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Re: --enable-auto-import extension

First: Woo Hoo!  Thanks for looking in to this problem Egor!!

egor duda wrote:

Clean way to handle such situations (other than convincing
Microsoft to change a loader) is to add some runtime support for
non-zero-offset imports.

The idea is to add a vector of "cygwin internal pseudo-relocation"
entries to executable data section and to perform relocations of
appropriate data manually at program startup.

But why is this cygwin-specific? It seems that it's equally applicable to mingw (e.g. native) DLLs, just as mingw's gcc can use the current auto-import feature, even though MSVC can't understand or use it...

Attached is a proof-of-concept patch to ld and simple testcase.

Tested, and works:

$ ./crtest
ptr=1125, addend=8
reloc target=401125
ptr=112b, addend=4
reloc target=40112b
data=111 222 333

If this idea is worthwhile, i think i should add more things to the

Absolutely. Of course, it would still need to be *rigorously* tested to insure that
a) DLLs built this way could still be linked-to by "regular" code that doesn't violate the current limitations. (e.g. suppose I as the cygintl-2.dll maintainer built the next cygintl-2.dll using this spiffy new ld.exe; so now, cygintl-2.dll has the extra reloc table.

Q1: will existing code that relied on the OLD cygintl-2.dll (without the additional reloc table) and does NOT try to access data-with-addend, STILL work if I drop in the new DLL? [e.g. runtime backwards compat]

Q2: Could I relink old code (that again, does NOT try to access data-with-addend) to the new DLL using an OLD ld.exe? (e.g. an enduser of cygintl-2.dll who hasn't updated their binutils) [e.g linktime backwards compat]

Q3: Is the new DLL usable by windows tools, provided a suitable import library is generated? (I'm thinking here of mingw folks who build DLLs and implibs for use by others with MSVC -- granted, MSVC can't use auto-import at all, much less your extention. But the same linker will be used even to build "regular" DLLs...we can't break that.)

1. Make cygreloc generation conditional via --enable-cygwin-reloc or
something like that.

At first, yes, it does need to be conditional -- and default to OFF, probably...

And, it should probably not be "cygwin" specific. --enable-data-reloc?

2. If linker creates at least one cygreloc entry, it should emit
reference to some external symbol, say 'cygwin_process_cygreloc' so
that if object contains non-empty cygreloc vector it'd be
guaranteed that it can't be linked with runtime without cygreloc

Okay, that takes care of "new style" exe accidentally linking at runtime to "old style" DLL. Still, that leaves compatibility questions about
existing "old style" EXE ---> "new style" DLL
linking a "new" old style EXE using the old linker ---> against a "new style" DLL

This chunk of code (in pe-dll.c)

+ if (pe_dll_extra_pe_debug)
+ printf ("creating cygreloc entry for %s (addend=%d)\n",
+ fixup_name, addend);
+ b = make_cygreloc_fixup_entry (name, fixup_name, addend, output_bfd);
+ add_bfd_to_link (b, b->filename, &link_info);

doesn't seem to get called in your example -- but it should, if I understand correctly...What's the deal?

Anyway, because I can't see any "creating cygreloc entry..." debug messages, I'm not quite sure exactly where the cygreloc vector GOES -- into the client .o, or into the DLL. I had assumed the DLL, but your point #2 above confuses that issue for me...
(cygreloc --> addend_reloc?)

3. Make relocations a bit more flexible by adding type and size
(possible 64-bit support?)

I dunno -- that's a tall order. This addend-offset problem affects structs and arrays -- which come in all SORTS of specific types with different field orders and sizes. Also, what about recursive offsets?

bob = a[2][3].baz ?

Granted, fixing 64 bit types (long long), simple arrays, and simple structs will go a LONG way to solving the problem in practical terms -- but until EVERY case is covered, we still need to detect the failure cases and warn at link time (not runtime).

Nice work so far, but it'll need LOTS of testing and verification, as you can well imagine. Unfortunately, my time will be VERY VERY limited over the next six weeks to help with this sort of thing -- or for any cygwin-related stuff. Thesis Defense approaches...


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