[PATCH] D79300: [ELF] Demote lazy symbols relative to a discarded section to Undefined

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: grimar, psmith, rprichard, ruiu.
Herald added subscribers: llvm-commits, arichardson, emaste.
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Herald added a project: LLVM.
MaskRay marked an inline comment as done.
MaskRay added inline comments.


================
Comment at: lld/ELF/InputFiles.cpp:1588
+  // symbols is in a valid but unspecified state. Make it empty and inform a
+  // future invocation of ObjFile<ELFT>::initializeSymbols() that this
+  // LazyObjFile has been processed.
----------------
s/has been processed/is being fetched/


Fixes PR45594.

In `ObjFile<ELFT>::initializeSymbols()`, for a defined symbol relative to
a discarded section (due to section group rules), it may have been
inserted as a lazy symbol. We need to demote it to an Undefined to
enable the `discarded section` error happened in a later pass.

Use the emptiness of `file->symbols` to represent that there is an
ongoing lazy symbol fetch and we should replace the current lazy symbol
with an Undefined, instead of calling `Symbol::resolve` (the lazy symbol
was added by an unrelated archive/lazy object).

As a side result, one small issue in start-lib-comdat.s is now fixed.
The hack motivating D51892 <https://reviews.llvm.org/D51892> will be unsupported: if
`.gnu.linkonce.t.__i686.get_pc_thunk.bx` is in an archive is referenced
by another section, this will likely be errored unless the function is
also defined in a regular object file.
(Bringing back rL330869 <https://reviews.llvm.org/rL330869> would error `undefined symbol` instead of the
more relevant `discarded section`.)

Note, glibc i386's crti.o still works (PR31215), because
`.gnu.linkonce.t.__x86.get_pc_thunk.bx` is in crti.o (one of the first
regular object files in a linker command line).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D79300

Files:
  lld/ELF/InputFiles.cpp
  lld/test/ELF/comdat-discarded-lazy.s
  lld/test/ELF/i386-linkonce.s
  lld/test/ELF/start-lib-comdat.s

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