I have the source code of a static library. I'm trying to compile it into a dynamic library. The source code has .c files and one .S file. While compiling I'm getting a relocation error from the assembly code. Going through the assembly code, I find out that this error is generated while calling a function from one of the C files. Assembly code segment is,
.extern dune_syscall_handler // I added it
__dune_syscall:
testq $1, %gs:IN_USERMODE
jnz 1f
pushq %r11
popfq
vmcall
jmp *%rcx
1:
/* first switch to the kernel stack */
movq %rsp, %gs:TMP
movq %gs:TRAP_STACK, %rsp
/* now push the trap frame onto the stack */
subq $TF_END, %rsp
movq %rcx, RIP(%rsp)
movq %r11, RFLAGS(%rsp)
movq %r10, RCX(%rsp) /* fixup to standard 64-bit calling ABI */
SAVE_REGS 0, 1
movq %gs:TMP, %rax
movq %rax, RSP(%rsp)
SET_G0_FS_BASE
/* re-enable interrupts and jump to the handler */
sti
movq %rsp, %rdi /* argument 0 */
lea dune_syscall_handler, %rax <-------------------- Causing error
call *%rax
SET_G3_FS_BASE
RESTORE_REGS 0, 1
movq RCX(%rsp), %r10
movq RFLAGS(%rsp), %r11
movq RIP(%rsp), %rcx
/* switch to the user stack and return to ring 3 */
movq RSP(%rsp), %rsp
sysretq
Causing the following error,
ld: dune.o: relocation R_X86_64_32S against `dune_syscall_handler' can not be used when
making a shared object; recompile with -fPIC
dune.o: error adding symbols: Bad value
dune_syscall_handler is defined in a separate C file. I use -fPIC flag while compiling. readelf shows the following,
$readelf -r dune.o
Relocation section '.rela.text' at offset 0x18c8 contains 2 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000219 00120000000b R_X86_64_32S 0000000000000000 dune_syscall_handler + 0
I guess Sym. Value suppose to filledup by linker during runtime? That's why it is empty? Listing symbols from the object shows,
$nm dune.o
0000000000000070 t __dune_retry
00000000000002e5 T dune_jump_to_user
000000000000016c T __dune_syscall
000000000000028e T __dune_syscall_end
U dune_syscall_handler
0000000000000100 a I
I'm new to assembly. My understanding is while trying to load the effective address of the C function dune_syscall_handler, the linker could not find it. I just can't figure out why. Can someone please tell me how can I call a C function from an assembly file? There is a similar post with a same tile How to call a C function from Assembly code. But I guess my problem is different.