Linux Applications Debugging Techniques/The interposition library

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The dynamic liker allows for interception of any function call an application makes to any shared library it uses. As such, interposition is a powerful technique allowing to tune performance, collect runtime statistics, or debug the application without having to instrument the code of that application.

As an example, we can use an interposition library to trace calls, with arguments' values and return codes.

Call tracing[edit]

Note that part of code below is 32-bit x86 and gcc 4.1/4.2 specific.

Code intrumentation[edit]

In the library, we want to address the following points:

  • when a function/method is entered and exited.
  • what were the call arguments when the function is entered.
  • what was the return code when the function is exited.
  • optionally, where was the function called from.

The first one is easy: if requested, the compiler will instrument functions and methods so that when a function/method is entered, a call to an instrumentation function is made and when the function is exited, a similar intrumentation call is made:

    void __cyg_profile_func_enter(void *func, void *callsite); 
    void __cyg_profile_func_exit(void *func, void *callsite);

This is achieved by compiling the code with the -finstrument-functions flag. The above two functions can be used for instance to collect data for coverage; or for profiling. We will use them to print a trace of function calls. Furthermore, we can isolate these two functions and the supporting code in an interposition library of our own. This library can be loaded when and if needed, thus leaving the application code basically unchanged.

Now when the function is entered we can get the arguments of the call:

    void __cyg_profile_func_enter( void *func, void *callsite )
        char buf_func[CTRACE_BUF_LEN+1] = {0};
        char buf_file[CTRACE_BUF_LEN+1] = {0}; 
        char buf_args[ARG_BUF_LEN + 1] = {0}; 
        pthread_t self = (pthread_t)0;
        int *frame = NULL;
        int nargs = 0;
        self = pthread_self(); 
        frame = (int *)__builtin_frame_address(1); /*of the 'func'*/
        /*Which function*/
        libtrace_resolve (func, buf_func, CTRACE_BUF_LEN, NULL, 0);
        /*From where.  KO with optimizations. */
        libtrace_resolve (callsite, NULL, 0, buf_file, CTRACE_BUF_LEN);
        nargs = nchr(buf_func, ',') + 1; /*Last arg has no comma after*/ 
        nargs += is_cpp(buf_func);       /*'this'*/
        if (nargs > MAX_ARG_SHOW)
            nargs = MAX_ARG_SHOW;
        printf("T%p: %p %s %s [from %s]\n", 
               self, (int*)func, buf_func, 
               args(buf_args, ARG_BUF_LEN, nargs, frame), 

And when the function is is exited, we get the return value:

    void __cyg_profile_func_exit( void *func, void *callsite )
        long ret = 0L; 
        char buf_func[CTRACE_BUF_LEN+1] = {0};
        char buf_file[CTRACE_BUF_LEN+1] = {0}; 
        pthread_t self = (pthread_t)0;
        self = pthread_self(); 
        /*Which function*/
        libtrace_resolve (func, buf_func, CTRACE_BUF_LEN, NULL, 0);
        printf("T%p: %p %s => %d\n", 
               self, (int*)func, buf_func, 

Since these two instrumentation functions are aware of addresses and we actually want the trace to be readable by humans, we need also a way to resolve symbol addresses to symbol names: this is what libtrace_resolve() does.

Binutils and libbfd[edit]

First, we have to have the symbols information handy. To achieve this, we compile our application with the -g flag. Then, we can map addresses to symbol names and this would normally require writing some code knowledgeable of the ELF format.

Luckily, the there is the binutils package which comes with a library that does just that: libbfd; and with a tool: addr2line. addr2line is a good example on how to use libbfd and I have simply used it to wrap around libbfd. The result is the libtrace_resolve() function.

Since the instrumentation functions are isolated in a stand-alone module, we tell this module the name of the instrumented executable through an environment variable (CTRACE_PROGRAM) that we set before running the program. This is needed to properly init libbfd to search for symbols.

Stack layout[edit]

To address the first point the work has been architecture-agnostic (actually libbfd is aware of the architecture, but things are hidden behind its API). However, to retrieve function arguments and return values we have to look at the stack, write a bit of architecture-specific code and exploit some gcc quirks. Again, the compilers I have used were gcc 4.1 and 4.2; later or previous versions might work differently. In short:

  • x86 dictates that stack grows down.
  • GCC dictates how the stack is used - a "typical" stack is depicted below.
  • each function has a stack frame marked by the ebp (base pointer) and esp (stack pointer) registers.
  • normally, we expect the eax register to contain the reurn code
               +------------+   |
               | arg 2      |   \
               +------------+    >- previous function's stack frame
               | arg 1      |   /
               +------------+   |
               | ret %eip   |   /
               | saved %ebp |   \
        %ebp-> +------------+   |
               |            |   |
               |   local    |   \
               | variables, |    >- current function's stack frame
               |    etc.    |   /
               |            |   |
               |            |   |
        %esp-> +------------+   /

In an ideal world, the code the compiler generates would make sure that upon instrumenting the exit of a function: the return value is set, then CPU registers pushed on the stack (to ensure the instrumentation function does not affects them), then call the instrumentation function and then pop the registers. This sequence of code would ensure we always get access to the

return value in the instrumentation function. The code generated by the compiler is a bit different...

Also, in practice, many of gcc's flags affect the stack layout and registers usage. The most obvious ones are:

  • -fomit-frame-pointer. This flag affects the stack offset where the arguments are to be found.
  • The optimization flags: -Ox; each of these flags aggregates a number of optimizations. These flags did not affected the stack, and, quite amazingly, arguments were always passsed to functions through the stack, regardless of the optimization level. One would have expected that some arguments would pe passed through registers - in which case getting these arguments would have proven to be difficult to impossible. However, these flags did complicated recovering the return code. However, on some architectures, these flags will suck in the -fomit-frame-pointer optimization.
  • In any case, be wary: other flags you use to compile your application may reserve surprises.
Function arguments[edit]

In my tests with the compilers, all arguments were invariably passed through the stack. Hence this is trivial business, affected to a small extent by the -fomit-frame-pointer flag - this flag will change the offset at which arguments start.

How many arguments a function has, how many arguments are on the stack? One way to infer somehow the number of arguments is based on its signature (for C++, beware of the 'this' hidden argument) and this is the technique used in __cyg_profile_func_enter().

Once we know the offset where arguments start on the stack and how many of them there are, we just walk the stack to retrieve their values:

    char *args(char *buf, int len, int nargs, int *frame)
        int i; 
        int offset;
        memset(buf, 0, len); 
        snprintf(buf, len, "("); 
        offset = 1; 
        for (i=0; i<nargs && offset<len; i++) {
            offset += snprintf(buf+offset, len-offset, "%d%s", 
                             i==nargs-1 ? " ...)" : ", ");
        return buf; 

Function return values[edit]

Obtaining the return value proved to be possible only when using the -O0 flag.

Let's look what happens when this method

    class B {
        virtual int m1(int i, int j) {printf("B::m1()\n"); f1(i); return 20;}

is instrumented with -O0:

     080496a2 <_ZN1B2m1Eii>:
     80496a2:    55                       push   %ebp
     80496a3:    89 e5                    mov    %esp,%ebp
     80496a5:    53                       push   %ebx
     80496a6:    83 ec 24                 sub    $0x24,%esp
     80496a9:    8b 45 04                 mov    0x4(%ebp),%eax
     80496ac:    89 44 24 04              mov    %eax,0x4(%esp)
     80496b0:    c7 04 24 a2 96 04 08     movl   $0x80496a2,(%esp)
     80496b7:    e8 b0 f4 ff ff           call   8048b6c <__cyg_profile_func_enter@plt>
     80496bc:    c7 04 24 35 9c 04 08     movl   $0x8049c35,(%esp)
     80496c3:    e8 b4 f4 ff ff           call   8048b7c <puts@plt>
     80496c8:    8b 45 0c                 mov    0xc(%ebp),%eax
     80496cb:    89 04 24                 mov    %eax,(%esp)
     80496ce:    e8 9d f8 ff ff           call   8048f70 <_Z2f1i>
==>  80496d3:    bb 14 00 00 00           mov    $0x14,%ebx
     80496d8:    8b 45 04                 mov    0x4(%ebp),%eax
     80496db:    89 44 24 04              mov    %eax,0x4(%esp)
     80496df:    c7 04 24 a2 96 04 08     movl   $0x80496a2,(%esp)
==>  80496e6:    e8 81 f5 ff ff           call   8048c6c <__cyg_profile_func_exit@plt>
     80496eb:    89 5d f8                 mov    %ebx,0xfffffff8(%ebp)
==>  80496ee:    eb 27                    jmp    8049717 <_ZN1B2m1Eii+0x75>
     80496f0:    89 45 f4                 mov    %eax,0xfffffff4(%ebp)
     80496f3:    8b 5d f4                 mov    0xfffffff4(%ebp),%ebx
     80496f6:    8b 45 04                 mov    0x4(%ebp),%eax
     80496f9:    89 44 24 04              mov    %eax,0x4(%esp)
     80496fd:    c7 04 24 a2 96 04 08     movl   $0x80496a2,(%esp)
     8049704:    e8 63 f5 ff ff           call   8048c6c <__cyg_profile_func_exit@plt>
     8049709:    89 5d f4                 mov    %ebx,0xfffffff4(%ebp)
     804970c:    8b 45 f4                 mov    0xfffffff4(%ebp),%eax
     804970f:    89 04 24                 mov    %eax,(%esp)
     8049712:    e8 15 f5 ff ff           call   8048c2c <_Unwind_Resume@plt>
==>  8049717:    8b 45 f8                 mov    0xfffffff8(%ebp),%eax
     804971a:    83 c4 24                 add    $0x24,%esp
     804971d:    5b                       pop    %ebx
     804971e:    5d                       pop    %ebp
     804971f:    c3                       ret

Note how the return code is moved into the ebx register - a bit unexpected, since, traditionally, the eax register is used for return codes - and then the instrumentation function is called. Good to retrieve the return value but to avoid that the ebx register gets clobbered in the instrumentation function, we save it upon entering the function and we restore it upon exit.

When the compilation is done with some degree of optimization (-O1...3; shown here is -O2), the code changes:

    080498c0 <_ZN1B2m1Eii>:
     80498c0:    55                       push   %ebp
     80498c1:    89 e5                    mov    %esp,%ebp
     80498c3:    53                       push   %ebx
     80498c4:    83 ec 14                 sub    $0x14,%esp
     80498c7:    8b 45 04                 mov    0x4(%ebp),%eax
     80498ca:    c7 04 24 c0 98 04 08     movl   $0x80498c0,(%esp)
     80498d1:    89 44 24 04              mov    %eax,0x4(%esp)
     80498d5:    e8 12 f4 ff ff           call   8048cec <__cyg_profile_func_enter@plt>
     80498da:    c7 04 24 2d 9c 04 08     movl   $0x8049c2d,(%esp)
     80498e1:    e8 16 f4 ff ff           call   8048cfc <puts@plt>
     80498e6:    8b 45 0c                 mov    0xc(%ebp),%eax
     80498e9:    89 04 24                 mov    %eax,(%esp)
     80498ec:    e8 af f7 ff ff           call   80490a0 <_Z2f1i>
     80498f1:    8b 45 04                 mov    0x4(%ebp),%eax
     80498f4:    c7 04 24 c0 98 04 08     movl   $0x80498c0,(%esp)
     80498fb:    89 44 24 04              mov    %eax,0x4(%esp)
==>  80498ff:    e8 88 f3 ff ff           call   8048c8c <__cyg_profile_func_exit@plt>
     8049904:    83 c4 14                 add    $0x14,%esp
==>  8049907:    b8 14 00 00 00           mov    $0x14,%eax
     804990c:    5b                       pop    %ebx
     804990d:    5d                       pop    %ebp
==>  804990e:    c3                       ret    
     804990f:    89 c3                    mov    %eax,%ebx
     8049911:    8b 45 04                 mov    0x4(%ebp),%eax
     8049914:    c7 04 24 c0 98 04 08     movl   $0x80498c0,(%esp)
     804991b:    89 44 24 04              mov    %eax,0x4(%esp)
     804991f:    e8 68 f3 ff ff           call   8048c8c <__cyg_profile_func_exit@plt>
     8049924:    89 1c 24                 mov    %ebx,(%esp)
     8049927:    e8 f0 f3 ff ff           call   8048d1c <_Unwind_Resume@plt>
     804992c:    90                       nop    
     804992d:    90                       nop    
     804992e:    90                       nop    
     804992f:    90                       nop

Note how the instrumentation function gets called first and only then the eax register is set with the return value. Thus, if we absolutely want the return code, we are forced to compile with -O0.

Sample output[edit]

Finally, below are the results. At at shell prompt type:

$ export CTRACE_PROGRAM=./cpptraced
$ LD_PRELOAD=./ ./cpptraced
T0xb7c0f6c0: 0x8048d34 main (0 ...) [from ]
./cpptraced: main(argc=1)
T0xb7c0ebb0: 0x80492d8 thread1(void*) (1 ...) [from ]
T0xb7c0ebb0: 0x80498b2 D (134605416 ...) [from cpptraced.cpp:91]
T0xb7c0ebb0: 0x8049630 B (134605416 ...) [from cpptraced.cpp:66]
T0xb7c0ebb0: 0x8049630 B => -1209622540 [from ]
T0xb7c0ebb0: 0x80498b2 D => -1209622540 [from ]
Hello World! It's me, thread #1!
./cpptraced: done.
T0xb7c0f6c0: 0x8048d34 main => -1212090144 [from ]
T0xb740dbb0: 0x8049000 thread2(void*) (2 ...) [from ]
T0xb740dbb0: 0x80498b2 D (134605432 ...) [from cpptraced.cpp:137]
T0xb740dbb0: 0x8049630 B (134605432 ...) [from cpptraced.cpp:66]
T0xb740dbb0: 0x8049630 B => -1209622540 [from ]
T0xb740dbb0: 0x80498b2 D => -1209622540 [from ]
Hello World! It's me, thread #2!
T0xb6c0cbb0: 0x8049166 thread3(void*) (3 ...) [from ]
T0xb6c0cbb0: 0x80498b2 D (134613288 ...) [from cpptraced.cpp:157]
T0xb6c0cbb0: 0x8049630 B (134613288 ...) [from cpptraced.cpp:66]
T0xb6c0cbb0: 0x8049630 B => -1209622540 [from ]
T0xb6c0cbb0: 0x80498b2 D => -1209622540 [from ]
Hello World! It's me, thread #3!
T0xb7c0ebb0: 0x80490dc wrap_strerror_r (134525680 ...) [from cpptraced.cpp:105]
T0xb7c0ebb0: 0x80490dc wrap_strerror_r => -1210887643 [from ]
T#1+M2 (Success)
T0xb740dbb0: 0x80495a0 D::m1(int, int) (134605432, 3, 4 ...) [from cpptraced.cpp:141]
T0xb740dbb0: 0x8049522 B::m2(int) (134605432, 14 ...) [from cpptraced.cpp:69]
T0xb740dbb0: 0x8048f70 f1 (14 ...) [from cpptraced.cpp:55]
f1 14
T0xb740dbb0: 0x8048ee0 f2(int) (74 ...) [from cpptraced.cpp:44]
f2 74
T0xb740dbb0: 0x8048e5e f3 (144 ...) [from cpptraced.cpp:36]
f3 144
T0xb740dbb0: 0x8048e5e f3 => 80 [from ]
T0xb740dbb0: 0x8048ee0 f2(int) => 70 [from ]
T0xb740dbb0: 0x8048f70 f1 => 60 [from ]
T0xb740dbb0: 0x8049522 B::m2(int) => 21 [from ]
T0xb740dbb0: 0x80495a0 D::m1(int, int) => 30 [from ]

Note how libbfd fails to resolve some addresses when the function gets inlined.