[PATCH 1/3] makedep: Align PE sections so they can be mmapped.

[Rémi Bernon]

On 5/29/20 2:57 PM, Jacek Caban wrote:
> On 27.05.2020 13:49, Rémi Bernon wrote:
>> On 5/25/20 5:26 PM, Jacek Caban wrote:
>>> On 25.05.2020 16:32, Rémi Bernon wrote:
>>>> I could also see that some CROSSDEBUG checks look for "split" prefix in
>>>> makedep, and some other set the variable to dwarf. It's not clear to me
>>>> what this variable is supposed to be set to (other than "pdb"). Does
>>>> "dwarf" mean separate file as well?
>>>
>>>
>>> No, "dwarf" means that debug info will be embedded in module PE 
>>> files, which has always been the default. "dwarf" part of 
>>> "split-dwarf" is important for configure script to apply 
>>> dwarf-specific flags. "dwarf," "split-dwarf" and "pdb" are the only 
>>> configs for which I know a real use, but in theory you could set it 
>>> to anything else as long as you take care of CROSSCFLAGS yourself. 
>>> "stabs" and "split-stabs" should be in theory achievable this way 
>>> (but I didn't try).
>>>
>>>
>>> Jacek
>>>
>>
>> Alright. So, "split*" means that the debug sections are extracted to a 
>> separate file, and, currently, this file is also a PE file, as the 
>> default objcopy operation is to use the same output format as the 
>> source file.
>>
>> Having the debug section in an ELF file instead in this case shouldn't 
>> be an issue right? Gdb and any other tool that were already able to 
>> parse DWARF from PE files should be able to parse DWARF from ELF 
>> without trouble. Using an ELF container allows perf to read it. I 
>> don't think there's anything that requires PE container with DWARF 
>> debug info, but maybe there is?
>>
>> Regarding the default name and location of this ELF module, I see that 
>> we already implement in dbghelp the same logic as gdb does, which 
>> includes looking for .debug/debugfile, where debugfile is the contents 
>> of GNU debuglink, so this should work fine as well, while at the same 
>> time making perf work OOTB.
>>
>> From a quick test, as long as the .debug folder is also copied 
>> alongside the dll in the prefix, both winedbg and winedbg --gdb are 
>> happy with the ELF container.
>>
>> So, in the end, although I understand the root issue is some 
>> limitation of perf itself, this feels much simpler to fix in Wine with 
>> these small tweaks that makes every tool happy, than to try 
>> implementing PE support in perf. I think it would require much more 
>> work there, and would be way harder to justify if it were to be 
>> upstreamed (this is clearly a Wine specific use-case and I'm not sure 
>> how eager perf maintainers would be to have huge code changes just for 
>> the sake of PE module parsing).
> 
> 
> Yes, I agree that it could all work, I even made sure that dbghelp.dll 
> can handle it when I was adding support for debug links in PE. I'm not 
> against (at least part) of the patchset. But the thing is that it's 
> quite a lot of perf-specific tweaking. Even if we take those patches, it 
> would still be interesting to be better supported by perf, so why not at 
> least evaluate it first.

Sure it's perf specific, but to be honest I'm not seeing any actual use 
case for this separate debug file except for perf compatibility. I'm not 
talking about the PDB here, just the separate PE module with DWARF debug 
infos.

I understand why debug symbols are usually split, for instance by OS 
distributions, to reduce the size of the packages, but it is usually 
done already using build wrappers. I'm maybe not aware of all use cases 
but right now I don't see any use of having the DWARF debug info being 
split after link for normal wine builds.

> 
> I took a quick look at perf sources and I'm not convinced that it would 
> be hard. It already has some abstraction, see dso_binary_type enum and 
> its usage. It already has special cases for things like 
> DSO_BINARY_TYPE__JAVA_JIT, so DSO_BINARY_TYPE__PE doesn't seem out of 
> place. The amount of PE understanding that perf would need to find debug 
> info is very minimal: it just needs to be able to enumerate sections.
> 

Well DSO_BINARY_TYPE__JAVA_JIT is really some ugly hack to make perf a 
little bit more flexible and let some other tool dump a symbol list with 
address ranges in a "perfmap" text file at runtime.

Other than that the dso_binary_type enumeration is mostly used to tell 
where to find the debug file, not really the format it's in. The code 
the assumes ELF format all over the place, even using libelf constants 
internally.

The "perfmap" support is just some plumbing on top of it, parsing the 
file and inserting ELF-like symbols in the internal list.

It's indeed possible to do the same thing for PE files, as implemented 
in the attached patches, using libbfd to parse them, but it still feels 
very hacky. Then I may be pessimistic but I also expect it to be a bit 
hard to justify such a change, as it is -in the other way this time- a 
wine-specific change in perf.

Also, although the basic features seems to work fine (perf annotate 
relies on external tools for disassembly for instance), it's limited to 
providing the symbol list and it's not going to be well integrated with 
the ELF/DWARF part of the code, so it may lack features.

> 
> The above is about two other patches. I didn't look at PE alignment fix, 
> but I just expect that it should be fixable upstream. With that in mind, 
> my comments to patches:
> 

I'm not sure to understand, you mean fixable in perf as well? I don't 
think it is easily fixable there either, perf uses mmap hooks to detect 
which files are mapped to which memory range, and if the section aren't 
aligned we copy them instead, which makes it loose track.

> 
> - PE alignment
> 
> The patch breaks build with llvm-mingw because lld doesn't support 
> --file-alignment. It's also not exactly free. Note that you could just 
> pass CROSSLDFLAGS=... to configure instead of patching Wine.
> 

Sure and all these patches can actually be implemented with some build 
system wrapper, I just think it'd be nice if perf could handle default 
wine builds.

> 
> - .debug directory
> 
> I don't have a strong opinion here. If it's indeed a standard, maybe we 
> should follow it. GDB documentation mentions both %s.debug and .debug/%s 
> and, not being a fan of hidden directories in build tree, I chose the 
> one that seemed nicer:
> 
> https://sourceware.org/gdb/current/onlinedocs/gdb/Separate-Debug-Files.html
> 
> 
> - symbols in ELF file
> 
> We could do that, creating an exotic mix then building on mac, with PE, 
> Mach-O and ELF files. Another option would be 
> CROSSLDFLAGS=-Wl,--objcopy,-Oelf64-x86-64, but it doesn't seem nice...
> 

I don't know how useful ELF or Mach-O format would be on mac, we could 
leave the PE format there if that's better. But again, I don't really 
understand the use case for this non-PDB split debug file.

> 
> Thanks,
> 
> Jacek
> 

Cheers,
-- 
Rémi Bernon <rbernon at codeweavers.com>
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