[all-commits] [llvm/llvm-project] fb1412: [ELF] --gdb-index: skip SHF_GROUP .debug_info

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: fb141292f4411448af41fc454c07f3903acb84dd
      https://github.com/llvm/llvm-project/commit/fb141292f4411448af41fc454c07f3903acb84dd
  Author: Fangrui Song <maskray at google.com>
  Date:   2020-08-13 (Thu, 13 Aug 2020)

  Changed paths:
    M lld/ELF/DWARF.cpp
    M lld/ELF/DWARF.h
    M lld/ELF/SyntheticSections.cpp
    A lld/test/ELF/gdb-index-dwarf5-type-unit.s

  Log Message:
  -----------
  [ELF] --gdb-index: skip SHF_GROUP .debug_info

-gdwarf-5 -fdebug-types-section may produce multiple .debug_info sections.  All
except one are type units (.debug_types before DWARF v5). When constructing
.gdb_index, we should ignore these type units. We use a simple heuristic: the
compile unit does not have the SHF_GROUP flag. (This needs to be revisited if
people place compile unit .debug_info in COMDAT groups.)

This issue manifests as a data race: because an object file may have multiple
.debug_info sections, we may concurrently construct `LLDDwarfObj` for the same
file in multiple threads. The threads may access `InputSectionBase::data()`
concurrently on the same input section. `InputSectionBase::data()` does a lazy
uncompress() and rewrites the member variable `rawData`. A thread running zlib
`inflate()` (transitively called by uncompress()) on a buffer with `rawData`
tampered by another thread may fail with `uncompress failed: zlib error: Z_DATA_ERROR`.

Even if no data race occurred in an optimistic run, if there are N .debug_info,
one CU entry and its address ranges will be replicated N times. The result
.gdb_index can be much larger than a correct one.

The new test gdb-index-dwarf5-type-unit.s actually has two compile units. This
cannot be produced with regular approaches (it can be produced with -r
--unique). This is used to demonstrate that the .gdb_index construction code
only considers the last non-SHF_GROUP .debug_info

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D85579