[lldb-dev] [RFC] OS Awareness in LLDB

Thu Nov 1 01:25:33 PDT 2018

I'm new in plugin ecosystem, so I have some misunderstanding. You wrote
that to comprehend mutexes we just need a data formatter, but how can we
get the raw data of all mutexes in our OS? I thought I was supposed to
write a generic code that will use a user-defined (specific for each OS)
way to collect all mutexes and then use some data formatter to show them.

On Thu, Nov 1, 2018 at 2:31 AM Jim Ingham <jingham at apple.com> wrote:

> As Jason pointed out, we also have the SystemRuntime Plugin.  That is
> intended to provide extra runtime available information based on the
> current system.  For instance, on Darwin we use it to present the
> libdispatch queue view of threads on the system, and to decorate threads
> that are doing work on behalf of some queue with the state of the thread
> that enqueued the work at the time the work  item was enqueued.
>
> If for instance you had a way to gather all the locks in the process
> (something we've been asked to do but I don't know how to do it on
> Darwin...), that would be the place to put that functionality.
>
> Jim
>
>
> > On Oct 31, 2018, at 4:05 PM, Jim Ingham via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> > Right now, the OS plugin only supports the job of adding threads.  And
> that makes more sense as a plugin, because for instance if you had a
> cooperative threading scheme that you were laying on top of the system
> threads in a User Space process, you can use the Operating System plugin to
> show you the cooperative threads.  This is not an abstract example...  I
> think it should stay with just that job.
> >
> > The place where lldb holds this sort of knowledge is supposed to be in
> the Platform class.  So for instance, to comprehend mutexes you really just
> need a data formatter.  The trick is that it has to be a data formatter
> that is provided by the platform.  Similarly you want to have frame
> recognizers for interesting lower-level calls in the system.  The machinery
> will shortly be there to do that, but loading the particular recognizers
> will either need to be done by hand, or coordinated by the Platform.  In
> general, I think most of the kinds of re-presentation you need to do to
> make OS objects and processes more comprehensible can be built as general
> mechanisms like the above.  Then the Platform can coordinate providing the
> set of more general transformations that are appropriate to the Platform
> you are targeting.
> >
> > Jim
> >
> >
> >
> >> On Oct 31, 2018, at 3:44 PM, Alexander Polyakov <polyakov.alx at gmail.com>
> wrote:
> >>
> >> So, if I understand you write, I can look at OS plugin and add a
> support of mutexes or memory pages for example?
> >>
> >> чт, 1 нояб. 2018 г. в 1:05, Jim Ingham <jingham at apple.com>:
> >> lldb has one feature - the "Operating System Plugin" that is
> specifically designed for debugging threads in kernel contexts.  The OS
> plugin allows a kernel to present it's notion of threads into
> lldb_private::Threads.  The xnu kernel macros have an implementation of
> this, as do some other embedded OS'es.  lldb actually gets used pretty
> extensively for debugging xnu - the Darwin kernel.
> >>
> >> Kuba is adding the notion of "Frame recognizers" which can give
> significance to particular functions when they appear on the stack (for
> instance displaying the first argument of read, etc. as a file handle even
> if you have no debug information for it.)  That's another way that you
> could express your understanding of the OS you are running on for debugger
> users.  Greg wrote a data formatter for the Darwin implementation of
> pthread_mutex that shows the thread that has the lock and some other
> information like that.  So data formatters are also a way lldb can express
> knowledge of the host OS.
> >>
> >> Jim
> >>
> >>> On Oct 31, 2018, at 12:52 PM, Leonard Mosescu via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >>>
> >>> Conceptually it's different levels of abstraction: a user-mode
> debugger handles processes, threads as first class concepts. In kernel-mode
> (or kernel land), these are just data structures that the code (the kernel)
> is managing. From a more pragmatic perspective, the difference is in where
> the debugging hooks are implemented and what interfaces are exposed (for
> example a kernel mode debugger can normally "poke" around any piece of
> memory and it has to be aware of things like VA mappings, while a user-mode
> debugger is only allowed to control a limited slice of the system - ex.
> control a sub-process through something like ptrace)
> >>>
> >>> Unless you're specifically looking at kernel debugging I'd stay away
> from that. For one thing, LLDB is mostly used as an user-mode debugger so
> the impact of any improvements would be bigger.
> >>>
> >>> Regarding the value of OS-awareness for user-mode debugging, I agree
> with Zach - for example windbg provides both kernel mode and user mode
> !locks commands. The only suggestion I'd add is to consider an expanded
> view of the "OS" to include runtime components which may not be technically
> part of what most people think of as the "OS": user-mode loaders and high
> level things like std::mutex, etc.
> >>>
> >>> On Wed, Oct 31, 2018 at 12:29 PM, Alexander Polyakov <
> polyakov.alx at gmail.com> wrote:
> >>> Looks like I don't completely understand what is the difference
> between user-mode and kernel-mode from the debugger's point of view. Could
> you please explain me this?
> >>>
> >>> On Wed, Oct 31, 2018 at 10:22 PM Zachary Turner <zturner at google.com>
> wrote:
> >>> I don’t totally agree with this. I think there are a lot of useful os
> awareness tasks in user mode. For example, you’re debugging a deadlock and
> want to understand the state of other mutexes, who owns them, etc. or you
> want to examine open file descriptors. In the case of a heap corruption you
> may wish to study the internal structures of your process’s heap, or even
> lower level, the os virtual memory page table structures.
> >>>
> >>> There’s quite a lot you can still do in user mode, but definitely
> there is more in kernel mode. As Leonard said, try put WinDbg as a lot of
> this stuff already exists so it’s a good reference
> >>> On Wed, Oct 31, 2018 at 12:08 PM Alexander Polyakov via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >>> Hi Leonard,
> >>>
> >>> I think it will be kernel-mode debugging since debugging an
> application in user mode is not an OS awareness imo. Of course, some of
> kernel's modules might run in user-mode, but it will be ok I think.
> >>>
> >>> Thanks for your reference, I'll take a look at it.
> >>>
> >>> Also, I found out that ARM supports OS awareness in their DS-5
> debugger. They have a mechanism for adding new operating systems. All you
> need to do is to describe OS' model (thread's or task's structure for
> example). I think that is how it might be done in LLDB.
> >>>
> >>> On Wed, Oct 31, 2018 at 9:26 PM Leonard Mosescu <mosescu at google.com>
> wrote:
> >>> Hi Alexander, are you interested in user-mode, kernel-mode debugging
> or both?
> >>>
> >>> Fore reference, the current state of the art regarding OS-awareness
> debugging is debugging tools for windows (windbg & co.). This is not
> surprising since the tools were developed alongside Windows. Obviously they
> are specific to Windows, but it's good example of how the OS-awareness
> might look like.
> >>>
> >>>
> >>> On Mon, Oct 29, 2018 at 11:37 AM, Alexander Polyakov via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >>> Hi lldb-dev,
> >>>
> >>> I'm a senior student at Saint Petersburg State University. The one of
> my possible diploma themes is "OS Awareness in LLDB". Generally, the OS
> awareness extends a debugger to provide a representation of the OS threads
> - or tasks - and other relevant data structures, typically semaphores,
> mutexes, or queues.
> >>>
> >>> I want to ask the community if OS awareness is interesting for LLDB
> users and developers? The main goal is to create some base on top of LLDB
> that can be extended to support awareness for different operating systems.
> >>>
> >>> Also, if you have a good article or other useful information about OS
> awareness, please share it with me.
> >>>
> >>> Thanks in advance!
> >>>
> >>> --
> >>> Alexander
> >>>
> >>> _______________________________________________
> >>> lldb-dev mailing list
> >>> lldb-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
> >>>
> >>>
> >>>
> >>>
> >>> --
> >>> Alexander
> >>> _______________________________________________
> >>> lldb-dev mailing list
> >>> lldb-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
> >>>
> >>>
> >>> --
> >>> Alexander
> >>>
> >>> _______________________________________________
> >>> lldb-dev mailing list
> >>> lldb-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
> >>
> >> --
> >> Alexander
> >
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
>
>

-- 
Alexander
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