> On Sep 25, 2017, at 1:30 AM, John McCall via swift-dev <swift-dev@swift.org> > wrote: > > >> On Sep 22, 2017, at 8:39 PM, Saleem Abdulrasool <compn...@compnerd.org >> <mailto:compn...@compnerd.org>> wrote: >> >> On Thu, Sep 21, 2017 at 10:28 PM, John McCall <rjmcc...@apple.com >> <mailto:rjmcc...@apple.com>> wrote: >> >>> On Sep 21, 2017, at 10:10 PM, Saleem Abdulrasool <compn...@compnerd.org >>> <mailto:compn...@compnerd.org>> wrote: >>> >>> On Thu, Sep 21, 2017 at 5:18 PM, John McCall <rjmcc...@apple.com >>> <mailto:rjmcc...@apple.com>> wrote: >>>> On Sep 21, 2017, at 1:26 PM, Saleem Abdulrasool via swift-dev >>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >>>> On Thu, Sep 21, 2017 at 12:04 PM, Joe Groff <jgr...@apple.com >>>> <mailto:jgr...@apple.com>> wrote: >>>> >>>> >>>>> On Sep 21, 2017, at 11:49 AM, Saleem Abdulrasool <compn...@compnerd.org >>>>> <mailto:compn...@compnerd.org>> wrote: >>>>> >>>>> On Thu, Sep 21, 2017 at 10:53 AM, Joe Groff <jgr...@apple.com >>>>> <mailto:jgr...@apple.com>> wrote: >>>>> >>>>> >>>>>> On Sep 21, 2017, at 9:32 AM, Saleem Abdulrasool via swift-dev >>>>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >>>>>> >>>>>> Hello, >>>>>> >>>>>> The current layout for the swift metadata for structure types, as >>>>>> emitted, seems to be unrepresentable in PE/COFF (at least for x86_64). >>>>>> There is a partial listing of the generated code following the message >>>>>> for reference. >>>>>> >>>>>> When building the standard library, LLVM encounters a relocation which >>>>>> cannot be represented. Tracking down the relocation led to the type >>>>>> metadata for SwiftNSOperatingSystemVersion. The metadata here is >>>>>> _T0SC30_SwiftNSOperatingSystemVersionVN. At +32-bytes we find the Kind >>>>>> (1). So, this is a struct metadata type. Thus at Offset 1 (+40 bytes) >>>>>> we have the nominal type descriptor reference. This is the relocation >>>>>> which we fail to represent correctly. If I'm not mistaken, it seems >>>>>> that the field is supposed to be a relative offset to the nominal type >>>>>> descriptor. However, currently, the nominal type descriptor is emitted >>>>>> in a different section (.rodata) as opposed to the type descriptor >>>>>> (.data). This cross-section relocation cannot be represented in the >>>>>> file format. >>>>>> >>>>>> My understanding is that the type metadata will be adjusted during the >>>>>> load for the field offsets. Furthermore, my guess is that the relative >>>>>> offset is used to encode the location to avoid a relocation for the load >>>>>> address base. In the case of windows, the based relocations are a >>>>>> given, and I'm not sure if there is a better approach to be taken. >>>>>> There are a couple of solutions which immediately spring to mind: moving >>>>>> the nominal type descriptor into the (RW) data segment and the other is >>>>>> to adjust the ABI to use an absolute relocation which would be rebased. >>>>>> Given that the type metadata may be adjusted means that we cannot emit >>>>>> it into the RO data segment. Is there another solution that I am >>>>>> overlooking which may be simpler or better? >>>>> >>>>> IIRC, this came up when someone was trying to port Swift to Windows on >>>>> ARM as well, and they were able to conditionalize the code so that we >>>>> used absolute pointers on Windows/ARM, and we may have to do the same on >>>>> Windows in general. It may be somewhat more complicated on Win64 since we >>>>> generally assume that relative references can be 32-bit, whereas an >>>>> absolute reference will be 64-bit, so some formats may have to change >>>>> layout to make this work too. I believe Windows' executable loader still >>>>> ultimately maps the final PE image contiguously, so alternatively, you >>>>> could conceivably build a Swift toolchain that used ELF or Mach-O or some >>>>> other format with better support for PIC as the intermediate object >>>>> format and still linked a final PE executable. Using relative references >>>>> should still be a win on Windows both because of the size benefit of >>>>> being 32-bit and the fact that they don't need to be slid when running >>>>> under ASLR or when a DLL needs to be rebased. >>>>> >>>>> >>>>> Yeah, I tracked down the relativePointer thing. There is a nice subtle >>>>> little warning that it is not fully portable :-). Would you happen to >>>>> have a pointer to where the adjustment for the absolute pointers on WoA >>>>> is? >>>>> >>>>> You are correct that the image should be contiugously mapped on Windows. >>>>> The idea of MachO as an intermediatary is rather intriguing. Thinking >>>>> longer term, maybe we want to use that as a global solution? It would >>>>> also provide a nicer autolinking mechanism for ELF which is the one >>>>> target which currently is missing this functionality. However, if Im not >>>>> mistaken, this would require a MachO linker (and the only current viable >>>>> MachO linker would be ld64). The MachO binary would then need to be >>>>> converted into ELF or COFF. This seems like it could take a while to >>>>> implement though, but would not really break ABI, so pushing that off to >>>>> later may be wise. >>>> >>>> Intriguingly, LLVM does support `*-*-win32-macho` as a target triple >>>> already, though I don't know what Mach-O to PE linker (if any) that's >>>> intended to be used with. We implemented relative references using >>>> current-position-relative offsets for Darwin and Linux both because that >>>> still allows for a fairly convenient pointer-like C++ API for working with >>>> relative offsets, and because the established toolchains on those >>>> platforms already have to support PIC so had most of the relocations we >>>> needed to make them work already; is there another base we could use for >>>> relative offsets on Windows that would fit in the set of relocations >>>> supported by standard COFF linkers? >>>> >>>> >>>> Yes, the `-windows-macho` target is used for UEFI :-). The MachO binary >>>> is translated later to PE/COFF as required by the UEFI specification. >>>> >>>> There are only two relocation types which can be used for relative >>>> displacements: __ImageBase relative (IMAGE_REL_*_ADDR32NB) and section >>>> relative (IMAGE_REL_*_SECREL) which are relative to the beginning of the >>>> section. The latter is why I mentioned that moving them into the same >>>> section could be a solution as that would allow the relative distance to >>>> be encoded. Unfortunately, the section relative relocation is relative to >>>> the section within which the symbol is. >>> >>> What's wrong with IMAGE_REL_AMD64_REL32? We'd have to adjust the >>> relative-pointer logic to store an offset from the end of the relative >>> pointer instead of the beginning, but it doesn't seem to have a section >>> requirement. >>> >>> Hmm, is it possible to use RIP relative addressing in data? If so, yes, >>> that could work. >> >> There's no inherent reason, but I wouldn't put it past the linker to fall >> over and die. But it should at least be section-agnostic about the target, >> since this is likely to be used for all sorts of PC-relative addressing. >> >> >> At least MC doesnt seem to like it. Something like this for example: >> >> ``` >> .data >> data: >> .long 0 >> >> .section .rodata >> rodata: >> .quad data(%rip) >> ``` >> >> Bails out due to the unexpected modifier. Now, theoretically, we could >> support that modififer, but it does seem pretty odd. >> >> Now, as it so happens, both PE and PE+ have limitations on the file size at >> 4GiB. This means that we are guaranteed that the relative difference is >> guaranteed to fit within 32-bits. This is where things get really >> interesting! >> >> We cannot generate the relocation because we are emitting the values at >> pointer width. However, the value that we are emitting is a relative >> offset, which we just determined to be limited to 32-bits in width. The >> thing is, the IMAGE_REL_AMD64_REL32 doesn't actually seem to care about the >> cross-setionness as you pointed out. So, rather than emitting a >> pointer-width value (`.quad`), we could emit a pad (`.long 0`) and follow >> that with the relative displacement (`.long <expr>`). This would be >> representable in the PE/COFF model. >> >> If I understand the layout correctly, the type metadata fields are supposed >> to be pointer sized. I assume that we would like to maintain that across >> the formats. It may be possible to alter the emission to change the >> relative pointer emission to emit a pair of longs instead for PE/COFF with a >> 64-bit pointer value. Basically, we cannot truncate the relocation to a >> IMAGE_REL_AMD64_REL32 but we could generate the appropriate relocation and >> pad to the desired width. >> >> Are there any pitfalls that I should be aware of trying to adjust the >> emission to do this? The only downsides that I can see is that the >> emission would need to be taret dependent (that is check the output object >> format and the target pointer width). >> >> Thanks for the hint John! It seems that was spot on :-). > > Honestly, I don't know that there's a great reason for this pointer to be > relative in the first place. The struct metadata will already have an > absolute pointer to the value witness table which requires load-time > relocation, so maybe we should just make this an absolute pointer, too, > unless we're seriously considering making that a relative pointer before > allocation.
I'm sorry, my mind wandered. "...making that (the value witness table pointer) a relative pointer before we reach ABI stability." John. > > In practice this will just be a rebase, not a full relocation, so it should > be relatively cheap. > > John. > _______________________________________________ > swift-dev mailing list > swift-dev@swift.org > https://lists.swift.org/mailman/listinfo/swift-dev
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