Deliberate deduplication of -rpath arguments in ld.bfd and ld.gold?

Deliberate deduplication of -rpath arguments in ld.bfd and ld.gold?

[Stephan Bergmann]

Over in the comments at <https://reviews.llvm.org/D129142> "[LLD][ELF] 
Drop duplicates from rpath" we wonder what the rationale might be that 
both ld.bfd and ld.gold apparently deliberately drop duplicated -rpath 
arguments with identical paths (as in

> $ cat test.c
> void f(void) {}
> $ gcc -c test.c
> $ ld.bfd -rpath=/path -rpath=/path -shared test.o
> $ readelf -d a.out | grep RUNPATH
>  0x000000000000001d (RUNPATH)            Library runpath: [/path]

for example).

For the relevant code in ld.bfd see `/* First see whether OPTARG is 
already in the path.  */` ever since 
<https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=252b5132c753830d5fd56823373aed85f2a0db63> 
"19990502 sourceware import") and for the relevant code in ld.gold see 
`// Eliminate duplicates.` since 
<https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=41f542e70b89b9ea2875c438b0c9a60ac46092dd> 
"Add support for -rpath."

Re: Deliberate deduplication of -rpath arguments in ld.bfd and ld.gold?

[Michael Matz]

Hello,

On Wed, 6 Jul 2022, Stephan Bergmann via Binutils wrote:

> Over in the comments at <https://reviews.llvm.org/D129142> "[LLD][ELF] Drop
> duplicates from rpath" we wonder what the rationale might be that both ld.bfd
> and ld.gold apparently deliberately drop duplicated -rpath arguments with
> identical paths

The rationale would be "avoid waste in the real world".  If you don't do 
that then you will find that when building certain software it will end up 
with hundreds of copies of things like "RUNPATH 
/usr/lib/my-strange-module-path/" because its build-system is strange, and 
blindly collects linker arguments from e.g. pkg-config files, but doesn't 
remove duplicates itself.  As the runtime linker doesn't do duplicate 
removal either (speed!) it would then end up looking into loading the same 
non-existing file that same hundred times.

Instead of fixing all these build systems, something simply has to remove 
the duplicates and the linker is the most sensible choice.


Ciao,
Michael.