[GHC] #10672: GHCi linker does not understand C++ exception tables on Windows

[GHC]

#10672: GHCi linker does not understand C++ exception tables on Windows
-------------------------------------+-------------------------------------
        Reporter:  lukexi            |                Owner:  Phyx-
            Type:  bug               |               Status:  patch
        Priority:  high              |            Milestone:  7.10.3
       Component:  Runtime System    |              Version:  7.10.1
  (Linker)                           |
      Resolution:                    |             Keywords:
Operating System:  Windows           |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9297 #10563      |  Differential Rev(s):  Phab:D1244
  #8237 #9907                        |
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Comment (by Thomas Miedema <thomasmiedema@…>):

 In [changeset:"620fc6f909cd6e51b5613454097ec1c9f323839a/ghc"
 620fc6f9/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="620fc6f909cd6e51b5613454097ec1c9f323839a"
 Make Windows linker more robust to unknown sections

 The Windows Linker has 3 main parts that this patch changes.

 1) Identification and classification of sections
 2) Adding of symbols to the symbols tables
 3) Reallocation of sections

 1.
 Previously section identification used to be done on a whitelisted
 basis. It was also exclusively being done based on the names of the
 sections. This meant that there was a bit of a cat and mouse game
 between `GCC` and `GHC`. Every time `GCC` added new sections there was a
 good chance `GHC` would break. Luckily this hasn't happened much in the
 past because the `GCC` versions `GHC` used were largely unchanged.

 The new code instead treats all new section as `CODE` or `DATA`
 sections, and changes the classifications based on the `Characteristics`
 flag in the PE header. By doing so we no longer have the fragility of
 changing section names. The one exception to this is the `.ctors`
 section, which has no differentiating flag in the PE header, but we know
 we need to treat it as initialization data.

 The check to see if the sections are aligned by `4` has been removed.
 The reason is that debug sections often time are `1 aligned` but do have
 relocation symbols. In order to support relocations of `.debug` sections
 this check needs to be gone. Crucially this assumption doesn't seem to
 be in the rest of the code. We only check if there are at least 4 bytes
 to realign further down the road.

 2.
 The second loop is iterating of all the symbols in the file and trying
 to add them to the symbols table. Because the classification of the
 sections we did previously are (currently) not available in this phase
 we still have to exclude the sections by hand. If they don't we will
 load in symbols from sections we've explicitly ignored the in # 1. This
 whole part should rewritten to avoid this. But didn't want to do it in
 this commit.

 3.
 Finally the sections are relocated. But for some reason the PE files
 contain a Linux relocation constant in them `0x0011` This constant as
 far as I can tell does not come from GHC (or I couldn't find where it's
 being set). I believe this is probably a bug in GAS. But because the
 constant is in the output we have to handle it. I am thus mapping it to
 the constant I think it should be `0x0003`.

 Finally, static linking *should* work, but won't. At least not if you
 want to statically link `libgcc` with exceptions support. Doing so would
 require you to link `libgcc` and `libstd++` but also `libmingwex`. The
 problem is that `libmingwex` also defines a lot of symbols that the RTS
 automatically injects into the symbol table. Presumably because they're
 symbols that it needs. like `coshf`. The these symbols are not in a
 section that is declared with weak symbols support. So if we ever want
 to get this working, we should either a) Ask mingw to declare the
 section as such, or b) treat all a imported symbols as being weak.
 Though this doesn't seem like it's a good idea..

 Test Plan:
 Running ./validate for both x86 and x86_64

 Also running the specific test case for #10672

 make TESTS="T10672_x86 T10672_x64"

 Reviewed By: ezyang, thomie, austin

 Differential Revision: https://phabricator.haskell.org/D1244

 GHC Trac Issues: #9907, #10672, #10563
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10672#comment:19>
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