Execute instructions inside ptrace tracee... might be one of the ugliest hacks I made to date..

b.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <wait.h>
#include <assert.h>
#include <errno.h>

#include <sys/signal.h>
#include <sys/types.h>
#include <sys/ptrace.h>
#include <sys/syscall.h>
#include <sys/user.h>

#define try_ptrace(req, pid, addr, data) err = ptrace((req), (pid), (addr), (data)); \
        if (err == -1 && errno != 0) { \
            perror("ptrace(" #req ", ..."); \
            printf("line %d", __LINE__); \
            exit(1); \
        }

#define CONCAT(a, b) a ## b

#define TRACEE_BEGIN(lbl) execute(&&CONCAT(lbl_code_begin, lbl), &&CONCAT(lbl_code_end, lbl), -1); \
    if (always_true) { \
        goto CONCAT(lbl_code_end, lbl); \
    } \
CONCAT(lbl_code_begin, lbl):

#define TRACEE_END(lbl) asm("int3"); \
        CONCAT(lbl_code_end, lbl):

#define TRACEE_ASM(code) TRACEE_BEGIN(__LINE__) \
                asm(code); \
            TRACEE_END(__LINE__)

#define ALIGN_UP(addr, align) (((addr)+align-1) & ~(align-1))
#define ALIGN_DOWN(addr, align) ((addr) & ~(align-1))

pid_t pid;
struct user_regs_struct reset_regs;
struct user_regs_struct regs;
volatile int always_true = 1;

void execute(const char * begin, const char * end, int ninstr) {
    assert((unsigned long)begin == ALIGN_DOWN((unsigned long)begin, sizeof(long)));
    assert((end - begin) > 0);
    long err;
    long buffer[512];
    const long * lbegin = (const long*)begin;
    const long * lend = (const long*)ALIGN_UP((unsigned long)end, sizeof(long));
    int nwords = lend - lbegin;

    try_ptrace(PTRACE_GETREGS, pid, 0, &regs);
    regs.rip = ALIGN_DOWN(regs.rip, sizeof(long));
    const long * rip = (const long*)regs.rip;
    for (int i = 0; i < nwords; i++) {
        errno = 0;
        try_ptrace(PTRACE_PEEKTEXT, pid, &rip[i], 0);
        buffer[i] = err;
        try_ptrace(PTRACE_POKETEXT, pid, &rip[i], lbegin[i]);
    }
    try_ptrace(PTRACE_SETREGS, pid, 0, &regs);

    if (ninstr != -1) {
        for (int i = 0; i < ninstr; i++) {
            try_ptrace(PTRACE_SINGLESTEP, pid, 0, 0);
            waitpid(pid, 0, 0);
        }
    } else {
        while (1) {
            int status;
            try_ptrace(PTRACE_SINGLESTEP, pid, 0, 0);
            waitpid(pid, &status, 0);
            if (WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP) {
                try_ptrace(PTRACE_GETREGS, pid, 0, &regs);
                errno = 0;
                try_ptrace(PTRACE_PEEKTEXT, pid, regs.rip, 0);
                if ((err & 0xff) == 0xcc) break;
            } else if (WIFEXITED(status)) {
                puts("child exit");
                exit(1);
            }
        }
    }

    for (int i = 0; i < nwords; i++) {
        try_ptrace(PTRACE_POKETEXT, pid, &rip[i], buffer[i]);
    }

    try_ptrace(PTRACE_GETREGS, pid, 0, &regs);
    try_ptrace(PTRACE_SETREGS, pid, 0, &reset_regs);
}

int main(int argc, char ** argv) {
    long err;
    if (argc != 2) {
        puts("argc != 2");
        exit(1);
    }
    pid = atoi(argv[1]);
    printf("pid tracee = %d\n", pid);
    try_ptrace(PTRACE_SEIZE, pid, 0, 0);
    try_ptrace(PTRACE_GETREGS, pid, 0, &reset_regs);

    TRACEE_ASM(
        "push 0x2048454c\n"
        "mov rax, 1\n"
        "mov rdi, 1\n"
        "mov rsi, rsp\n"
        "mov rdx, 4\n"
        "syscall\n"
        "pop rax\n"
    )

    try_ptrace(PTRACE_SETREGS, pid, 0, &reset_regs);
    try_ptrace(PTRACE_CONT, pid, 0, SIGCONT);
    return 0;
}