Dec 15 -- Let the ELF Note Everything

Article:  https://ibr.cs.tu-bs.de/advent/15-pty/
Workload: ~114 source-code lines

15-pty/Makefile

+PROG = scribble
+
+${PROG}: ${PROG}.c
+	gcc $< -o  $@ -Wall -g -lpthread
+
+run: ${PROG}
+	./${PROG} OUT IN bash
+
+strace: ${PROG}
+	strace -f ./${PROG} OUT IN  bash
+
+clean:
+	rm -f ./${PROG}

15-pty/scribble.c

+#define _GNU_SOURCE
+#include <stdio.h>
+#include <spawn.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <pthread.h>
+#include <sys/types.h>
+#include <sys/ioctl.h>
+#include <sys/wait.h>
+#include <termios.h>
+#include <unistd.h>
+#include <stdlib.h>
+
+#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while(0)
+
+// This function starts a program within a PTY by setting the process'
+// stdin, stdout, and stderr to the same pseudo terminal file
+// descriptor. Please note that pty_fd is the open file descriptor of
+// the child (e.g. /dev/pts/7)
+static pid_t exec_in_pty(char *argv[], int pty_fd) {
+    // We use posix_spawnp(3) to perform the process creation. For the
+    // redirection of std{in,out,err}, we use a file action which
+    // performs a dup(2) *after* the fork. For details, please look at
+    // the man page of posix_spawnp.
+    //
+    //     dup2(pty_fd, STDIN_FILENO);
+    posix_spawn_file_actions_t fa;
+    posix_spawn_file_actions_init(&fa);
+    posix_spawn_file_actions_adddup2(&fa, pty_fd, STDIN_FILENO);
+    posix_spawn_file_actions_adddup2(&fa, pty_fd, STDOUT_FILENO);
+    posix_spawn_file_actions_adddup2(&fa, pty_fd, STDERR_FILENO);
+
+
+    // In order to allow shells within our pseudo terminal, we have to
+    // spawn the new process as a new session leader. While this is an
+    // interesting topic on its own, we won't cover it today.
+    posix_spawnattr_t attr;
+    posix_spawnattr_init(&attr);
+    posix_spawnattr_setflags(&attr, POSIX_SPAWN_SETSID);
+
+    // "magic variable": array of pointers to environment variantes.
+    // This symbol is described in environ(2)
+    extern char **environ;
+
+    // We spawn the process with posix_spawnp(3). Please note the
+    // spawnp searches the PATH variable and allows us to write:
+    //
+    // ./scribble OUT IN bash
+    //
+    // instead of
+    //
+    // ./scribble OUT IN /bin/bash
+    pid_t pid;
+    if (posix_spawnp(&pid, argv[0], &fa, &attr, argv,  environ) != 0)
+        die("posix_spawn");
+
+    // Cleanup everything
+    posix_spawn_file_actions_destroy(&fa);
+    posix_spawnattr_destroy(&attr);
+
+    return pid;
+}
+
+// For new interactive processes to work correctly within our pty, we
+// have to set a few options. As this is also quite cumbersome to find
+// out, we will only look at the result here.
+static struct termios orig_termios; // <- We store the old configuration
+
+// Restore the original termios setting
+void restore_terminal() {
+    tcsetattr(STDIN_FILENO, TCSAFLUSH, &orig_termios);
+}
+
+// Configure our terminal
+void configure_terminal() {
+    // Credits go to:
+    // https://viewsourcecode.org/snaptoken/kilo/02.enteringRawMode.html
+    // see also termios(3)
+
+    //  Get the original termios setting
+    tcgetattr(STDIN_FILENO, &orig_termios);
+    // Register an atexit(3) handler to surely restore it when the process exits
+    atexit(restore_terminal);
+
+    // We delete the flags and configure the terminal
+    // ~ECHO:   Disable direct echoing of input by the terminal 
+    // ~ICANON: Disable canonical mode (line buffered, line editing, etc..)
+    // ~ISIG:   Disable C-c to send a signal
+    struct termios raw = orig_termios;
+    raw.c_lflag &= ~(ECHO | ICANON | ISIG );
+    tcsetattr(STDIN_FILENO, TCSAFLUSH, &raw);
+}
+
+// FIXME: Create a thread handler body that copies data from src_fd to two
+//        destination file descriptors.
+
+
+int main(int argc, char *argv[]) {
+    if (argc < 4) {
+        fprintf(stderr, "usage: %s OUT IN CMD [ARG ...]", argv[0]);
+        return -1;
+    }
+    char *OUT    = argv[1];
+    char *IN     = argv[2];
+    char **CMD   = &argv[3];
+
+    (void) IN; (void) OUT; (void) CMD; (void) exec_in_pty;
+    // FIXME: Open OUT and IN file
+    // FIXME: Create a new primary PTY device (see pty(7))
+    // FIXME: Get the PTN with ioctl(fd, TIOCGPTN, &pts)
+    // FIXME: Open the child pty end (/dev/pts/{PTN})
+
+    // FIXME: Spawn CMD into secondary_fd pty
+    // FIXME: Create two threads to copy data around
+    // FIXME: Use the main thread to waitpid(2) for the child to exit.
+}