llvm dev - Nov 2021 - objcopy --prefix-symbols and undefined symbols

Hey folks,

I went to implement objcopy's --prefix-symbol support for MachO binaries
and was a bit surprised by the behavior of it with elf binaries (which
matches binutils' objcopy as well). Prefixing applies to all symbols,
including undefined symbols, meaning something as simple as this example,
will not work:

```
% cat /tmp/main.c
#include <stdio.h>

int main() {
  printf("hi\n");
}
% clang /tmp/main.c -o /tmp/main.o
% llvm-objcopy --prefix-symbols=bar /tmp/main.o
% clang /tmp/main.o
/usr/bin/ld: error in /tmp/main.o(.eh_frame); no .eh_frame_hdr table will
be created
/usr/bin/ld:
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crt1.o:
in function `_start':
(.text+0x24): undefined reference to `main'
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
```

While prefixing `main` specifically might not be a common use case, my
expected use case for this would be to prefix symbols in a static library,
which has a similar issue for undefined symbols:

```
/usr/bin/ld: prefixed.o:(.data+0x6b0): undefined reference to `barmunmap'
/usr/bin/ld: prefixed.o:(.data+0x6c8): undefined reference to `barmremap'
/usr/bin/ld: prefixed.o:(.data+0x6f8): undefined reference to `barreadlink'
/usr/bin/ld: prefixed.o:(.data+0x710): undefined reference to `barlstat64'
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
```

I must be understanding the purpose of this flag in general so I'm curious
if someone could clarify this use case to see if it's worth following
through on MachO support for.

Thanks!
--
Keith Smiley
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Hi Keith,

llvm-objcopy's ELF behaviour is closely modelled on GNU objcopy's
behaviour, with the intent that it is a drop-in replacement for that tool.
There are a limited number of exceptions to this, but --prefix-symbols is
not one of them. When using GNU objcopy v2.30, I get the following
behaviour:

$ cat test.cpp
void foo();
int main(){
   foo();
}
$ gcc -c test.cpp
$ nm test.o
                 U _GLOBAL_OFFSET_TABLE_
0000000000000000 T main
                 U _Z3foov
$ objcopy test.o test.prefix.o --prefix-symbols=bar
$ nm test.prefix.o
                 U bar_GLOBAL_OFFSET_TABLE_
0000000000000000 T barmain
                 U bar_Z3foov

I agree with you that I don't know under what situations this behaviour can
be useful, apart from some pretty niche cases. Perhaps worth raising on the
GNU mailing list?

James


On Sat, 13 Nov 2021 at 01:02, Keith Smiley via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hey folks,
>
> I went to implement objcopy's --prefix-symbol support for MachO
binaries
> and was a bit surprised by the behavior of it with elf binaries (which
> matches binutils' objcopy as well). Prefixing applies to all symbols,
> including undefined symbols, meaning something as simple as this example,
> will not work:
>
> ```
> % cat /tmp/main.c
> #include <stdio.h>
>
> int main() {
>   printf("hi\n");
> }
> % clang /tmp/main.c -o /tmp/main.o
> % llvm-objcopy --prefix-symbols=bar /tmp/main.o
> % clang /tmp/main.o
> /usr/bin/ld: error in /tmp/main.o(.eh_frame); no .eh_frame_hdr table will
> be created
> /usr/bin/ld:
> /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crt1.o:
> in function `_start':
> (.text+0x24): undefined reference to `main'
> clang: error: linker command failed with exit code 1 (use -v to see
> invocation)
> ```
>
> While prefixing `main` specifically might not be a common use case, my
> expected use case for this would be to prefix symbols in a static library,
> which has a similar issue for undefined symbols:
>
> ```
> /usr/bin/ld: prefixed.o:(.data+0x6b0): undefined reference to
`barmunmap'
> /usr/bin/ld: prefixed.o:(.data+0x6c8): undefined reference to
`barmremap'
> /usr/bin/ld: prefixed.o:(.data+0x6f8): undefined reference to
`barreadlink'
> /usr/bin/ld: prefixed.o:(.data+0x710): undefined reference to
`barlstat64'
> clang: error: linker command failed with exit code 1 (use -v to see
> invocation)
> ```
>
> I must be understanding the purpose of this flag in general so I'm
curious
> if someone could clarify this use case to see if it's worth following
> through on MachO support for.
>
> Thanks!
> --
> Keith Smiley
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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