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POTrace/wrap open system call
 

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  1. POTrace/wrap open system call
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    <p>I have the following test code:</p> <pre><code>#include &lt;stdio.h&gt; int main(void) { fprintf(stderr, "This is a test.\n"); int ret = open("somefile.log", 1); fprintf(stderr, "Return value is %d.\n", ret); return 0; } </code></pre> <p>Compiled with <code>gcc -m64 test.c -o test</code></p> <p>If I run <code>truss ./test</code>, I eventaully see the following output:</p> <pre><code>getrlimit(RLIMIT_STACK, 0xFFFFFFFF7FFFE280) = 0 getpid() = 1984 [1983] setustack(0xFFFFFFFF7EE002C0) fstat(2, 0xFFFFFFFF7FFFDAA0) = 0 This is a test. write(2, " T h i s i s a t e".., 16) = 16 open("somefile.log", O_WRONLY) = 3 Return value is write(2, " R e t u r n v a l u e".., 16) = 16 3. write(2, " 3 .\n", 3) = 3 _exit(0) </code></pre> <p>I would like to hook the open system call and execute some code before the call to open finishes. I have read about using ptrace to do this, however I do not have sys/ptrace.h on this system (solaris). I see documentation stating that /proc debugging interfaces should be used instead of ptrace(), but I haven't been able to figure out how to do what I want using procfs. </p> <p>Does anyone know if this is possible? If so, how?</p> <p>As a side note, I have also tried using the LD_PRELOAD trick to implement the open system call in my own shared library and have it call dlsym to find the address of the regular open system call. I wasn't able to figure out 100% why this doesn't work, but it seemed related to the calls being inlined and not using an address table to lookup those functions. However somehow <code>truss</code> is able to detect calls to open().</p> <p>This was my code for that attempt:</p> <p><code>cat wrap_open.c</code></p> <pre><code>#define _GNU_SOURCE #include &lt;dlfcn.h&gt; #include &lt;stdio.h&gt; static int (*next_open) (const char *path, int oflag, /* mode_t mode */) = NULL; int open(const char *path, int oflag) { char *msg; if(next_open == NULL) { fprintf(stderr, "wrapping open\n"); next_open = dlsym(RTLD_NEXT, "open"); if((msg = dlerror()) != NULL) { fprintf(stderr, "open: dlopen failed: %s\n", msg); } else { fprintf(stderr, "open: wrapping done\n"); } } fprintf(stderr, "open: opening %s\n", msg); fflush(stderr); return next_open(path, oflag); } </code></pre> <p>Compiled with <code>gcc -fPIC -shared -Wl,-soname,libwrap_open.so.1 -ldl -o libwrap_open.so.1.0</code> Executed with <code>LD_PRELOAD_32=./libwrap_open.so.1.0 ./test</code></p> <p>I don't get any output from the shared library here. Only normal program output.</p> <p>Any help or pointers is appreciated. Thanks in advance.</p>
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    1. COYour attempt with LD_PRELOAD might have failed because the name of the libc function could be different (not `open()`). With truss option -u, you can trace the libc.so function calls as well, and see which libc function is used.
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      1. USLaszlo Valko
    2. COI tried this and get some lines looking like
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      1. USJustin
    3. CO`/1@1: -> libc:open(0x10a50, 0x1, 0x0, 0xa13c8)` `/1@1: -> libc:_save_nv_regs(0xff362c40, 0x0, 0x0, 0x0) /1@1: <- libc:_save_nv_regs() = 0xff362c40 /1@1: -> libc:_ti_bind_guard(0x1, 0x0, 0x1, 0xff3fa984) ... /1@1: -> libc:_open(0x10a50, 0x1, 0x0, 0x0) /1@1: -> libc:__open(0x10a50, 0x1, 0x0, 0x0) /1: open("somefile.log", O_WRONLY) = 3 /1@1: <- libc:__open() = 3 /1@1: <- libc:_open() = 3` I'm having trouble figuring out what the name is. Would it be open(char* a, char* b, char* c, char* d) ?
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