JVM TI agent that continuously bombards C2 Compiler threads with no-op signals

signaler-dbg.c

#define _GNU_SOURCE

#include <jvmti.h>
#include <dirent.h>
#include <dlfcn.h>
#include <fcntl.h>
#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>

static int is_compiler_thread(int tid) {
    char buf[64];
    snprintf(buf, sizeof(buf), "/proc/self/task/%d/comm", tid);
    int fd = open(buf, O_RDONLY);
    if (fd == -1) {
        return 0;
    }

    ssize_t r = read(fd, buf, sizeof(buf));
    close(fd);

    return r > 0 && strncmp(buf, "C2 Compiler", 11) == 0;
}

static void* signaling_loop(void* unused) {
    pid_t self = getpid();
    unsigned int counter = 0;

    DIR* dir = opendir("/proc/self/task");
    if (dir == NULL) {
        return NULL;
    }

    while (1) {    
        struct dirent* entry;
        while ((entry = readdir(dir)) != NULL) {
            if (entry->d_name[0] != '.') {
                int tid = atoi(entry->d_name);
                if (is_compiler_thread(tid)) {
                    syscall(__NR_tgkill, self, tid, SIGPROF);
                }
            }
        }
        rewinddir(dir);
    }
}

#define BUF_SIZE 262144
struct {
    void* pc;
    int tid;
} pcdesc[BUF_SIZE];

static unsigned int ptr = 0;

void px(int tid) {
    unsigned int p = ptr;
    for (unsigned int i = 0; i < p; i++) {
        if (pcdesc[i].tid == tid) {
            const char* sym = ".";
            Dl_info info;
            if (dladdr(pcdesc[i].pc, &info) && info.dli_sname != NULL) {
                sym = info.dli_sname;
            }
            printf("%5d  %p  %s\n", i, pcdesc[i].pc, sym);
        }
    }
}


static void noop_handler(int signo, siginfo_t* siginfo, void* ucontext) {
    unsigned int p = __sync_fetch_and_add(&ptr, 1) & (BUF_SIZE - 1);
    pcdesc[p].pc = (void*)((ucontext_t*)ucontext)->uc_mcontext.pc;
    pcdesc[p].tid = syscall(__NR_gettid);
}

static void start_agent_thread() {
    struct sigaction sa;
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = noop_handler;
    sa.sa_flags = SA_SIGINFO | SA_RESTART;
    sigaction(SIGPROF, &sa, NULL);

    pthread_t thread;
    if (pthread_create(&thread, NULL, signaling_loop, NULL) == 0) {
        printf("signaling_loop started\n");
    }
}

jint JNICALL Agent_OnLoad(JavaVM* vm, char* options, void* reserved) {
    start_agent_thread();
    return 0;
}

jint JNICALL Agent_OnAttach(JavaVM* vm, char* options, void* reserved) {
    start_agent_thread();
    return 0;
}

signaler.c

#include <dirent.h>
#include <fcntl.h>
#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>

static int is_compiler_thread(int tid) {
    char buf[64];
    snprintf(buf, sizeof(buf), "/proc/self/task/%d/comm", tid);
    int fd = open(buf, O_RDONLY);
    if (fd == -1) {
        return 0;
    }

    ssize_t r = read(fd, buf, sizeof(buf));
    close(fd);

    return r > 0 && strncmp(buf, "C2 Compiler", 11) == 0;
}

static void* signaling_loop(void* unused) {
    pid_t self = getpid();
    unsigned int counter = 0;

    DIR* dir = opendir("/proc/self/task");
    if (dir == NULL) {
        return NULL;
    }

    while (1) {    
        struct dirent* entry;
        while ((entry = readdir(dir)) != NULL) {
            if (entry->d_name[0] != '.') {
                int tid = atoi(entry->d_name);
                if (is_compiler_thread(tid)) {
                    syscall(__NR_tgkill, self, tid, SIGPROF);
                }
            }
        }
        rewinddir(dir);
    }
}

static void noop_handler(int signo, siginfo_t* siginfo, void* ucontext) {
}

__attribute__((visibility("default")))
int Agent_OnLoad(void* vm, char* options, void* reserved) {
    struct sigaction sa;
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = noop_handler;
    sa.sa_flags = SA_SIGINFO | SA_RESTART;
    sigaction(SIGPROF, &sa, NULL);

    pthread_t thread;
    if (pthread_create(&thread, NULL, signaling_loop, NULL) == 0) {
        printf("Signaler started\n");
    }
    return 0;
}