What do /proc/fd file descriptors show? - c++

Question

Learning about the /proc/ directory today, in particular I'm interested in the security implications of having all the information about a process semi-publicly available, so I wrote a simple program that does some simple whatnot that allows me to explore some properties of the /proc/ directory:
#include <iostream>
#include <unistd.h>
#include <fcntl.h>
using namespace std;
extern char** environ;
void is_linux() {
#ifdef __linux
cout << "this is running on linux" << endl;
#endif
}
int main(int argc, char* argv[]) {
is_linux();
cout << "hello world" << endl;
int fd = open("afile.txt", O_RDONLY | O_CREAT, 0600);
cout << "afile.txt open on: " << fd << endl;
cout << "current pid: " << getpid() << endl;;
cout << "launch arguments: " << endl;
for (int index = 0; index != argc; ++index) {
cout << argv[index] << endl;
}
cout << "program environment: " << endl;
for (char** entry = environ; *entry; ++entry) {
cout << *entry << endl;
}
pause();
}
Interestingly though (to me anyway), when I check the file-descriptors folder (/pid/<PID#>/fd), I see this:
root#excalibur-VirtualBox:/proc/1546/fd# ls -l
total 0
lrwx------ 1 root root 64 Nov 7 09:12 0 -> /dev/null
lrwx------ 1 root root 64 Nov 7 09:12 1 -> /dev/null
lrwx------ 1 root root 64 Nov 7 09:12 2 -> /dev/null
lrwx------ 1 root root 64 Nov 7 09:12 3 -> socket:[11050]
why do the file descriptors point to /dev/null? Is that to prevent user's from being able to inject content into a file without actually being the process itself, or am I off base on that? And even more curious, why does the file descriptor to an open file point to a socket? That seems really odd. If anyone can shed some light on this for me, I would really appreciate it. Thanks!

Answer 1

You are definitely looking at the wrong /proc directory (for other PID or on another computer). The contents of /proc/<pid>/fd for your program should look like here:
lrwx------ 1 user group 64 Nov 7 22:15 0 -> /dev/pts/4
lrwx------ 1 user group 64 Nov 7 22:15 1 -> /dev/pts/4
lrwx------ 1 user group 64 Nov 7 22:15 2 -> /dev/pts/4
lr-x------ 1 user group 64 Nov 7 22:15 3 -> /tmp/afile.txt
Here we can see that file descriptors 0, 1, and 2 are shown as symbolic links to the pseudo terminal in which the program is running. It could be /dev/null if you started your program with input, output, and error redirection. The file descriptor #3 points to the file afile.txt which is currently opened.