fcntl(F_SETLK) for setting byte range locks is broken leading to data corruption

[ned14]

My WSL is the Creator's Update 1703. uname -r reports 4.4.0-43-Microsoft

I'm seeing fcntl(F_SETLK) permit multiple exclusive locks on WSL which is bad, it's instant data corruption for anything relying on this to work. This test launches four child processes and uses a shared memory map to check if fcntl(F_SETLK) is working correctly. This test passes on real Linux, fails badly on WSL:

integration/afio/shared_fs_mutex_byte_ranges/exclusives:
Tests that afio::algorithm::shared_fs_mutex::byte_ranges implementation implements exclusive locking
CHECK ((i.results[0] == 'o' && i.results[1] == 'k')) FAILED at /home/ned/windocs/boostish/afio/test/tests/shared_fs_mutex.cpp:283
Child reports Child 0 released exclusive lock to find child 3 had stolen my lock!
CHECK ((i.results[0] == 'o' && i.results[1] == 'k')) FAILED at /home/ned/windocs/boostish/afio/test/tests/shared_fs_mutex.cpp:283
Child reports Child 1 granted exclusive lock when child 2 already has exclusive lock!
CHECK ((i.results[0] == 'o' && i.results[1] == 'k')) FAILED at /home/ned/windocs/boostish/afio/test/tests/shared_fs_mutex.cpp:283
Child reports Child 2 released exclusive lock to find child 1 had stolen my lock!
CHECK ((i.results[0] == 'o' && i.results[1] == 'k')) FAILED at /home/ned/windocs/boostish/afio/test/tests/shared_fs_mutex.cpp:283
Child reports Child 3 granted exclusive lock when child 0 already has exclusive lock!
4 checks passed  4 checks failed  duration 5150 ms

While you're at it, if you could implement the non-insane fcntl(F_OFD_SETLK) locks that would be very useful. The fcntl(F_SETLK) type locks are stupid.

[therealkenc]

Same as #1712 (message), FWIW.

[ned14]

The issue referenced looks like lock files, not byte range locks. Another test in my test suite tests lock files and it passes on WSL. Still, the issues could be related.

It should be trivially easy to convert the new ORD locks to LockFileEx. The older POSIX locks would require a little more work to emulate the broken semantics required by POSIX. In any case they should return ENOTSUPP or something so implementations can choose a backup locking system like lock files.

[0xabu]

FWIW, recent builds of QEMU require F_OFD_SETLK / F_OFD_GETLK for disk image files. It would be nice to have them implemented.

[trawick]

I see this fcntl(F_SETLK) issue with libapr's apr_file_lock() API. A child process is able to run this sequence successfully while the parent holds the lock.

[pid   702] open("data/testfile.lock", O_WRONLY|O_CLOEXEC) = 3
[pid   702] fcntl(3, F_GETFD)           = 0x1 (flags FD_CLOEXEC)
[pid   702] fcntl(3, F_SETLK, {l_type=F_WRLCK, l_whence=SEEK_SET, l_start=0, l_len=0}) = 0

[apenwarr]

A simple test case for fcntl-style file locking semantics can be found in "locky.c" linked from this blog post: https://apenwarr.ca/log/20101213

[rgal]

Still seeing this on Version 1803 (4.4.0-17134-Microsoft).

Are there plans to fix this?

[remisharrock]

So any update on this ?

here are some codes in C not working (using lockf)

/******************************************************

In this file, we implement a simple mutual exclusion
between processes accessing a shared file.

********************************************************/

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <setjmp.h>
#include <signal.h>
#include <sys/file.h>

void on_alrm(int sig_nb);
jmp_buf ctxt;

int open_ret;

int main (int argc, char *argv[])
{
  int lock_ret, write_ret ;
  char taped_word[256];
  if (argc != 3)
  {
    printf("Wrong usage : %s file_name time_out! \n", argv[0]);
    exit(2);
  }
  /*-----------------------------------------------------
  To check that the program works correctly, display
  outputs on shell windows for which we first obtained
  the name using the tty command. Pass the file name
  as an argument to the program.
  ----------------------------------------------------- */

  int timeout = atoi(argv[2]);

  if ((open_ret = open (argv[1], O_RDWR)) == -1)
  {
    perror ("open 2");
    exit (1);
  }

  while (1)
  {
    sigsetjmp(ctxt, 1);
    /* Avoid starvation */
    sleep(1);
    /*  Implement a mutual exclusion  */
     lseek(open_ret, 0, SEEK_SET);
    lock_ret = flock(open_ret, LOCK_EX);
    printf ("Pid %d : entered the critical section\n", (int)getpid() );
    signal(SIGALRM, on_alrm);
    alarm(timeout);

    /*----------------------------------------------
    Begining of the critical section
    ----------------------------------------------*/
    while ( fgets(taped_word, sizeof(taped_word), stdin) )
    {
      /*----------------------------------------------
      The program stays in the read/write loop until
      the user tape "end".
      ----------------------------------------------*/
      if (!strncmp (taped_word, "end", 3)) break;
      write_ret = write (open_ret, taped_word, strlen(taped_word));
      if (write_ret < 0)
      {
        printf ("Error when writing in %s\n", argv[1]);
        break;
      }
    }

    /*----------------------------------------------
    End of the critical section
    ----------------------------------------------*/
    lseek(open_ret, 0, SEEK_SET);
    /* Release the second lock */
   lock_ret = flock(open_ret, LOCK_UN);

    printf ("Pid %d : arret temporaire d'utilisation de %s\n",
    (int)getpid(), argv[1]);

    /*  To avoid starvation  */
    sleep(1);

    printf ("Finished critical section, lock returned =  %d\n", lock_ret);
  }  /* fin de la boucle while */
}

void on_alrm(int sig_nb)
{
  lseek(open_ret, 0, SEEK_SET);
  /* Release the second lock */
  flock(open_ret, LOCK_UN);
  printf("Jump: re-execute before critical section\n");
  siglongjmp(ctxt, sig_nb);
}

/******************************************************
On UNIX, usage of locks when accessing a file verrou
is not mandatory (advisory locking). To make it
mandatory (mandatory locking), it is necessary
(if you have the wright to do so) to modify the access
wrights on the file (cf. man lockf et man chmod).
Depending on the operating systems, commands that follow
should do the job:
chmod 2644
chmod +l
********************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>

int main (int argc, char *argv[]){
  int i=0, sentence_size, sleep_index_in_sentence,
  deadlock_nb=0,  /* counter for the number of detected deadlocks */
  correct_iteration_nb =0,  /* counter for the number of correct iteration
                             * of the critical section */
  lockf_ret =0, /* value returned by call to lockf */
  file_1, file_2; /* shared resources */
  char sentence[256];

  if (argc != 3){
    printf(" Wrong usage : %s file_1_name file_2_name ! \n", argv[0]);
    exit(2);
  }

  sprintf(sentence, " -- Writen by pid %d -- \n", (int)getpid() );
  sentence_size=strlen(sentence);

  /* Open the two files */

  if ((file_1 = open (argv[1], O_RDWR)) == -1) {
    perror ("open file_1");
    exit (1);
  }
  if ((file_2 = open (argv[2], O_RDWR)) == -1) {
    perror ("open file_2");
    exit (1);
  }

  while(1)
  {
    /* Take the first lock */
    lockf_ret = lockf(file_1, F_LOCK, 0);
    if (lockf_ret != 0)  perror ("lockf file_1");
    printf ("Pid %d : entered critical section 1 (%s), lockf_ret=%d\n", (int)getpid(), argv[1], lockf_ret );

    // simulate execution
    sleep(random() %4);

    /* Take the second lock */
    lockf_ret = lockf(file_2, F_LOCK, 0);

    /* If a deadlock is detected, release the first lock
       execute sleep, increment deadlock_nb, and try to
      take it again by restarting the loop */

    if (lockf_ret != 0)  {
      perror ("lockf file_2");
      if (errno == EDEADLK){
        printf ("Pid %d : ulock %s (deadlock_nb=%d)\n", (int)getpid(),  argv[1], deadlock_nb);
        deadlock_nb++;
        /** Release the first lock */
        lockf_ret = lockf(file_1, F_ULOCK, 0);
        sleep(deadlock_nb);
        /* continue the loop */
        continue;
      }
    }

    deadlock_nb=0;
    correct_iteration_nb++;
    printf ("Pid %d : entered critical section 2 (%s), lockf_ret=%d, correct_iteration_nb =%d\n",
    (int)getpid(), argv[2], lockf_ret, correct_iteration_nb);

    /*----------------------------------------------
    Begining of the critical section
    ----------------------------------------------*/

    sleep_index_in_sentence=random()%sentence_size;

    for (i=0; i <sentence_size ;i++){
      if (i == sleep_index_in_sentence)sleep(random()%4);
      if (write (file_1, sentence+i, sizeof (char)) < 0){
        printf ("ERROR: writing to %s failed\n", argv[1]);
        break;
      }
    }

    for (i=0; i <sentence_size;i++){
      if (i == sleep_index_in_sentence)sleep(random()%4);
      if (write (file_2, sentence+i, sizeof (char) ) < 0){
        printf ("ERROR: writing to %s failed\n", argv[2]);
        break;
      }
    }

    /*----------------------------------------------
    End of the critical section
    ----------------------------------------------*/

    lseek(file_2, 0, SEEK_SET);
    /* Release the second lock */
    lockf_ret = lockf(file_2, F_ULOCK, 0);
    if (lockf_ret != 0)  perror ("ulockf file_2");
    printf ("Pid %d : exit critical section 2 (%s), lockf_ret=%d\n", (int)getpid(), argv[2], lockf_ret );

    lseek(file_1, 0, SEEK_SET);
    /* Release the first lock */
    lockf_ret = lockf(file_1, F_ULOCK, 0);
    if (lockf_ret != 0)  perror ("ulockf file_1");
    printf ("Pid %d : exit critical section 1 (%s), lockf_ret=%d\n", (int)getpid(), argv[1], lockf_ret );

  }  /* end of for */

  return 0;

}

[edrevo]

@benhillis, do you know if this applies to WSL2? In gittup/tup#394 they seem to have fixed the issue moving to WSL2, but com-lihaoyi/mill#874 seems to indicate this bug is still present in WSL2.

[therealkenc]

Yes all the functls are good on WSL2. Its a real ext4 filesystem running on real Linux kernel.

[fredericoabraham]

Was it fixed for WSL1?

[gertvanantwerpentno]

Why will this not be fixed in WSL-1. The WSL-2 environment cannot be used on older CPU-environments and WSL-1 is unusable for me when this is not fixed. Why does Microsoft not fix a bug in a product it still distributes on the newest Windows versions? WSL-2 is not a replacement for WSL-1, says Microsoft. They can be used as different products. This is really terrible, as it is a simple bug that exists already for years.

[StephenAtty]

I'm in the same situation as @gertvanantwerpentno I do not want to move to WSL-2 for various reasons and without this being fixed in WSL-1 I'm basically stuck.

[fredericoabraham]

Me too, @StephenAtty .

[saurav3199]

Will, there be a solution or a workaround for WSL1. It's annoying sometimes

[StephenAtty]

@saurav3199 = It's not been fixed. They never had any intention of fixing it. They want you to move onto WSL2 which is a full virtual machine.

So using WSL as light weight linux development environment is a dead duck.

[maxprehl]

Another WSL1 user here. Recently hit this issue with a systemd update on Debian sid. Really hoping this gets implemented or worked around at some point! https://superuser.com/a/1805742/1298503

[Vampire]

It is really unlucky that this bug will not get fixed. :-(
WSL 1 is not obsolete, for example on GitHub Actions WSL 2 is simply technically not possible, so there you can only use WSL 1.
And currently if you for example use kali-linux and try to install sudo it fails due to this bug.
Or if you try to use ubuntu-24.04 and do an apt update, it also fails due to this bug.
:'''-(

[kkovacs]

Another victim of this issue here, ubuntu-24.04 and apt upgrade. (In my case, I'm using WSL1 because a stupid corporate VPN breaks my networking with WSL2.)

Not very elegant, but my workaround was to momentarily convert to WSL2 using (surprisingly, it's not much time):

wsl.exe --set-version Ubuntu-24.04 2

, then let apt fix the problem with:

apt install -f

, then convert back to WSL1, with:

wsl.exe --set-version Ubuntu-24.04 1

It works now (I guess until the next polkitd update...)