gcc(4.5.3) + ld(2.21.1) with -fno-common option reports link error with multiple definition not only global variable but also function

[Takehiko NOZAKI]

Generally RTLD shouldn’t depends on libc’s function because there’s no libc function are loaded on memory until RTLD itself do.

our RTLD implementation(=ld.elf_so) uses some libc’s function(such as mem* and str*), it is safe because not dynamic link by -lc but static link by -lc_pic(PIC archive).

but there’s different between ld.elf_so and libc_pic.a of exit(3) function, prior don’t call __cxa_finalize set by__attribute__((destructor)) but latter do it, these two version’s difference comes from ifdef _LIBC.

void (*__cleanup)(void);

void
exit(int status)
{

#ifdef _LIBC
        __cxa_finalize(NULL);
#endif
        if (__cleanup)
                (*__cleanup)();
        _exit(status);
}

so ld_elf.so has own version of exit.o and -lc_pic(exit.pico) shold be ignored it.

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but this two version of exit(3) cause conflict by COPTS+=-fcommon.

gcc   -symbolic  -shared -nostartfiles -nodefaultlibs -Wl,-static -Wl,--warn-shared-textrel -Wl,-e,.rtld_start -o ld.elf_so rtld_start.o mdreloc.o rtld.o reloc.o symbol.o xmalloc.o xprintf.o debug.o map_object.o load.o search.o headers.o paths.o expand.o tls.o symver.o stack_protector.o exit.o -L/usr/src/lib/libc -L/usr/lib -lc_pic
/usr/src/lib/libc/libc_pic.a(exit.pico): In function `exit':
/usr/src/lib/libc/stdlib/exit.c:55: multiple definition of `exit'
exit.o:exit.c:(.text+0x0): first defined here
*** Error code 1

Stop.

‌

[Takehiko NOZAKI]

this is a false alarm caused by __cleanup is a tentative definition without storage class and initializer(see 6.9.2).

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if -fcommon option is set, tentative definition may places common block and all tentative definition has same name in common block are assembled to single global variable when link time.

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on the other hand -fno-common, tentative definition promoted to external linkage per compile unit, so throw link time error with multiple definition.

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but this case, ld(1) should do link ld.elf_so’s exit.o and should completely ignore libc_pic.a’s exit.pico. this is a bug of gcc/binutils. this case is never been conflict between multiple definitions.

‌

to fix this problem:

1. rename ld.elf_so’s exit(3) to another name (rtld_exit and so on). we already have rtld_die() function
2. cheating ld, set __cleanup as tentiative definition on common block by setting __attribute__((__common__)) or COPTS.exit.c+=-fcommon
3. cheating ld, set option “-z muldefs” and ignore the error
4. upgrade latest gcc/binutils, there’s no problems on N HEAD(gcc10.4.0/binutils2.39)

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[Takehiko NOZAKI]

BUGFIX: Issue #341 - gcc(4.5.3) + ld(2.21.1) with -fno-common option reports link error with multiple definition not only global variable but also function add linker option -Wl,-r,muldefs until we upgrade base gcc/binutils

→ <<cset 29b47a7df9b9>>

[Takehiko NOZAKI]

reopen, to avoid conflict crunchgen(1) by -Wl,-z,muldefs make binary huge size(or libhack.o is not working?)