phanikishoreg / dynload_exp

An experiment to learn more about dynamic loading with `dlopen` and `dlmopen`.

Dynamic Loading (libdl) Experiments

In an executable, if we have some global functions/variables,

1. Are those accessible in the shared-objects? here hello and world objects? Yes, they are accessible within the SOs. Tested with variables base and bound and whatever function.

2. Does the compiler optimize to use a register for frequently accessed global variables outside of the shared object with -fPIC? Yes, it does use registers for the global variables outside of the shared-object and not do a PIC load on each access. An inline function check_bound() accesses base and incr_bound() accesses the bound global variables from the executable and the assembler output shows that compiler optimizes the use of these global variables to access using rbx and r14 registers. Below is the assembly for that.

hello_main():
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:32
        }
}

int hello_main(void)
{
        printf("%p:%p %p:%u\n", &base, base, &bound, bound);
 800:   41 56                   push   %r14
 802:   53                      push   %rbx
 803:   50                      push   %rax
 804:   48 8b 1d ad 07 20 00    mov    0x2007ad(%rip),%rbx        # 200fb8 <base>
 80b:   48 8b 13                mov    (%rbx),%rdx
 80e:   4c 8b 35 db 07 20 00    mov    0x2007db(%rip),%r14        # 200ff0 <bound>
 815:   45 8b 06                mov    (%r14),%r8d
 818:   48 8d 3d c2 01 00 00    lea    0x1c2(%rip),%rdi        # 9e1 <_fini+0x9>
 81f:   31 c0                   xor    %eax,%eax
 821:   48 89 de                mov    %rbx,%rsi
 824:   4c 89 f1                mov    %r14,%rcx
 827:   e8 a4 fe ff ff          callq  6d0 <printf@plt>
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:34

        printf("Hello!\n");
 82c:   48 8d 3d cc 01 00 00    lea    0x1cc(%rip),%rdi        # 9ff <_fini+0x27>
 833:   e8 78 fe ff ff          callq  6b0 <puts@plt>
check_bound():
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:23
        if (base) { 
 838:   48 83 3b 00             cmpq   $0x0,(%rbx)
 83c:   74 1c                   je     85a <hello_main+0x5a>
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:24
                x++;
 83e:   8b 05 04 08 20 00       mov    0x200804(%rip),%eax        # 201048 <check_bound.x>
incr_bound():
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:15
        bound++;
 844:   41 83 06 01             addl   $0x1,(%r14)
check_bound():
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:24
                x++;
 848:   83 c0 01                add    $0x1,%eax
 84b:   89 05 f7 07 20 00       mov    %eax,0x2007f7(%rip)        # 201048 <check_bound.x>
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:26
                if (x % 2) more_mem();
 851:   a8 01                   test   $0x1,%al
 853:   74 05                   je     85a <hello_main+0x5a>
 855:   e8 66 fe ff ff          callq  6c0 <more_mem@plt>
hello_main():
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:36
        check_bound();
        printf("Hello!\n");
 85a:   48 8d 3d 9e 01 00 00    lea    0x19e(%rip),%rdi        # 9ff <_fini+0x27>
 861:   e8 4a fe ff ff          callq  6b0 <puts@plt>
check_bound():
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:23
        if (base) { 
 866:   48 83 3b 00             cmpq   $0x0,(%rbx)
 86a:   74 1c                   je     888 <hello_main+0x88>
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:24
                x++;
 86c:   8b 05 d6 07 20 00       mov    0x2007d6(%rip),%eax        # 201048 <check_bound.x>
incr_bound():
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:15
        bound++;
 872:   41 83 06 01             addl   $0x1,(%r14)
check_bound():
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:24
                x++;
 876:   83 c0 01                add    $0x1,%eax
 879:   89 05 c9 07 20 00       mov    %eax,0x2007c9(%rip)        # 201048 <check_bound.x>
/home/xxxxx/research/me/misc/test/dynload_exp/hello.c:26
                if (x % 2) more_mem();
 87f:   a8 01                   test   $0x1,%al

COMPILE

make clean all

RUN

. libenv.sh

./load hello.so world.so

#or

./load hello.so

load application loads hello.so and world.so shared objects and their dependencies.

Example output

./load hello.so world.so

produces

xxxxx@xxxxx:~/research/me/misc/test/dynload_exp$ ./load hello.so world.so
base: 0x938a20 bound: 16384
Hello 0x7fe2e7d535f0 0x7fe2e7d53620 0x7fe2e7d53650!
a's a is 1
a's b is 1
a's c is 1
Whatever:1
0x602088:0x938a20 0x602090:16384
Hello!
Hello!
base: 0x93ce40 bound: 24578
Hello!
Hello!
base: 0x93ce40 bound: 32772
Hello!
Hello!
base: 0x93ce40 bound: 40966
Hello!
0x602088:0x93ce40 0x602090:40967
Hello!
base: 0x93ce40 bound: 49160
Hello!
Hello!
base: 0x93ce40 bound: 57354
Hello!
Hello!
base: 0x93ce40 bound: 65548
Hello!
Hello!
base: 0x93ce40 bound: 73742
World 0x7fe2e794f5f0 0x7fe2e794f620 0x7fe2e794f650!
b's a is 2
b's b is 2
b's c is 2
Whatever:2
World!
World!
0x400940 0x4008e0 0x400930
x's a is 3
x's b is 3
x's c is 3
xxxxx@xxxxx:~/research/me/misc/test/dynload_exp$