[llvm-bugs] [Bug 28924] New: Missing relocations when linking i686 with -fpie

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28924

            Bug ID: 28924
           Summary: Missing relocations when linking i686 with -fpie
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: ELF
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm at sjor.sg
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Compiling for i686-unknown-linux-gnu with -fpie, it looks like lld is missing
some relocations, or applying them incorrectly, causing the use of external
global variables to fail. I boiled a bug down to this, but am not sure if I can
go further, as this is starting to go well outside my expertise.

This is using LLVM r278216, Clang r278213, LLD r278205.

Thanks for investigating!

-------8<-------

$ cat header.h
extern int (*global_function)();

static inline int do_global_call() {
    return global_function();
}
$ cat unit1.c
#include "header.h"
#include <stdio.h>

int (*global_function)();

int global_implementation() {
    return 1234;
}

void cu2_call();

int main() {
    global_function = global_implementation;
    printf("Global call value: %d\n", do_global_call());
    cu2_call();
}
$ cat unit2.c
#include "header.h"
#include <stdio.h>

void cu2_call() {
    printf("Compilation unit 2 global call value: %d\n", do_global_call());
}
$ i686-unknown-linux-gnu-clang -Wall -fpie unit1.c -c -o unit1.o
$ i686-unknown-linux-gnu-clang -Wall -fpie unit2.c -c -o unit2.o
$ i686-unknown-linux-gnu-clang -fpie -fuse-ld=lld
-Wl,--allow-multiple-definition unit1.o unit2.o -o testbin.lld
duplicate symbol: __x86.get_pc_thunk.bx in
/usr/lib/gcc/i586-linux-gnu/4.9/../../../i386-linux-gnu/crti.o and
/usr/lib/i386-linux-gnu/libc_nonshared.a(elf-init.oS)
$ i686-unknown-linux-gnu-clang -fpie unit1.o unit2.o -o testbin.gld
$ ./testbin.lld
Global call value: 1234
[1]    21726 segmentation fault  ./testbin.lld
$ ./testbin.gld
Global call value: 1234
Compilation unit 2 global call value: 1234

-------8<------

$ readelf -r testbin.lld

Relocation section '.rel.plt' at offset 0x36c contains 2 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00013014  00000307 R_386_JUMP_SLOT   000112c0   __libc_start_main
00013018  00000207 R_386_JUMP_SLOT   00000000   printf

$ readelf -r testbin.gld

Relocation section '.rel.dyn' at offset 0x278 contains 1 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
08049784  00000206 R_386_GLOB_DAT    00000000   __gmon_start__

Relocation section '.rel.plt' at offset 0x280 contains 3 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
08049794  00000107 R_386_JUMP_SLOT   00000000   printf
08049798  00000207 R_386_JUMP_SLOT   00000000   __gmon_start__
0804979c  00000307 R_386_JUMP_SLOT   00000000   __libc_start_main

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