linux-ext4 - [Bug 218850] New: Unexpected failure when write to a file with two file descriptor

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Message-ID: <bug-218850-13602@https.bugzilla.kernel.org/>
Date: Thu, 16 May 2024 15:02:08 +0000
From: bugzilla-daemon@...nel.org
To: linux-ext4@...r.kernel.org
Subject: [Bug 218850] New: Unexpected failure when write to a file with two
 file descriptor

https://bugzilla.kernel.org/show_bug.cgi?id=218850

            Bug ID: 218850
           Summary: Unexpected failure when write to a file with two file
                    descriptor
           Product: File System
           Version: 2.5
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: ext4
          Assignee: fs_ext4@...nel-bugs.osdl.org
          Reporter: zhangchi_seg@...il.nju.edu.cn
        Regression: No

Created attachment 306300
  --> https://bugzilla.kernel.org/attachment.cgi?id=306300&action=edit
reproduce.c

Hi,

I mounted an ext4 image, created a file, and created a link to it, then I wrote
to these two files, and I failed with a specific read and write order. I can
reproduce this with the latest linux kernel
https://git.kernel.org/torvalds/t/linux-6.9-rc7.tar.gz

The following is the triggering script:
```
dd if=/dev/zero of=ext4-0.img bs=1M count=120
mkfs.ext4 ext4-0.img
g++ -static reproduce.c
losetup /dev/loop0 ext4-0.img
mkdir /root/mnt
./a.out
```

After run the script, you will see the error message:

```
write failure
```

The contents of `reproduce.c` :
```
#include <assert.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdarg.h>
#include <stddef.h>
#include <unistd.h>
#include <pthread.h>
#include <errno.h>
#include <dirent.h>

#include <string>

#include <sys/mount.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/wait.h>
#include <sys/xattr.h>
#include <sys/mount.h>
#include <sys/statfs.h>
#include <fcntl.h>

#define ALIGN 4096

void* align_alloc(size_t size) {
    void *ptr = NULL;
    int ret = posix_memalign(&ptr, ALIGN, size);
    if (ret) {
      printf("align error\n");
      exit(1);
    }
    return ptr;
}

int main()
{
    char *buf_15 = (char*)align_alloc(4096*20);
    memset(buf_15, 'a', 4096*20);

    char *buf_4 = (char*)align_alloc(4096*20);
    memset(buf_4, 'a', 4096*20);

    mount("/dev/loop0", "/root/mnt", "f2fs", 0, "");

    creat("/root/mnt/a", S_IRWXG);
    link("/root/mnt/a", "/root/mnt/b");
    int fd_a = open("/root/mnt/a", O_RDWR);
    int fd_b = open("/root/mnt/b", O_RDWR | O_DIRECT);

    lseek(fd_a, 100, SEEK_SET); 
    write(fd_a, buf_15, 9900); 

    read(fd_b, buf_4, 73728); 

    int state = write(fd_b, buf_15, 65536); 
    if (state == -1) {
        printf("write failure\n");
    }

    return 0;
}

```

If I move the statement `read(fd_b, buf_4, 73728); ` before the first write
operation, or modify the size `73728` to a smaller one, such as `63728`, then
this script will not fail.

Did I do anything wrong?

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