Re: INFO: rcu detected stall in ext4_file_write_iter

[Theodore Y. Ts'o]

TL;DR: This doesn't appear to be ext4 specific, and seems to involve
an unholy combination of the perf_event_open(2) and sendfile(2) system
calls.

On Mon, Feb 25, 2019 at 10:50:05PM -0800, syzbot wrote:
> syzbot found the following crash on:
> 
> HEAD commit:    8a61716ff2ab Merge tag 'ceph-for-5.0-rc8' of git://github...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=161b71d4c00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=7132344728e7ec3f
> dashboard link: https://syzkaller.appspot.com/bug?extid=7d19c5fe6a3f1161abb7
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=103908f8c00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=105e5cd0c00000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+7d19c5fe6a3f1161abb7@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> audit: type=1400 audit(1550814986.750:36): avc:  denied  { map } for
> pid=8058 comm="syz-executor004" path="/root/syz-executor004991115"
> dev="sda1" ino=1426 scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
> tcontext=unconfined_u:object_r:user_home_t:s0 tclass=file permissive=1
> hrtimer: interrupt took 42841 ns
> rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> rcu: 	(detected by 1, t=10502 jiffies, g=5873, q=2)
> rcu: All QSes seen, last rcu_preempt kthread activity 10502
> (4295059997-4295049495), jiffies_till_next_fqs=1, root ->qsmask 0x0
> syz-executor004 R  running task    26448  8069   8060 0x00000000

This particular repro seems to induce similar failures when I tried it
with xfs and btrfs as well as ext4.

The repro seems to involve the perf_event_open(2) and sendfile(2)
system calls, and killing the process which is performing the
sendfile(2).  The repro also uses the sched_setattr(2) system call,
but when I commented it out, the failure still happened, so this
appears to be another case of "Syzkaller?  We don't need to bug
developers with a minimal test case!  Open source developers are a
free unlimited resource, after all!"  :-)

Commenting out the perf_event_open(2) does seem to make the problem go
away.

Since there are zillions of ways to self-DOS a Linux server without
having to resert to exotic combination of system calls, this isn't
something I'm going to prioritize for myself, but I'm hoping someone
else has time and curiosity.

					- Ted
// autogenerated by syzkaller (https://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <dirent.h>
#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mount.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

#include <linux/futex.h>

unsigned long long procid;

static void sleep_ms(uint64_t ms)
{
  usleep(ms * 1000);
}

static uint64_t current_time_ms(void)
{
  struct timespec ts;
  if (clock_gettime(CLOCK_MONOTONIC, &ts))
    exit(1);
  return (uint64_t)ts.tv_sec * 1000 + (uint64_t)ts.tv_nsec / 1000000;
}

static void use_temporary_dir(void)
{
  char tmpdir_template[] = "./syzkaller.XXXXXX";
  char* tmpdir = mkdtemp(tmpdir_template);
  if (!tmpdir)
    exit(1);
  if (chmod(tmpdir, 0777))
    exit(1);
  if (chdir(tmpdir))
    exit(1);
}

static void thread_start(void* (*fn)(void*), void* arg)
{
  pthread_t th;
  pthread_attr_t attr;
  pthread_attr_init(&attr);
  pthread_attr_setstacksize(&attr, 128 << 10);
  int i;
  for (i = 0; i < 100; i++) {
    if (pthread_create(&th, &attr, fn, arg) == 0) {
      pthread_attr_destroy(&attr);
      return;
    }
    if (errno == EAGAIN) {
      usleep(50);
      continue;
    }
    break;
  }
  exit(1);
}

#define BITMASK(bf_off, bf_len) (((1ull << (bf_len)) - 1) << (bf_off))
#define STORE_BY_BITMASK(type, htobe, addr, val, bf_off, bf_len)               \
  *(type*)(addr) =                                                             \
      htobe((htobe(*(type*)(addr)) & ~BITMASK((bf_off), (bf_len))) |           \
            (((type)(val) << (bf_off)) & BITMASK((bf_off), (bf_len))))

typedef struct {
  int state;
} event_t;

static void event_init(event_t* ev)
{
  ev->state = 0;
}

static void event_reset(event_t* ev)
{
  ev->state = 0;
}

static void event_set(event_t* ev)
{
  if (ev->state)
    exit(1);
  __atomic_store_n(&ev->state, 1, __ATOMIC_RELEASE);
  syscall(SYS_futex, &ev->state, FUTEX_WAKE | FUTEX_PRIVATE_FLAG);
}

static void event_wait(event_t* ev)
{
  while (!__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE))
    syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0, 0);
}

static int event_isset(event_t* ev)
{
  return __atomic_load_n(&ev->state, __ATOMIC_ACQUIRE);
}

static int event_timedwait(event_t* ev, uint64_t timeout)
{
  uint64_t start = current_time_ms();
  uint64_t now = start;
  for (;;) {
    uint64_t remain = timeout - (now - start);
    struct timespec ts;
    ts.tv_sec = remain / 1000;
    ts.tv_nsec = (remain % 1000) * 1000 * 1000;
    syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0, &ts);
    if (__atomic_load_n(&ev->state, __ATOMIC_RELAXED))
      return 1;
    now = current_time_ms();
    if (now - start > timeout)
      return 0;
  }
}

#define FS_IOC_SETFLAGS _IOW('f', 2, long)
static void remove_dir(const char* dir)
{
  DIR* dp;
  struct dirent* ep;
  int iter = 0;
retry:
  while (umount2(dir, MNT_DETACH) == 0) {
  }
  dp = opendir(dir);
  if (dp == NULL) {
    if (errno == EMFILE) {
      exit(1);
    }
    exit(1);
  }
  while ((ep = readdir(dp))) {
    if (strcmp(ep->d_name, ".") == 0 || strcmp(ep->d_name, "..") == 0)
      continue;
    char filename[FILENAME_MAX];
    snprintf(filename, sizeof(filename), "%s/%s", dir, ep->d_name);
    while (umount2(filename, MNT_DETACH) == 0) {
    }
    struct stat st;
    if (lstat(filename, &st))
      exit(1);
    if (S_ISDIR(st.st_mode)) {
      remove_dir(filename);
      continue;
    }
    int i;
    for (i = 0;; i++) {
      if (unlink(filename) == 0)
        break;
      if (errno == EPERM) {
        int fd = open(filename, O_RDONLY);
        if (fd != -1) {
          long flags = 0;
          if (ioctl(fd, FS_IOC_SETFLAGS, &flags) == 0)
            close(fd);
          continue;
        }
      }
      if (errno == EROFS) {
        break;
      }
      if (errno != EBUSY || i > 100)
        exit(1);
      if (umount2(filename, MNT_DETACH))
        exit(1);
    }
  }
  closedir(dp);
  int i;
  for (i = 0;; i++) {
    if (rmdir(dir) == 0)
      break;
    if (i < 100) {
      if (errno == EPERM) {
        int fd = open(dir, O_RDONLY);
        if (fd != -1) {
          long flags = 0;
          if (ioctl(fd, FS_IOC_SETFLAGS, &flags) == 0)
            close(fd);
          continue;
        }
      }
      if (errno == EROFS) {
        break;
      }
      if (errno == EBUSY) {
        if (umount2(dir, MNT_DETACH))
          exit(1);
        continue;
      }
      if (errno == ENOTEMPTY) {
        if (iter < 100) {
          iter++;
          goto retry;
        }
      }
    }
    exit(1);
  }
}

static void kill_and_wait(int pid, int* status)
{
  kill(-pid, SIGKILL);
  kill(pid, SIGKILL);
  int i;
  for (i = 0; i < 100; i++) {
    if (waitpid(-1, status, WNOHANG | __WALL) == pid)
      return;
    usleep(1000);
  }
  DIR* dir = opendir("/sys/fs/fuse/connections");
  if (dir) {
    for (;;) {
      struct dirent* ent = readdir(dir);
      if (!ent)
        break;
      if (strcmp(ent->d_name, ".") == 0 || strcmp(ent->d_name, "..") == 0)
        continue;
      char abort[300];
      snprintf(abort, sizeof(abort), "/sys/fs/fuse/connections/%s/abort",
               ent->d_name);
      int fd = open(abort, O_WRONLY);
      if (fd == -1) {
        continue;
      }
      if (write(fd, abort, 1) < 0) {
      }
      close(fd);
    }
    closedir(dir);
  } else {
  }
  while (waitpid(-1, status, __WALL) != pid) {
  }
}

#define SYZ_HAVE_SETUP_TEST 1
static void setup_test()
{
  prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
  setpgrp();
}

#define SYZ_HAVE_RESET_TEST 1
static void reset_test()
{
  int fd;
  for (fd = 3; fd < 30; fd++)
    close(fd);
}

struct thread_t {
  int created, call;
  event_t ready, done;
};

static struct thread_t threads[16];
static void execute_call(int call);
static int running;

static void* thr(void* arg)
{
  struct thread_t* th = (struct thread_t*)arg;
  for (;;) {
    event_wait(&th->ready);
    event_reset(&th->ready);
    execute_call(th->call);
    __atomic_fetch_sub(&running, 1, __ATOMIC_RELAXED);
    event_set(&th->done);
  }
  return 0;
}

static void execute_one(void)
{
  int i, call, thread;
  int collide = 0;
again:
  for (call = 0; call < 5; call++) {
    for (thread = 0; thread < (int)(sizeof(threads) / sizeof(threads[0]));
         thread++) {
      struct thread_t* th = &threads[thread];
      if (!th->created) {
        th->created = 1;
        event_init(&th->ready);
        event_init(&th->done);
        event_set(&th->done);
        thread_start(thr, th);
      }
      if (!event_isset(&th->done))
        continue;
      event_reset(&th->done);
      th->call = call;
      __atomic_fetch_add(&running, 1, __ATOMIC_RELAXED);
      event_set(&th->ready);
      if (collide && (call % 2) == 0)
        break;
      event_timedwait(&th->done, 45);
      break;
    }
  }
  for (i = 0; i < 100 && __atomic_load_n(&running, __ATOMIC_RELAXED); i++)
    sleep_ms(1);
  if (!collide) {
    collide = 1;
    goto again;
  }
}

static void execute_one(void);

#define WAIT_FLAGS __WALL

static void loop(void)
{
  int iter;
  for (iter = 0;; iter++) {
    char cwdbuf[32];
    sprintf(cwdbuf, "./%d", iter);
    if (mkdir(cwdbuf, 0777))
      exit(1);
    int pid = fork();
    if (pid < 0)
      exit(1);
    if (pid == 0) {
      if (chdir(cwdbuf))
        exit(1);
      setup_test();
      execute_one();
      reset_test();
      exit(0);
    }
    int status = 0;
    uint64_t start = current_time_ms();
    for (;;) {
      if (waitpid(-1, &status, WNOHANG | WAIT_FLAGS) == pid)
        break;
      sleep_ms(1);
      if (current_time_ms() - start < 5 * 1000)
        continue;
      kill_and_wait(pid, &status);
      break;
    }
    remove_dir(cwdbuf);
  }
}
#ifndef __NR_sched_setattr
#define __NR_sched_setattr 314
#endif

uint64_t r[1] = {0xffffffffffffffff};

void execute_call(int call)
{
  long res;
  switch (call) {
  case 0:
    memcpy((void*)0x20000040, "./bus\000", 6);
    res = syscall(__NR_open, 0x20000040, 0x1fe, 0);
    if (res != -1)
      r[0] = res;
    break;
  case 1:
    memcpy((void*)0x200001c0, "\x15\x8a\xdd\x00", 4);
    syscall(__NR_write, r[0], 0x200001c0, 4);
    break;
  case 2:
    *(uint32_t*)0x20000800 = 1;
    *(uint32_t*)0x20000804 = 0x70;
    *(uint8_t*)0x20000808 = 0;
    *(uint8_t*)0x20000809 = 0;
    *(uint8_t*)0x2000080a = 0;
    *(uint8_t*)0x2000080b = 0;
    *(uint32_t*)0x2000080c = 0;
    *(uint64_t*)0x20000810 = 0x50a;
    *(uint64_t*)0x20000818 = 0;
    *(uint64_t*)0x20000820 = 0;
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 0, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 1, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 2, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 3, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 4, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 7, 5, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 6, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 7, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 8, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 9, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 10, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 11, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 12, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 13, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0x7fff, 14, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 15, 2);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 17, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 18, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 19, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 20, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 21, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 22, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 23, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 24, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 25, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 26, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 27, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 28, 1);
    STORE_BY_BITMASK(uint64_t, , 0x20000828, 0, 29, 35);
    *(uint32_t*)0x20000830 = 0;
    *(uint32_t*)0x20000834 = 0;
    *(uint64_t*)0x20000838 = 0;
    *(uint64_t*)0x20000840 = 0;
    *(uint64_t*)0x20000848 = 0;
    *(uint64_t*)0x20000850 = 0;
    *(uint32_t*)0x20000858 = 0;
    *(uint32_t*)0x2000085c = 0;
    *(uint64_t*)0x20000860 = 0;
    *(uint32_t*)0x20000868 = 0xfffffffd;
    *(uint16_t*)0x2000086c = 0;
    *(uint16_t*)0x2000086e = 0;
    syscall(__NR_perf_event_open, 0x20000800, 0, -1, -1, 0);
    break;
  case 3:
    *(uint64_t*)0x20000000 = 0;
    syscall(__NR_sendfile, r[0], r[0], 0x20000000, 0x8080fffffffe);
    break;
#if 0
  case 4:
    *(uint32_t*)0x200002c0 = 0;
    *(uint32_t*)0x200002c4 = 2;
    *(uint64_t*)0x200002c8 = 0;
    *(uint32_t*)0x200002d0 = 0;
    *(uint32_t*)0x200002d4 = 3;
    *(uint64_t*)0x200002d8 = 0;
    *(uint64_t*)0x200002e0 = 0;
    *(uint64_t*)0x200002e8 = 0;
    syscall(__NR_sched_setattr, 0, 0x200002c0, 0);
    break;
#endif
  }
}
int main(void)
{
  syscall(__NR_mmap, 0x20000000, 0x1000000, 3, 0x32, -1, 0);
  for (procid = 0; procid < 6; procid++) {
    if (fork() == 0) {
      use_temporary_dir();
      loop();
    }
  }
  sleep(1000000);
  return 0;
}