[llvm-bugs] [Bug 45277] New: llvm-objcopy generates unhelpful PE/COFF debug data for gnu-debuglink

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45277

            Bug ID: 45277
           Summary: llvm-objcopy generates unhelpful PE/COFF debug data
                    for gnu-debuglink
           Product: tools
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: llvm-objcopy/strip
          Assignee: unassignedbugs at nondot.org
          Reporter: vit9696 at avp.su
                CC: alexander.v.shaposhnikov at gmail.com,
                    jake.h.ehrlich at gmail.com,
                    jh7370.2008 at my.bristol.ac.uk,
                    llvm-bugs at lists.llvm.org, rupprecht at google.com

After stripping DWARF debug information one can use --add-gnu-debuglink to link
the resulting file with the original file containing the debug information:

$ cp file.dll file.debug
$ llvm-objcopy --strip-unneeded file.dll
$ llvm-objcopy --add-gnu-debuglink=$(pwd)/file.debug file.dll

When working with PE/COFF files this functionality is unfortunately very
limited:
https://github.com/llvm/llvm-project/blob/f69eba07726a9fe084812aa224309d62c4bdd2e4/llvm/tools/llvm-objcopy/COFF/COFFObjcopy.cpp#L84-L90

1. Unlike ELF, which gets a section with .gnu_debuglink name, PE/COFF files get
anonymous sections like /1234. This makes it very hard to reliably locate the
section, as it does not even have a magic, just a filename and a CRC32 hash.

.gnu_debuglink is 14 bytes long, while PE/COFF has 8 bytes maximum for the
section name. However, even if we cannot use this name, any other unique value
will work just fine (e.g. .dbglink or .debug).

2. .gnu_debuglink section is not an expected way to link PE with the debug
information. In general for PE/COFF a CodeView entry in debug DataDirectory is
used in either PDB 2.0 (NB10) or PDB 7.0 (RSDS) format.

For DWARF this is not added by llvm-objcopy, but it is likely desired, as
several tools for e.g. UEFI firmware debugging rely on at least some kind of
CodeView entry to be present. 

In MinGW mode LLD already generates a dummy PDB 7.0 entry:
https://github.com/llvm/llvm-project/blob/8620bb9534342176ac739e2a587e4cecf437310c/lld/COFF/Writer.cpp#L1823-L1831

For non-LLVM projects, such as EDK II GenFw utility used for building UEFI
firmware PE files from ELFs, it is common to add a PDB 2.0 (NB10) entry:
https://github.com/tianocore/edk2/blob/b219e2c/MdePkg/Include/IndustryStandard/PeImage.h#L614

Perhaps, this can be adopted in llvm-objcopy as well.

3. Tools like GenFw also embed full file path instead of just the base name,
which makes it much easier to locate the file on the host during the debugging
session.

I believe GNU objcopy for ELF also strips the path, but for convenience reasons
we can make an option to keep it. E.g.
--add-gnu-debuglink=/path/to/filename,/path/to/embedded/filename.

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