strange behavior when trying to compile a source with tcc against gcc generated .o file

Question

I am trying to compile a source with tcc (ver 0.9.26) against a gcc-generated .o file, but it has strange behavior. The gcc (ver 5.3.0)is from MinGW 64 bit.

More specifically, I have the following two files (te1.c te2.c). I did the following commands on windows7 box

c:\tcc> gcc -c te1.c
c:\tcc> objcopy -O  elf64-x86-64 te1.o   #this is needed because te1.o from previous step is in COFF format, tcc only understand ELF format
c:\tcc> tcc te2.c te1.o
c:\tcc> te2.exe
567in dummy!!!

Note that it cut off 4 bytes from the string 1234567in dummy!!!\n. Wonder if what could have gone wrong.

Thanks Jin

========file te1.c===========

#include <stdio.h>

void dummy () {
    printf1("1234567in dummy!!!\n");
}

========file te2.c===========

#include <stdio.h>

void printf1(char *p) {
    printf("%s\n",p);
}
extern void dummy();
int main(int argc, char *argv[]) {
    dummy();
    return 0;
}

Update 1

Saw a difference in assembly between te1.o (te1.c compiled by tcc) and te1_gcc.o (te1.c compiled by gcc). In the tcc compiled, I saw lea -0x4(%rip),%rcx, on the gcc compiled, I saw lea 0x0(%rip),%rcx. Not sure why.

C:\temp>objdump -d te1.o

te1.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <dummy>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 81 ec 20 00 00 00    sub    $0x20,%rsp
   b:   48 8d 0d fc ff ff ff    lea    -0x4(%rip),%rcx        # e <dummy+0xe>
  12:   e8 fc ff ff ff          callq  13 <dummy+0x13>
  17:   c9                      leaveq
  18:   c3                      retq
  19:   00 00                   add    %al,(%rax)
  1b:   00 01                   add    %al,(%rcx)
  1d:   04 02                   add    $0x2,%al
  1f:   05 04 03 01 50          add    $0x50010304,%eax

C:\temp>objdump -d te1_gcc.o

te1_gcc.o:     file format pe-x86-64


Disassembly of section .text:

0000000000000000 <dummy>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 83 ec 20             sub    $0x20,%rsp
   8:   48 8d 0d 00 00 00 00    lea    0x0(%rip),%rcx        # f <dummy+0xf>
   f:   e8 00 00 00 00          callq  14 <dummy+0x14>
  14:   90                      nop
  15:   48 83 c4 20             add    $0x20,%rsp
  19:   5d                      pop    %rbp
  1a:   c3                      retq
  1b:   90                      nop
  1c:   90                      nop
  1d:   90                      nop
  1e:   90                      nop
  1f:   90                      nop

Update2

Using a binary editor, I changed the machine code in te1.o (produced by gcc) and changed lea 0(%rip),%rcx to lea -0x4(%rip),%rcx and using the tcc to link it, the resulted exe works fine. More precisely, I did

c:\tcc> gcc -c te1.c
c:\tcc> objcopy -O  elf64-x86-64 te1.o 
c:\tcc> use a binary editor to the change the bytes from (48 8d 0d 00 00 00 00) to (48 8d 0d fc ff ff ff)
c:\tcc> tcc te2.c te1.o
c:\tcc> te2
1234567in dummy!!!

Update 3

As requested, here is the output of objdump -r te1.o

C:\temp>gcc -c te1.c

C:\temp>objdump -r te1.o

te1.o:     file format pe-x86-64

RELOCATION RECORDS FOR [.text]:
OFFSET           TYPE              VALUE
000000000000000b R_X86_64_PC32     .rdata
0000000000000010 R_X86_64_PC32     printf1


RELOCATION RECORDS FOR [.pdata]:
OFFSET           TYPE              VALUE
0000000000000000 rva32             .text
0000000000000004 rva32             .text
0000000000000008 rva32             .xdata



C:\temp>objdump -d te1.o

te1.o:     file format pe-x86-64


Disassembly of section .text:

0000000000000000 <dummy>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 83 ec 20             sub    $0x20,%rsp
   8:   48 8d 0d 00 00 00 00    lea    0x0(%rip),%rcx        # f <dummy+0xf>
   f:   e8 00 00 00 00          callq  14 <dummy+0x14>
  14:   90                      nop
  15:   48 83 c4 20             add    $0x20,%rsp
  19:   5d                      pop    %rbp
  1a:   c3                      retq
  1b:   90                      nop
  1c:   90                      nop
  1d:   90                      nop
  1e:   90                      nop
  1f:   90                      nop

Answer 1

Has nothing to do with tcc or calling conventions. It has to do with different linker conventions for elf64-x86-64 and pe-x86-64 formats.

With PE, the linker will subtract 4 implicitly to calculate the final offset.

With ELF, it does not do this. Because of this, 0 is the correct initial value for PE, and -4 is correct for ELF.

Unfortunately, objcopy does not convert this -> bug in objcopy.

Answer 2

add

extern void printf1(char *p);

to your te1.c file

Or: the compiler will assume argument 32 bit integer since there's no prototype, and pointers are 64-bit long.

Edit: this is still not working. I found out that the function never returns (since calling the printf1 a second time does nothing!). Seems that the 4 first bytes are consumed as return address or something like that. In gcc 32-bit mode it works fine. Sounds like a calling convention problem to me but still cannot figure it out. Another clue: calling printf from te1.c side (gcc, using tcc stdlib bindings) crashes with segv.

I disassembled the executable. First part is repeated call from tcc side

  40104f:       48 8d 05 b3 0f 00 00    lea    0xfb3(%rip),%rax        # 0x402009
  401056:       48 89 45 f8             mov    %rax,-0x8(%rbp)
  40105a:       48 8b 4d f8             mov    -0x8(%rbp),%rcx
  40105e:       e8 9d ff ff ff          callq  0x401000
  401063:       48 8b 4d f8             mov    -0x8(%rbp),%rcx
  401067:       e8 94 ff ff ff          callq  0x401000
  40106c:       48 8b 4d f8             mov    -0x8(%rbp),%rcx
  401070:       e8 8b ff ff ff          callq  0x401000
  401075:       48 8b 4d f8             mov    -0x8(%rbp),%rcx
  401079:       e8 82 ff ff ff          callq  0x401000
  40107e:       e8 0d 00 00 00          callq  0x401090
  401083:       b8 00 00 00 00          mov    $0x0,%eax
  401088:       e9 00 00 00 00          jmpq   0x40108d
  40108d:       c9                      leaveq
  40108e:       c3                      retq

Second part is repeated (6 times) call to the same function. As you can see the address is different (shifted by 4 bytes, like your data) !!! It kind of works just once because the 4 first instructions are the following:

 401000:       55                      push   %rbp
 401001:       48 89 e5                mov    %rsp,%rbp

so stack is destroyed if those are skipped!!

  40109f:       48 89 45 f8             mov    %rax,-0x8(%rbp)
  4010a3:       48 8b 45 f8             mov    -0x8(%rbp),%rax
  4010a7:       48 89 c1                mov    %rax,%rcx
  4010aa:       e8 55 ff ff ff          callq  0x401004
  4010af:       48 8b 45 f8             mov    -0x8(%rbp),%rax
  4010b3:       48 89 c1                mov    %rax,%rcx
  4010b6:       e8 49 ff ff ff          callq  0x401004
  4010bb:       48 8b 45 f8             mov    -0x8(%rbp),%rax
  4010bf:       48 89 c1                mov    %rax,%rcx
  4010c2:       e8 3d ff ff ff          callq  0x401004
  4010c7:       48 8b 45 f8             mov    -0x8(%rbp),%rax
  4010cb:       48 89 c1                mov    %rax,%rcx
  4010ce:       e8 31 ff ff ff          callq  0x401004
  4010d3:       48 8b 45 f8             mov    -0x8(%rbp),%rax
  4010d7:       48 89 c1                mov    %rax,%rcx
  4010da:       e8 25 ff ff ff          callq  0x401004
  4010df:       48 8b 45 f8             mov    -0x8(%rbp),%rax
  4010e3:       48 89 c1                mov    %rax,%rcx
  4010e6:       e8 19 ff ff ff          callq  0x401004
  4010eb:       90                      nop