Will an executable access shared-libraries' global variable via GOT? - c

Question

I was learning dynamic linking recently and gave it a try:
dynamic.c
int global_variable = 10;
int XOR(int a) {
return global_variable;
}
test.c
#include <stdio.h>
extern int global_variable;
extern int XOR(int);
int main() {
global_variable = 3;
printf("%d\n", XOR(0x10));
}
The compiling commands are:
clang -shared -fPIC -o dynamic.so dynamic.c
clang -o test test.c dynamic.so
I was expecting that in executable test the main function will access global_variable via GOT. However, on the contrary, the global_variable is placed in test's data section and XOR in dynamic.so access the global_variable indirectly.
Could anyone tell me why the compiler didn't ask the test to access global_variable via GOT, but asked the shared object file to do so?

Answer 1

Part of the point of a shared library is that one copy gets loaded into memory, and multiple processes can access that one copy. But every program has its own copy of each of the library's variables. If they were accessed relative to the library's GOT then those would instead be shared among the processes using the library, just like the functions are.
There are other possibilities, but it is clean and consistent for each executable to provide for itself all the variables it needs. That requires the library functions to access all of its variables with static storage duration (not just external ones) indirectly, relative to the program. This is ordinary dynamic linking, just going the opposite direction from what you usually think of.

Answer 2

Turns out my clang produced PIC by default so it messed with results.
I will leave updated answer here, and the original can be read below it.
After digging a bit more into the topic i have noticed that compilation of test.c does not generate a .got section by itself. You can check it by compiling the executable into an object file and omitting the linking step for now (-c option):
clang -c -o test.o test.c
If you inspect the sections of resulting object file with readelf -S you will notice that there is no .got in there:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
0000000000000035 0000000000000000 AX 0 0 1
[ 2] .rela.text RELA 0000000000000000 00000210
0000000000000060 0000000000000018 I 11 1 8
[ 3] .data PROGBITS 0000000000000000 00000075
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .bss NOBITS 0000000000000000 00000075
0000000000000000 0000000000000000 WA 0 0 1
[ 5] .rodata PROGBITS 0000000000000000 00000075
0000000000000004 0000000000000000 A 0 0 1
[ 6] .comment PROGBITS 0000000000000000 00000079
0000000000000013 0000000000000001 MS 0 0 1
[ 7] .note.GNU-stack PROGBITS 0000000000000000 0000008c
0000000000000000 0000000000000000 0 0 1
[ 8] .note.gnu.pr[...] NOTE 0000000000000000 00000090
0000000000000030 0000000000000000 A 0 0 8
[ 9] .eh_frame PROGBITS 0000000000000000 000000c0
0000000000000038 0000000000000000 A 0 0 8
[10] .rela.eh_frame RELA 0000000000000000 00000270
0000000000000018 0000000000000018 I 11 9 8
[11] .symtab SYMTAB 0000000000000000 000000f8
00000000000000d8 0000000000000018 12 4 8
[12] .strtab STRTAB 0000000000000000 000001d0
000000000000003e 0000000000000000 0 0 1
[13] .shstrtab STRTAB 0000000000000000 00000288
0000000000000074 0000000000000000 0 0 1
This means that the entirety of .got section present in the test executable actually comes from dynamic.so, as it is PIC and uses GOT.
Would it be possible to compile dynamic.so as non-PIC as well? Turns out it apparently used to be 10 years ago (the article compiles examples to 32-bits, they dont have to work on 64 bits!). Linked article describes how a non-PIC shared library was relocated at load time - basically, every time an address that needed to be relocated after loading was present in machine code, it was instead set to zeroes and a relocation of a certain type was set in the library. During loading of the library the loader filled the zeros with actual runtime address of data/code that was needed. It is important to note that it cannot be applied in your though as 64-bit shared libraries cannot be made out of non-PIC (Source).
If you compile dynamic.so as a shared 32-bit library instead and do not use the -fPIC option (you usually need special repositories enabled to compile 32-bit code and have 32-bit libc installed):
gcc -m32 dynamic.c -shared -o dynamic.so
You will notice that:
// readelf -s dynamic.so
(... lots of output)
27: 00004010 4 OBJECT GLOBAL DEFAULT 19 global_variable
// readelf -S dynamic.so
(... lots of output)
[17] .got PROGBITS 00003ff0 002ff0 000010 04 WA 0 0 4
[18] .got.plt PROGBITS 00004000 003000 00000c 04 WA 0 0 4
[19] .data PROGBITS 0000400c 00300c 000008 00 WA 0 0 4
[20] .bss NOBITS 00004014 003014 000004 00 WA 0 0 1
global_variable is at offset 0x4010 which is inside .data section. Also, while .got is present (at offset 0x3ff0), it only contains relocations coming from other sources than your code:
// readelf -r
Offset Info Type Sym.Value Sym. Name
00003f28 00000008 R_386_RELATIVE
00003f2c 00000008 R_386_RELATIVE
0000400c 00000008 R_386_RELATIVE
00003ff0 00000106 R_386_GLOB_DAT 00000000 _ITM_deregisterTM[...]
00003ff4 00000206 R_386_GLOB_DAT 00000000 __cxa_finalize#GLIBC_2.1.3
00003ff8 00000306 R_386_GLOB_DAT 00000000 __gmon_start__
00003ffc 00000406 R_386_GLOB_DAT 00000000 _ITM_registerTMCl[...]
This article introduces GOT as part of introduction on PIC, and i have found that to be the case in plenty of places, which would suggest that indeed GOT is only used by PIC code although i am not 100% sure of it and i recommend researching the topic more.
What does this mean for you? A section in the first article i linked called "Extra credit #2" contains an explanation for a similar scenario. Although it is 10 years old, uses 32-bit code and the shared library is non-PIC it shares some similarities with your situation and might explain the problem you presented in your question.
Also keep in mind that (although similar) -fPIE and -fPIC are two separate options with slightly different effects and that if your executable during inspection is not loaded at 0x400000 then it probably is compiled as PIE without your knowledge which might also have impact on results. In the end it all boils down to what data is to be shared between processes, what data/code can be loaded at arbitrary address, what has to be loaded at fixed address etc. Hope this helps.
Also two other answers on Stack Overflow which seem relevant to me: here and here. Both the answers and comments.
Original answer:
I tried reproducing your problem with exactly the same code and compilation commands as the ones you provided, but it seems like both main and XOR use the GOT to access the global_variable. I will answer by providing example output of commands that i used to inspect the data flow. If your outputs differ from mine, it means there is some other difference between our environments (i mean a big difference, if only addresses/values are different then its ok). Best way to find that difference is for you to provide commands you originally used as well as their output.
First step is to check what address is accessed whenever a write or read to global_variable happens. For that we can use objdump -D -j .text test command to disassemble the code and look at the main function:
0000000000001150 <main>:
1150: 55 push %rbp
1151: 48 89 e5 mov %rsp,%rbp
1154: 48 8b 05 8d 2e 00 00 mov 0x2e8d(%rip),%rax # 3fe8 <global_variable>
115b: c7 00 03 00 00 00 movl $0x3,(%rax)
1161: bf 10 00 00 00 mov $0x10,%edi
1166: e8 d5 fe ff ff call 1040 <XOR#plt>
116b: 89 c6 mov %eax,%esi
116d: 48 8d 3d 90 0e 00 00 lea 0xe90(%rip),%rdi # 2004 <_IO_stdin_used+0x4>
1174: b0 00 mov $0x0,%al
1176: e8 b5 fe ff ff call 1030 <printf#plt>
117b: 31 c0 xor %eax,%eax
117d: 5d pop %rbp
117e: c3 ret
117f: 90 nop
Numbers in the first column are not absolute addresses - instead they are offsets relative to the base address at which the executable will be loaded. For the sake of explanation i will refer to them as "offsets".
The assembly at offset 0x115b and 0x1161 comes directly from the line global_variable = 3; in your code. To confirm that, you could compile the program with -g for debug symbols and invoke objdump with -S. This will display source code above corresponding assembly.
We will focus on what these two instructions are doing. First instruction is a mov of 8 bytes from a location in memory to the rax register. The location in memory is given as relative to the current rip value, offset by a constant 0x2e8d. Objdump already calculated the value for us, and it is equal to 0x3fe8. So this will take 8 bytes present in memory at the 0x3fe8 offset and store them in the rax register.
Next instruction is again a mov, the suffix l tells us that data size is 4 bytes this time. It stores a 4 byte integer with value equal to 0x3 in the location pointed to by the current value of rax (not in the rax itself! brackets around a register such as those in (%rax) signify that the location in the instruction is not the register itself, but rather where its contents are pointing to!).
To summarize, we read a pointer to a 4 byte variable from a certain location at offset 0x3fe8 and later store an immediate value of 0x3 at the location specified by said pointer. Now the question is: where does that offset of 0x3fe8 come from?
It actually comes from GOT. To show the contents of the .got section we can use the objdump -s -j .got test command. -s means we want to focus on actual raw contents of the section, without any disassembling. The output in my case is:
test: file format elf64-x86-64
Contents of section .got:
3fd0 00000000 00000000 00000000 00000000 ................
3fe0 00000000 00000000 00000000 00000000 ................
3ff0 00000000 00000000 00000000 00000000 ................
The whole section is obviously set to zero, as GOT is populated with data after loading the program into memory, but what is important is the address range. We can see that .got starts at 0x3fd0 offset and ends at 0x3ff0. This means it also includes the 0x3fe8 offset - which means the location of global_variable is indeed stored in GOT.
Another way of finding this information is to use readelf -S test to show sections of the executable file and scroll down to the .got section:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
(...lots of sections...)
[22] .got PROGBITS 0000000000003fd0 00002fd0
0000000000000030 0000000000000008 WA 0 0 8
Looking at the Address and Size columns, we can see that the section is loaded at offset 0x3fd0 in memory and its size is 0x30 - which corresponds to what objdump displayed. Note that in readelf ouput "Offset" is actually the offset into the file form which the program is loaded - not the offset in memory that we are interested in.
by issuing the same commands on the dynamic.so library we get similar results:
00000000000010f0 <XOR>:
10f0: 55 push %rbp
10f1: 48 89 e5 mov %rsp,%rbp
10f4: 89 7d fc mov %edi,-0x4(%rbp)
10f7: 48 8b 05 ea 2e 00 00 mov 0x2eea(%rip),%rax # 3fe8 <global_variable##Base-0x38>
10fe: 8b 00 mov (%rax),%eax
1100: 5d pop %rbp
1101: c3 ret
So we see that both main and XOR use GOT to find the location of global_variable.
As for the location of global_variable we need to run the program to populate GOT. For that we can use GDB. We can run our program in GDB by invoking it this way:
LD_LIBRARY_PATH="$LD_LIBRARY_PATH:." gdb ./test
LD_LIBRARY_PATH environment variable tells linker where to look for shared objects, so we extend it to include the current directory "." so that it may find dynamic.so.
After the GDB loads our code, we may invoke break main to set up a breakpoint at main and run to run the program. The program execution should pause at the beginning of the main function, giving us a view into our executable after it was fully loaded into memory, with GOT populated.
Running disassemble main in this state will show us the actual absolute offsets into memory:
Dump of assembler code for function main:
0x0000555555555150 <+0>: push %rbp
0x0000555555555151 <+1>: mov %rsp,%rbp
=> 0x0000555555555154 <+4>: mov 0x2e8d(%rip),%rax # 0x555555557fe8
0x000055555555515b <+11>: movl $0x3,(%rax)
0x0000555555555161 <+17>: mov $0x10,%edi
0x0000555555555166 <+22>: call 0x555555555040 <XOR#plt>
0x000055555555516b <+27>: mov %eax,%esi
0x000055555555516d <+29>: lea 0xe90(%rip),%rdi # 0x555555556004
0x0000555555555174 <+36>: mov $0x0,%al
0x0000555555555176 <+38>: call 0x555555555030 <printf#plt>
0x000055555555517b <+43>: xor %eax,%eax
0x000055555555517d <+45>: pop %rbp
0x000055555555517e <+46>: ret
End of assembler dump.
(gdb)
Our 0x3fe8 offset has turned into an absolute address of equal to 0x555555557fe8. We may again check that this location comes from the .got section by issuing maintenance info sections inside GDB, which will list a long list of sections and their memory mappings. For me .got is placed in this address range:
[21] 0x555555557fd0->0x555555558000 at 0x00002fd0: .got ALLOC LOAD DATA HAS_CONTENTS
Which contains 0x555555557fe8.
To finally inspect the address of global_variable itself we may examine the contents of that memory by issuing x/xag 0x555555557fe8. Arguments xag of the x command deal with the size, format and type of data being inspected - for explanation invoke help x in GDB. On my machine the command returns:
0x555555557fe8: 0x7ffff7fc4020 <global_variable>
On your machine it may only display the address and the data, without the "<global_variable>" helper, which probably comes from an extension i have installed called pwndbg. It is ok, because the value at that address is all we need. We now know that the global_variable is located in memory under the address 0x7ffff7fc4020. Now we may issue info proc mappings in GDB to find out what address range does this address belong to. My output is pretty long, but among all the ranges listed there is one of interest to us:
0x7ffff7fc4000 0x7ffff7fc5000 0x1000 0x3000 /home/user/test_got/dynamic.so
The address is inside of that memory area, and GDB tells us that it comes from the dynamic.so library.
In case any of the outputs of said commands are different for you (change in a value is ok - i mean a fundamental difference like addresses not belonging to certain address ranges etc.) please provide more information about what exactly did you do to come to the conclusion that global_variable is stored in the .data section - what commands did you invoke and what outputs they produced.