Problem with reading procfs

Discussion in 'C Programming' started by Simone, Dec 11, 2007.

  1. Simone

    Simone Guest

    Hi, I'm developing an application that access the procfs and then
    read/write /proc/<PID>/mem of a process to change the content of a
    file
    owned by that process.

    The sequential steps performed by the application are the followings:

    1) catch, by using ptrace, of the "read" system call
    2) read the file content directly on /proc/PID/mem
    3) modify this content
    4) write back the modified content on /proc/PD/mem

    The problem is that it returns "Error 22 Invalid argument" every time
    it tries to read (or write) on procfs.
    (I've already patched the kernel to allow read/write operation on
    /proc/<pid>/mem and I've tried the pread/pwrite functions, but they
    have the same behavior).

    This is an extract of my code:


    ....
    int eax_reg, ecx_reg, ret;
    char * buffer;
    char orig_buf[eax_reg+1];
    char proc_path[MAX_FILE_NAME+1];

    // traced_proc is the pid_t of the traced process
    // get number of bytes to read
    eax_reg = ptrace(PTRACE_PEEKUSER, traced_proc, 4*EAX, NULL);
    // get traced process space buffer
    ecx_reg = ptrace(PTRACE_PEEKUSER, traced_proc, 4*ECX, NULL);

    buffer = malloc(sizeof(char)*eax_reg);
    memset(orig_buf,0,eax_reg+1);
    memset(proc_path, 0, sizeof(proc_path));

    sprintf(proc_path, "/proc/%d/mem", traced_proc);
    fd_proc = open(proc_path, O_RDWR);

    lseek(fd_proc, ecx_reg, SEEK_SET);
    int ret = read(fd_proc, buffer, eax_reg);
    if (ret < 0) {
    perror("read");
    fprintf(stderr, "Failed to read: %d - %s\n", errno, strerror(errno));
    }
    ....


    if I run the application with strace, the output for the code shown
    above is:

    open("/proc/13271/mem", O_RDWR) = 3
    lseek(3, 3086368768, SEEK_SET) = 3086368768
    read(3, 0xbfca0940, 280) = -1 EINVAL (Invalid argument)

    Can someone tell me where I'm wrong?
    Thanks in advance for any help...
     
    Simone, Dec 11, 2007
    #1
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  2. In article <>,
    Simone <> wrote:
    >Hi, I'm developing an application that access the procfs and then
    >read/write /proc/<PID>/mem of a process to change the content of a
    >file
    >owned by that process.


    Everything you describe is operating system dependant, and so should
    be addressed to a newsgroup that is specific to your OS.

    Even different OSs that support procfs have differences in how to
    use them properly.
    --
    "I will speculate that [...] applications [...] could actually see a
    performance boost for most users by going dual-core [...] because it
    is running the adware and spyware that [...] are otherwise slowing
    down the single CPU that user has today" -- Herb Sutter
     
    Walter Roberson, Dec 11, 2007
    #2
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  3. Simone

    Flash Gordon Guest

    Simone wrote, On 11/12/07 21:29:
    > Hi, I'm developing an application that access the procfs and then
    > read/write /proc/<PID>/mem of a process to change the content of a


    <snip>

    > open("/proc/13271/mem", O_RDWR) = 3
    > lseek(3, 3086368768, SEEK_SET) = 3086368768
    > read(3, 0xbfca0940, 280) = -1 EINVAL (Invalid argument)
    >
    > Can someone tell me where I'm wrong?
    > Thanks in advance for any help...


    This is all very Linux specific (not even Posix I believe) so you should
    ask in one of the Linux groups after checking the relevant FAQs/charters
    and a weeks worth of postings to see what goes on.

    Note that just because someone once posted about a topic on a group that
    does not mean the post was topical, and it might be that the responses
    (if any) were just pointing out that it was not topical, so you need to
    read posts not just look at subject lines.

    For more information about what a lot of regulars consider to be topical
    and why see http://clc-wiki.net/wiki/Introduction_to_comp.lang.c
    --
    Flash Gordon
     
    Flash Gordon, Dec 11, 2007
    #3
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