[libunwind] c9cffdd - [libunwind] Fix usage of `_dl_find_object` on 32-bit x86

[Adrian Vogelsgesang via cfe-commits]

Author: Adrian Vogelsgesang
Date: 2022-09-16T06:29:49-07:00
New Revision: c9cffdde393f646acf62d6160e7fa6613e038508

URL: https://github.com/llvm/llvm-project/commit/c9cffdde393f646acf62d6160e7fa6613e038508
DIFF: https://github.com/llvm/llvm-project/commit/c9cffdde393f646acf62d6160e7fa6613e038508.diff

LOG: [libunwind] Fix usage of `_dl_find_object` on 32-bit x86

On 32-bit x86, `_dl_find_object` also returns a `dlfo_eh_dbase` address.
So far, compiling against a version of `_dl_find_object` which returns a
`dlfo_eh_dbase` was blocked using a `#if` + `#error`. This commit now
removes this compile time assertion and simply ignores the returned
`dlfo_eh_dbase`. All test cases are passing on a 32-bit build now.

According to https://www.gnu.org/software/libc/manual/html_node/Dynamic-Linker-Introspection.html,
`dlfo_eh_dbase` should be the base address for all DW_EH_PE_datarel
relocations. However, glibc/elf/dl-find_object.h says that eh_dbase
is the relocated DT_PLTGOT value. I don't understand how those two
statements fit together, but to fix 32-bit x86, ignoring `dlfo_eh_dbase`
seems to be good enough.

Fixes #57733

Differential Revision: https://reviews.llvm.org/D133846

Added: 
    

Modified: 
    libunwind/src/AddressSpace.hpp

Removed: 
    


################################################################################
diff  --git a/libunwind/src/AddressSpace.hpp b/libunwind/src/AddressSpace.hpp
index 315289cd4211b..b0135b0c0519c 100644
--- a/libunwind/src/AddressSpace.hpp
+++ b/libunwind/src/AddressSpace.hpp
@@ -584,11 +584,6 @@ inline bool LocalAddressSpace::findUnwindSections(pint_t targetAddr,
   // support for _dl_find_object on other unwind formats is not implemented,
   // yet.
 #if defined(DLFO_STRUCT_HAS_EH_DBASE) & defined(_LIBUNWIND_SUPPORT_DWARF_INDEX)
-  // We expect to run on a platform which does not use a base address for
-  // exception information.
-#if DLFO_STRUCT_HAS_EH_DBASE
-#error dlfo_eh_dbase is not supported for DWARF-based unwinding
-#endif
   // We expect `_dl_find_object` to return PT_GNU_EH_FRAME.
 #if DLFO_EH_SEGMENT_TYPE != PT_GNU_EH_FRAME
 #error _dl_find_object retrieves an unexpected section type