I am writing an Elf Loader for ARM/ARM64. While processing the dynamic relocations I became a bit confused by some of the terms/symbols in the documentation I am following.
On Pg.14 it is stated,
"S (when used on its own) is the address of the symbol."
"P is the address of the place being relocated (derived from r_offset)."
"Delta(S) if S is a normal symbol, resolves to the difference between the static link address of S and the
execution address of S. If S is the null symbol (ELF symbol index 0), resolves to the difference between the
static link address of P and the execution address of P."
From what I gather, I believe the "execution address" of S (or P) to be the address of the symbol in the process's memory space but am unsure what is meant by "static link address".
If someone can clarify the terminology that would be great, thank you.
what is meant by "static link address".
A non-PIE executable is linked to load at a particular address. For example, on x86_64 Linux default static link address is 0x400000:
echo "int main() { return 0; }" | gcc -xc - -no-pie
readelf -Wl a.out | grep LOAD
LOAD 0x000000 0x0000000000400000 0x0000000000400000 0x000618 0x000618 R E 0x200000
LOAD 0x000e50 0x0000000000600e50 0x0000000000600e50 0x0001d8 0x0001e0 RW 0x200000
This binary is linked with static link address of 0x400000, and symbols in it reflect that:
nm a.out | grep ' main'
0000000000400487 T main
This executable must be loaded at 0x400000, and will not work correctly if loaded anywhere else.
Note that default non-PIE static link address
is different for different architectures (i386 default is 0x8048000), and
can be changed at static link time via linker script and/or linker flags.
Contrast this with a PIE executable, which is typically linked at static link address 0:
echo "int main() { return 0; }" | gcc -xc - -fPIE -pie
readelf -Wl a.out | grep LOAD
LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x0007d8 0x0007d8 R E 0x200000
LOAD 0x000e18 0x0000000000200e18 0x0000000000200e18 0x000210 0x000218 RW 0x200000
nm a.out | grep ' main'
00000000000005fa T main
So the static link address of main is 0x400487 in the non-PIE case, and 0x5fa in the PIE case.
Related
#include <stdio.h>
void func() {}
int main() {
printf("%p", &func);
return 0;
}
This program outputted 0x55c4cda9464a
Supposing that func will be stored in the .text section, and according to this figure, from CS:APP:
I suppose that the address of func would be somewhere near the starting address of the .text section, but this address is somewhere in the middle. Why is this the case? Local variables stored on the stack have addresses near 2^48 - 1, but I tried to disassemble different C codes and the instructions were always located somewhere around that 0x55... address.
gcc, when configured with --enable-default-pie1 (which is the default), produces Position Independent Executables(PIE). Which means the load address isn't same as what linker specified at compile-time (0x400000 for x86_64). This is a security mechanism so that Address Space Layout Randomization (ASLR) 2 can be enabled. That is, gcc compiles with -pie option by default.
If you compile with -no-pie option (gcc -no-pie file.c), then you can see the address of func is closer to 0x400000.
On my system, I get:
$ gcc -no-pie t.c
$ ./a.out
0x401132
You can also check the load address with readelf:
$ readelf -Wl a.out | grep LOAD
LOAD 0x000000 0x0000000000400000 0x0000000000400000 0x000478 0x000478 R 0x1000
LOAD 0x001000 0x0000000000401000 0x0000000000401000 0x0001f5 0x0001f5 R E 0x1000
LOAD 0x002000 0x0000000000402000 0x0000000000402000 0x000158 0x000158 R 0x1000
LOAD 0x002e10 0x0000000000403e10 0x0000000000403e10 0x000228 0x000230 RW 0x1000
1 you can check this with gcc --verbose.
2 You may also notice that address printed by your program is different in each run. That's because of ASLR. You can disable it with:
$ echo 0 | sudo tee /proc/sys/kernel/randomize_va_space
ASLR is enabled on Linux by default.
I have an object file of a C program which prints hello world, just for the question.
I am trying to understand using readelf utility or gdb or hexedit(I can't figure which tool is a correct one) where in the file does the code of function "main" starts.
I know using readelf that symbol _start & main occurs and the address where it is mapped in a virtual memory. Moreover, I also know what the size of .text section and the of coruse where entry point specified, i.e the address which the same of text section.
The question is - Where in the file does the code of function "main" starts? I tought that is the entry point and the offset of the text section but how I understand it the sections data, bss, rodata should be ran before main and it appears after section text in readelf.
Also I tought we should sum the size all the lines till main in symbol table, but I am not sure at all if it is correct.
Additional question which follow up this one is if I want to replace main function with NOP instrcutres or plant one ret instruction in my object file. how can I know the offset where I can do it using hexedit.
So, let's go through it step by step.
Start with this C file:
#include <stdio.h>
void printit()
{
puts("Hello world!");
}
int main(void)
{
printit();
return 0;
}
As the comments look like you are on x86, compile it as 32-bit non-PIE executable like this:
$ gcc -m32 -no-pie -o test test.c
The -m32 option is needed, because I am working at a x86-64 machine. As you already know, you can get the virtual memory address of main using readelf, objdump or nm, for example like this:
$ nm test | grep -w main
0804918d T main
Obviously, 804918d can not be an offset in the file that is just 15 kB big. You need to find the mapping between virtual memory addresses and file offsets. In a typical ELF file, the mapping is included twice. Once in a detailed form for linkers (as object files are also ELF files) and debuggers, and a second time in a condensed form that is used by the kernel for loading programs. The detailed form is the list of sections, consisting of section headers, and you can view it like this (the output is shortened a bit, to make the answer more readable):
$ readelf --section-headers test
There are 29 section headers, starting at offset 0x3748:
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[...]
[11] .init PROGBITS 08049000 001000 000020 00 AX 0 0 4
[12] .plt PROGBITS 08049020 001020 000030 04 AX 0 0 16
[13] .text PROGBITS 08049050 001050 0001c1 00 AX 0 0 16
[14] .fini PROGBITS 08049214 001214 000014 00 AX 0 0 4
[15] .rodata PROGBITS 0804a000 002000 000015 00 A 0 0 4
[...]
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
p (processor specific)
Here you find that the .text section starts at (virtual) address 08049050 and has a size of 1c1 bytes, so it ends at address 08049211. The address of main, 804918d is in this range, so you know main is a member of the text section. If you subtract the base of the text section from the address of main, you find that main is 13d bytes into the text section. The section listing also contains the file offset where the data for the text section starts. It's 1050, so the first byte of main is at offset 0x1050 + 0x13d == 0x118d.
You can do the same calculation using program headers:
$ readelf --program-headers test
[...]
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000034 0x08048034 0x08048034 0x00160 0x00160 R 0x4
INTERP 0x000194 0x08048194 0x08048194 0x00013 0x00013 R 0x1
[Requesting program interpreter: /lib/ld-linux.so.2]
LOAD 0x000000 0x08048000 0x08048000 0x002e8 0x002e8 R 0x1000
LOAD 0x001000 0x08049000 0x08049000 0x00228 0x00228 R E 0x1000
LOAD 0x002000 0x0804a000 0x0804a000 0x0019c 0x0019c R 0x1000
LOAD 0x002f0c 0x0804bf0c 0x0804bf0c 0x00110 0x00114 RW 0x1000
[...]
The second load line tells you that the area 08049000 (VirtAddr) to 08049228 (VirtAddr + MemSiz) is readable and executable, and loaded from offset 1000 in the file. So again you can calculate that the address of main is 18d bytes into this load area, so it has to reside at offset 0x118d inside the executable. Let's test that:
$ ./test
Hello world!
$ echo -ne '\xc3' | dd of=test conv=notrunc bs=1 count=1 seek=$((0x118d))
1+0 records in
1+0 records out
1 byte copied, 0.0116672 s, 0.1 kB/s
$ ./test
$
Overwriting the first byte of main with 0xc3, the opcode for return (near) on x86, causes the program to not output anything anymore.
_start normally belongs to a module ( a *.o file) that is fixed (it is called differently on different systems, but a common name is crt0.o which is written in assembler.) That fixed code prepares the stack (normally the arguments and the environment are stored in the initial stack segment by the execve(2) system call) the mission of crt0.s is to prepare the initial C stack frame and call main(). Once main() ends, it is responsible of getting the return value from main and calling all the atexit() handlers to finish calling the _exit(2) system call.
The linking of crt0.o is normally transparent due to the fact that you always call the compiler to do the linking itself, so you normally don't have to add crt0.o as the first object module, but the compiler knows (lately, all this stuff has grown considerably, since we depend on architecture and ABIs to pass parameters between functions)
If you execute the compiler with the -v option, you'll get the exact command line it uses to call the linker and you'll get the secrets of the final memory map your program has on its first stages.
I'm experimenting with LD_PRELOAD/dlopen and faced a confusion regarding symbol lookup. Consider the following 2 libraries:
libshar
shared.h
int sum(int a, int b);
shared.c
int sum(int a, int b){
return a + b;
}
libshar2
shared.h
int sum(int a, int b);
shared.c
int sum(int a, int b){
return a + b + 10000;
}
and executable bin_shared:
#include <dlfcn.h>
#include "shared.h"
int main(void){
void *handle = dlopen("/home/me/c/build/libshar2.so", RTLD_NOW | RTLD_GLOBAL);
int s = sum(2 + 3);
printf("s = %d", s);
}
linking the binary with libshar and libdl I considered the following 2 cases:
LD_PRELOAD is empty
The program prints 5.
Why does the dynamic linker decide to lookup the sum function in the libshar, not libshar2? Both of them are loaded and contain the needed symbol:
0x7ffff73dc000 0x7ffff73dd000 0x1000 0x0 /home/me/c/build/libshar2.so
0x7ffff73dd000 0x7ffff75dc000 0x1ff000 0x1000 /home/me/c/build/libshar2.so
0x7ffff75dc000 0x7ffff75dd000 0x1000 0x0 /home/me/c/build/libshar2.so
0x7ffff75dd000 0x7ffff75de000 0x1000 0x1000 /home/me/c/build/libshar2.so
#...
0x7ffff7bd3000 0x7ffff7bd4000 0x1000 0x0 /home/me/c/build/libshar.so
0x7ffff7bd4000 0x7ffff7dd3000 0x1ff000 0x1000 /home/me/c/build/libshar.so
0x7ffff7dd3000 0x7ffff7dd4000 0x1000 0x0 /home/me/c/build/libshar.so
0x7ffff7dd4000 0x7ffff7dd5000 0x1000 0x1000 /home/me/c/build/libshar.so
LD_PRELOAD = /path/to/libshar2.so
The program prints 10005. This is expected, but again I noticed that both libshar.so and libshar2.so are loaded:
0x7ffff79d1000 0x7ffff79d2000 0x1000 0x0 /home/me/c/build/libshar.so
0x7ffff79d2000 0x7ffff7bd1000 0x1ff000 0x1000 /home/me/c/build/libshar.so
0x7ffff7bd1000 0x7ffff7bd2000 0x1000 0x0 /home/me/c/build/libshar.so
0x7ffff7bd2000 0x7ffff7bd3000 0x1000 0x1000 /home/me/c/build/libshar.so
0x7ffff7bd3000 0x7ffff7bd4000 0x1000 0x0 /home/me/c/build/libshar2.so
0x7ffff7bd4000 0x7ffff7dd3000 0x1ff000 0x1000 /home/me/c/build/libshar2.so
0x7ffff7dd3000 0x7ffff7dd4000 0x1000 0x0 /home/me/c/build/libshar2.so
0x7ffff7dd4000 0x7ffff7dd5000 0x1000 0x1000 /home/me/c/build/libshar2.so
The LD_PRELOAD case seems to be explained in ld.so(8):
LD_PRELOAD
A list of additional, user-specified, ELF shared objects to be loaded
before all others. The items of the list can be separated by spaces
or colons. This can be used to selectively override functions in
other shared objects. The objects are searched for using the rules
given under DESCRIPTION.
Why does the dynamic linker decide to lookup the sum function in the libshar, not libshar2?
Dynamic linkers on UNIX attempt to emulate what would have happened if you linked with archive libraries.
In the case of empty LD_PRELOAD, the symbol search order is (when the symbol is referenced by the main binary; rules get more complicated when the symbol is referenced by the DSO): the main binary, directly linked DSOs in the order they are listed on the link line, dlopened DSOs in the order they were dlopened.
LD_PRELOAD = /path/to/libshar2.so
The program prints 10005. This is expected,
Non-empty LD_PRELOAD modifies the search order by inserting any libraries listed after the main executable, and before any directly linked DSOs.
but again I noticed that both libshar.so and libshar2.so are loaded:
Why is that a surprise? The dynamic linker loads all libraries listed in LD_PRELOAD, and then all libraries that you directly linked against (as explained before).
dlopen can't (nor can anything else) change the definition of (global) symbols already present at the time of the call. It can only make available new ones that did not exist before.
The (sloppy) formalization of this is in the specification for dlopen:
Symbols introduced into the process image through calls to dlopen() may be used in relocation activities. Symbols so introduced may duplicate symbols already defined by the program or previous dlopen() operations. To resolve the ambiguities such a situation might present, the resolution of a symbol reference to symbol definition is based on a symbol resolution order. Two such resolution orders are defined: load order and dependency order. Load order establishes an ordering among symbol definitions, such that the first definition loaded (including definitions from the process image file and any dependent executable object files loaded with it) has priority over executable object files added later (by dlopen()). Load ordering is used in relocation processing. Dependency ordering uses a breadth-first order starting with a given executable object file, then all of its dependencies, then any dependents of those, iterating until all dependencies are satisfied. With the exception of the global symbol table handle obtained via a dlopen() operation with a null pointer as the file argument, dependency ordering is used by the dlsym() function. Load ordering is used in dlsym() operations upon the global symbol table handle.
Note that LD_PRELOAD is nonstandard functionality and thus not described here, but on implementations that offer it, LD_PRELOAD acts with load order after the main program but before any shared libraries loaded as dependencies.
I found the following post (How to generate gcc debug symbol outside the build target?) on how to split a the compiled file and the debugging symbols.
However, I cannot find any useful information in the debugging file.
For example,
My helloWorld code is:
#include<stdio.h>
int main(void) {
int a;
a = 5;
printf("The memory address of a is: %p\n", (void*) &a);
return 0;
}
I ran gcc -g -o hello hello.c
objcopy --only-keep-debug hello hello.debug
gdb -s main.debug -e main
In gdb, anything I tried won't give me any information on a, I cannot find its address, I cannot find the main function address
For example :
(gdb) info variables
All defined variables:
Non-debugging symbols:
0x0000000000400618 _IO_stdin_used
0x0000000000400710 __FRAME_END__
0x0000000000600e3c __init_array_end
0x0000000000600e3c __init_array_start
0x0000000000600e40 __CTOR_LIST__
0x0000000000600e48 __CTOR_END__
0x0000000000600e50 __DTOR_LIST__
0x0000000000600e58 __DTOR_END__
0x0000000000600e60 __JCR_END__
0x0000000000600e60 __JCR_LIST__
0x0000000000600e68 _DYNAMIC
0x0000000000601000 _GLOBAL_OFFSET_TABLE_
0x0000000000601028 __data_start
0x0000000000601028 data_start
0x0000000000601030 __dso_handle
0x0000000000601038 __bss_start
0x0000000000601038 _edata
0x0000000000601038 completed.6603
0x0000000000601040 dtor_idx.6605
0x0000000000601048 _end
Am I doing something wrong? Am I understanding the debug file incorrectly? Is there even a way to find out an address of compiled variable/function from a saved debugging information?
int a is a stack variable so it does not have a fixed address unless you are in a call to that specific function. Furthermore, each call to that function will allocate its own variable.
When we say "debugging symbols" we usually mean functions and global variables. A local variable is not a "symbol" in this context. In fact, if you compile with optimisations enabled int a would almost certainly be optimised to a register variable so it would not have an address at all, unless you forced it to be written to memory by doing some_function(&a) or similar.
You can find the address of main just by writing print main in GDB. This is because functions are implicitly converted to pointers in C when they appear in value context, and GDB's print uses C semantics.
I register a token destructor function with
static void cleanup __attribute__ ((destructor));
The function just prints a debug message; the token program runs fine (main() just prints another message; token function prints upon exit).
When I look at the file with
nm ./a.out,
I see:
08049f10 d __DTOR_END__
08049f0c d __DTOR_LIST__
However, the token destructor function's address should be at 0x08049f10 - an address which contains 0, indicating end of destructor list, as I can check using:
objdump -s ./a.out
At 0x08049f0c, I see 0xffffffff, as is expected for this location. It is my understanding that what I see in the elf file would mean that no destructor was registered; but it is executed with one.
If someone could explain, I'd appreciate. Is this part of the security suite to prevent inserting malicious destructors? How does the compiler keep track of the destructors' addresses?
My system:
Ubuntu 12.04.
elf32-i386
Kernel: 3.2.0-30-generic-pae
gcc version: 4.6.3
DTOR_LIST is the start of a table of desctructors. Have a look which section it is in (probably .dtors):
~> objdump -t test | grep DTOR_LIST
0000000000600728 l O .dtors 0000000000000000 __DTOR_LIST__
Then dump that section with readelf (or whatever):
~> readelf --hex-dump=.dtors test
Hex dump of section '.dtors':
0x00600728 ffffffff ffffffff 1c054000 00000000 ..........#.....
0x00600738 00000000 00000000 ........
Which in my test case contains a couple of presumably -1, a real pointer, and then zero termination.