[llvm-bugs] [Bug 40004] New: llvm-objcopy should strip undefined symbols if they are no longer referenced following --only-section

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40004

            Bug ID: 40004
           Summary: llvm-objcopy should strip undefined symbols if they
                    are no longer referenced following --only-section
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: llvm-objcopy
          Assignee: unassignedbugs at nondot.org
          Reporter: jh7370.2008 at my.bristol.ac.uk
                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

This may apply more widely when stripping specific sections, but I've not
tried. Currently, llvm-objcopy does not remove undefined symbols if the section
with the source of the reference (i.e. a relocation section) is stripped
following --only-section. This will make the object unusable for linking, even
though it probably should be usable under certain situations. It also is
different to what GNU objcopy does:

Input:
// bar.cpp
extern int bar;

int main(){
    return bar;
}

Build with:
clang.exe --target=x86_64-pc-linux -c bar.cpp -g

GNU:
objcopy -j .debug_str bar.o foo.o
readelf foo.o -s

Symbol table '.symtab' contains 2 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 SECTION LOCAL  DEFAULT    1

LLVM:
llvm-objcopy bar.o foo.o -j .debug_str
readelf -s foo.o

Symbol table '.symtab' contains 4 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS bar.cpp
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1
     3: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND bar <-- BAD

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