[lldb-dev] postmortem debugging (core/minidump) & modules

[Leonard Mosescu via lldb-dev]

Thanks Greg! It makes sense and looking at the code it's already
implemented along those lines: Target::GetSharedModule() defaults to
Platform::GetSharedModule() if the initial attempt to get the module fails.

The part I'd like to understand is if there's a precedence for modules
which don't have any accessible file image (local or remote). Is everything
expected to work if we create placeholder Module & ModuleSpecs?
(it seems that the current implementation assumes that we have a file
somewhere. Ex. even creating a Module from a ModuleSpec will still try to
map the source ModuleSpec to some files).

At Apple, we call "dsymForUUID <UUID>" which, if global defaults were set
> to point at Apple's build servers, would go out and download the correct
> file for us and store it locally in a cache for future use.


Just curious, what happens if the download fails? Is the corresponding
module skipped? (is this strictly about the dSYMs or the same mechanism
works for the Mach-O binaries?)

That way if you create a target that is a minidump on a different system
> (macOS, linux, etc), the platform would be remote-windows.
>

Not sure if I understand this one, core & minidumps are currently not using
any of the the remote debugging machinery, right? Are you suggesting
changing that?

On Tue, Apr 10, 2018 at 11:56 AM, Greg Clayton <clayborg at gmail.com> wrote:

>
>
> On Apr 10, 2018, at 11:32 AM, Leonard Mosescu via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
>
> I'm looking at how the LLDB minidump reader creates the list of modules:
>
> void ProcessMinidump::ReadModuleList() {
>
> ...
>
> const auto file_spec = FileSpec(name.getValue(), true);
> ModuleSpec module_spec = file_spec;
> Status error;
> lldb::ModuleSP module_sp = GetTarget().GetSharedModule(module_spec, &
> error);
> if (!module_sp || error.Fail()) {
> continue;
> }
>
> ...
>
> }
>
>
> LLDB currently will insist on finding a local image for the module, which
> is usually not the case for postmortem debugging on machines different than
> the the the one where the minidump was created.
>
> I don't see an obvious way to model modules which have no local image
> (which is still different than the remote scenario where there is a remote
> module image), am I missing anything?
>
>
> The lldb_private::Platform is responsible for digging up any binaries for
> a given target, so this code should be grabbing the platform from the
> target and using that to get the shared module. That way if you create a
> target that is a minidump on a different system (macOS, linux, etc), the
> platform would be remote-windows.
>
> That being said "ModuleSpec" should be filled in with more than just the
> path. It should specify the UUID info from the mini dump that specifies
> exactly which version the file that you want. That way if the file on the
> current system exists, it won't return it unless the path matches. I assume
> the mini dump has each module's UUID information? If so, set it. If not the
> file format assumes you will be running the dumping the same machine and it
> should be updated to include this information. The platform code can then
> use this UUID info to possible go and fetch the right version from a UUID
> database, or how ever the platform wants to provide access to certain
> binaries. At Apple, we call "dsymForUUID <UUID>" which, if global defaults
> were set to point at Apple's build servers, would go out and download the
> correct file for us and store it locally in a cache for future use.
>
> So fill in the UUID in ModuleSpec and modify the
> Platform::GetSharedModule() for your platform to do the right thing is the
> correct way to go. ProcessMinidump should switch over to using the
> Platform::GetSharedModule() instead of the target one, or use it after the
> target one if the target returns an invalid module.
>
> Let me know if you have any more questions,
>
> Greg
>
>
> Thanks!
> Lemo.
>
>
>
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