oprala wrote:
> On 05/29/2014 01:44 PM, Janani Venkataraman wrote:
>> Hi,
>>
>> We have developed a tool called "gencore" which captures the core of
>> an application without
>> disrupting its process. The dump is collected non-disruptively and
>> this tool currently supports
>> s390, x86 and power systems.
>>
>> THE TOOL:
>>
>> The tool can perform non-disruptive third party dumps. The tool also
>> contains a library "libgencore"
>> which helps applicationsto trigger self dumps.
>>
>> The tool can perform:
>>
>> 1) Third party dump: The pid of the process to dumped is given along
>> with name of the core-file to
>> be created.
>>
>> eg.
>>
>> [janani@localhost]:gencore 6616 core.test
>>
>> 2) Self dump: The programs can request a self-dump using gencore()
>> API, provided throughlibgencore. This
>> is implemented through a daemon which listens on a UNIX Filesocket for
>> such requests. The daemon is started
>> immediately post installation. The program which requires the dump
>> makes use of the gencore() API and provides
>> the name of the core-file as a parameter.
>>
>> eg.
>>
>> /* Opening the library, in this case the library is present in the
>> /usr/lib64 */
>> lib = dlopen("libgencore.so", RTLD_LAZY);
>>
>> gencore = dlsym(lib, "gencore");
>>
>> Call the API:
>> gencore("/home/janani/core_test").
>>
>> BASIC IDEA:
>>
>> The basic idea is that the threads of the process are held using
>> ptrace calls and the dump is generated in the
>> ELF format using the /proc/pid filesystem.
>>
>> PATCH SET:
>> We have designed this tool based on the discussions with linux kernel
>> community. The patches have been posted
>> at: https://lkml.org/lkml/2014/3/20/138
>>
>> Do you think this can be part of the util-linux bundle? We can tweak
>> it to make it work as a package in util-linux.
>>
>> Let us know your reviews and comments.
>>
>> Thanks.
>> Janani
>>
>> -- 
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> Interesting,
> but how is this different from attaching to a process with GDB and using
> the gcore command? Or to automate it more, using the gcore script that
> comes with GDB?
> Cheers,
> Ondrej
> 

There are two major issues with that.

1) GDB uses PTRACE_ATTACH and hence the process gets a SIGSTOP.

2) A process cannot initiate the request to dump itself, say from a
signal handler. (since fork() is not signal safe)

Thanks
Suzuki