Packed relative relocations can produce unloadable Fil-C shared libraries

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This is AI-assisted. Model: gpt-6-astra

While porting Ruby 4.0.9 to Fil-C, I found that packing libruby's relative relocations saves about 19 MiB of mapped metadata without removing safety checks. But enabling packing globally breaks loading of Ruby's monitor.so, so our build currently limits it to the interpreter and libruby.

Fixing this would let us extend packing to native extensions. Would making packing the default for the matched Linux/glibc toolchain also be worth considering?

With optfil 0.687 on Linux ARM64, a tiny shared library linked with -Wl,-z,pack-relative-relocs fails dlopen:

./relr-packed.so: DT_RELR without GLIBC_ABI_DT_RELR dependency

It has DT_RELR and a libc.so.6666 dependency, but its only version requirement is GLIBC_2.17 on libyolocimpl.so. The missing ABI requirement triggers glibc's loader check. Packed libruby records that requirement and loads successfully.

Could the linker emit the required ABI dependency for this arrangement, rather than disabling the loader check? The precise fix still needs investigation.

Standalone reproducer: relr-dlopen.c

#ifdef LIBRARY

int answer(void)
{
    return 42;
}

#else

#include <assert.h>
#include <dlfcn.h>
#include <stdio.h>

int main(int argc, char **argv)
{
    assert(argc == 2);
    void *library = dlopen(argv[1], RTLD_NOW | RTLD_LOCAL);
    if (!library) {
        fprintf(stderr, "%s\n", dlerror());
        return 1;
    }
    int (*function)(void) = dlsym(library, "answer");
    const char *error = dlerror();
    if (error) {
        fprintf(stderr, "%s\n", error);
        return 1;
    }
    assert(function() == 42);
    assert(!dlclose(library));
    puts("loaded and called answer()");
    return 0;
}

#endif

/opt/fil/bin/filcc -O2 -Wall -Wextra -Werror relr-dlopen.c -ldl -o relr-dlopen
/opt/fil/bin/filcc -O2 -fPIC -shared -DLIBRARY relr-dlopen.c -o relr-plain.so
/opt/fil/bin/filcc -O2 -fPIC -shared -DLIBRARY relr-dlopen.c \
  -Wl,-z,pack-relative-relocs -o relr-packed.so
./relr-dlopen ./relr-plain.so
./relr-dlopen ./relr-packed.so
readelf -d relr-packed.so
readelf -V relr-packed.so

The plain library prints loaded and called answer() and exits zero; the packed library prints the loader error and exits one. Bundled linker: GNU ld 2.47.20260726.

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