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I am looking at statically linked linux x86 stripped binary. I noticed that there are .got and .plt sections.

I wonder what does a statically linked binary need got and plt sections for ? Anyone ?

  • They shouldn't have these sections at all. I did try on my own and I confirm that the section .got and .got.plt are still present even when -static is given to the compiler. But, the .dynamic section is not present. My guess is that they are just empty sections when -static is given. – perror Jun 2 '13 at 15:34
  • 2
    static is dead, a relevant stack overflow question: stackoverflow.com/questions/3430400/… – NirIzr Jun 2 '13 at 17:29
  • @Nirlzr: Interesting, I didn't know that. So, in fact, the only way to get rid of the got/plt tables is to use the -nostdlib option. – perror Jun 2 '13 at 17:39
  • @perror: ill give -nodefaultlibs a try first. but you might have to provide your own implementations of any standard functions the compiler might require (memcpy, etc...) – NirIzr Jun 2 '13 at 17:59
  • @Nirtlzr: Yes, with this option you will have to rebuild the whole libc (no printf, scanf, and so on). – perror Jun 2 '13 at 20:00
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+50

There are a plethora of things programmers do not know about how ELF binaries work internally. And, unfortunately, there's almost no solid references apart from two or three which broadly cover the subject. Many tools (linkers, loaders, assemblers, debuggers, ...) remain a mystery for most of you. When it comes to linkers and loaders, the main reference is Linkers and Loaders by John R. Levine (http://linker.iecc.com/). Another reliable source of information is the official ELF binary format documentation. But these are merely introductions to how a certain, or most, technologies work.

Now, here's an answer to your question (why are the GOT and PLT sections still included in static ELF binaries?): PERFORMANCE.

More explanations ... Suppose you have this C code:

    #include <stdio.h>
    #include <string.h>

    int main(int argc, char **argv)
    {
        char str[1024];

        strcpy(str, argv[1]);
        printf("%s\n", str);

        return 0;
    }

No need to be a genius to figure out that all it does is copy a command line parameter into a string and print it out. Here's the main function in assembly:

     000000000040105e <main>:
     40105e:   55                      push   rbp
     40105f:   48 89 e5                mov    rbp,rsp
     401062:   48 81 ec 10 04 00 00    sub    rsp,0x410
     401069:   89 bd fc fb ff ff       mov    DWORD PTR [rbp-0x404],edi
     40106f:   48 89 b5 f0 fb ff ff    mov    QWORD PTR [rbp-0x410],rsi
     401076:   48 8b 85 f0 fb ff ff    mov    rax,QWORD PTR [rbp-0x410]
     40107d    48 83 c0 08             add    rax,0x8
     401081:   48 8b 10                mov    rdx,QWORD PTR [rax]
     401084:   48 8d 85 00 fc ff ff    lea    rax,[rbp-0x400]
     40108b:   48 89 d6                mov    rsi,rdx
     40108e:   48 89 c7                mov    rdi,rax
     401091:   e8 3a f2 ff ff          call   4002d0 <__rela_iplt_end+0x38>
     401096:   48 8d 85 00 fc ff ff    lea    rax,[rbp-0x400]
     40109d:   48 89 c7                mov    rdi,rax
     4010a0:   e8 fb 09 00 00          call   401aa0 <_IO_puts>
     4010a5:   b8 00 00 00 00          mov    eax,0x0
     4010aa:   c9                      leave
     4010ab:   c3                      ret
     4010ac:   0f 1f 40 00             nop    DWORD PTR [rax+0x0]

Notice that at the address 401091 you have a call to a function stored in the PLT (the label is more expressive). Amazingly, at this address 4002d0 you'll find a jump to something stored in the GOT (see below).

     4002d0:   ff 25 f2 2f 2c 00       jmp QWORD PTR [rip+0x2c2ff2] # 6c32c8 <_GLOBAL_OFFSET_TABLE_+0x20>

At that exact location in the GOT, you'll find calls to functions stored in sections such as the following:

    00000000004187d0 <handle_amd>:
    4187d0:    53                      push   rbx
    4187d1:    b8 00 00 00 80          mov    eax,0x80000000
    4187d6:    0f a2                   cpuid
    4187d8:    81 ff c4 00 00 00       cmp    edi,0xc4
    4187de:    7f 40                   jg     418820 <handle_amd+0x50>
    4187e0:    31 d2                   xor    edx,edx
    4187e2:    81 ff bf 00 00 00       cmp    edi,0xbf
    4187e8:    0f 9d c2                setge  dl
    4187eb:    81 ea fb ff ff 7f       sub    edx,0x7ffffffb
    4187f1:    39 c2                   cmp    edx,eax
    4187f3:    77 2b                   ja     418820 <handle_amd+0x50>
    4187f5:    89 d0                   mov    eax,edx
    4187f7:    0f a2                   cpuid
    4187f9:    81 ff bb 00 00 00       cmp    edi,0xbb
    4187ff:    7e 27                   jle    418828 <handle_amd+0x58>
    418801:    81 ef bc 00 00 00       sub    edi,0xbc
    418807:    83 ff 08                cmp    edi,0x8
    41880a:    0f 87 48 01 00 00       ja     418958 <handle_amd+0x188>
    418810:    48 8d 35 c9 0b 08 00    lea    rsi,[rip+0x80bc9]        # 4993e0 <__PRETTY_FUNCTION__.4767+0x20>
    418817:    48 63 04 be             movsxd rax,DWORD PTR [rsi+rdi*4]
    41881b:    48 01 c6                add    rsi,rax
    41881e:    ff e6                   jmp    rsi
    418820:    31 c0                   xor    eax,eax
    418822:    5b                      pop    rbx
    418823:    c3                      ret

First, look at the section's name. Second, if you look closely at the code you'll notice that this function identifies the CPU - by dissecting the return values of the cpuid instruction (4187d6 and 4187f7) - (more accurately the micro architecture and other features such as cache size, ...) you're running your ELF binary on, and then decides which implementation suites that configuration best. This way, the strcpy function called in the above C code will always be the fastest possible, whatever architecture you're on (Intel: Nehalem, Sandy Bridge, Ivy Bridge, Haswell, ...; AMD: Phenom, Opteron, ...; ...). Keep in mind that those fast implementations have been hand optimized and fine tuned for each of the possible target architectures.

So that's what the PLT and GOT sections are used for in your static ELF binary file.

Now, if you want to investigate this yourself, you should compile the C code above with GCC version 4.9 (which is the one I used) using the -static and -g3 (debug symbols) flags. Then, disassemble the binary file using objdump and the -D switch in order to have all the ELF sections. You can then go through all the sections and explore the assembly code. You can also run the binary file using gdb and set breakpoints at key locations and run the program step by step.

3

@yaspr's answer is great, since this question got some bounty of "Looking for an answer drawing from credible and/or official sources.", let me try to provide some references here.

Generally in my understanding, .PLT and .GOT tables are required here because of performance issues.

BinCFI is published on last year's top 2 computer security conference.

Since the purpose of PLT stubs is to dispatch cross module calls, it would seem that the targets can only be exported symbols from other modules. However, recent versions of gcc support a new function type called gnu indirect function, which allows a function to have many different implementations, with the most suitable one selected at runtime based on factors such as the CPU type. Currently, many glibc low level functions such as memcpy, strcmp and strlen use this feature. To support this feature, a library exports a chooser function that selects at runtime which of the many implementations is going to be used. These implementation functions may not be exported at all.

Some other references on how to leverage this feature are listed here.

  • Well, I avoided citing those references because they can be quite confusing. They are centered around the ifunc attribute which was not the subject of this question, rather the existence of GOT and PLT in a statically linked binary (Agner's CPU blog is a valuable source of info). But of course, additional information is always good, if used wisely ! The thing is, everything I've written in my post I discovered way before the references you posted when I was working on patching/instrumenting static ELF binaries for performance evaluation. – yaspr Oct 8 '14 at 9:14

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