How to get command line arguments inside LD_PRELOAD library

Question

I would like to get argv from an LD_PRELOAD library, for instance, let us assume we call

    LD_PRELOAD=/path/to/my/fopen ./program input

Inside my custom fopen I would like to get "input", hence the argv[1] of my program (also the argv[2] and so on). Is it possible? How?

Answer 1

Read entire /proc/self/cmdline file. Command line arguments are separated with \0.

See man proc for full details.

Linux example without C standard library or error handling:

#include <unistd.h>
#include <fcntl.h>
#include <linux/limits.h>

int main() {
    ssize_t i, n;
    char cmdline[ARG_MAX];
    int cmdline_fd = open("/proc/self/cmdline", O_RDONLY);
    n = read(cmdline_fd, cmdline, sizeof cmdline);
    for(i = 0; i < n; ++i)
        if(!cmdline[i])
            cmdline[i] = ' ';
    cmdline[n - 1] = '\n';
    write(STDOUT_FILENO, cmdline, n);
    return 0;
}

Python version for reference:

cmdline = open('/proc/self/cmdline', 'r').read().replace('\0', ' ').strip()
print(cmdline)