[Bug 218830] New: lseek on closed file does not trigger an error and affect other files

[Bug 218830] New: lseek on closed file does not trigger an error and affect other files

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=218830

            Bug ID: 218830
           Summary: lseek on closed file does not trigger an error and
                    affect other files
           Product: File System
           Version: 2.5
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: ext4
          Assignee: fs_ext4@kernel-bugs.osdl.org
          Reporter: zhangchi_seg@smail.nju.edu.cn
        Regression: No

Created attachment 306289
  --> https://bugzilla.kernel.org/attachment.cgi?id=306289&action=edit
reproduce.c

Hi,

I have a file and lseek on it after calling the close(), but it dose not
trigger an EBADF error. Then I open and write to another file, but the write
operation trigger an "Invalid argument" error. 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 running the script, you will see an error message:
```
write failure: (Invalid argument)
```

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()
{
    mount("/dev/loop0", "/root/mnt", "ext4", 0, "");

    creat("/root/mnt/a", S_IRWXU);
    creat("/root/mnt/b", S_IRWXU);
    int fd_a = open("/root/mnt/a", O_RDWR); 
    close(fd_a); 
    int fd_b = open("/root/mnt/b", O_RDWR | O_DIRECT); 
    int state = lseek(fd_a, 7208, SEEK_SET); 
    if (state == -1) {
      printf("lseek failure: (%s)\n", strerror(errno));
    }

    char *buf = (char*)align_alloc(4096);
    memset(buf, 'a', 4096);
    state = write(fd_b, buf, 4096);
    if (state == -1) {
      printf("write failure: (%s)\n", strerror(errno));
    }

    close(fd_b); 
    return 0;
}
```

I also found that if I remove the `O_DIRECT` flag of file b, the write
operation will not trigger an error, but the contents of b become garbled.

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[Bug 218830] lseek on closed file does not trigger an error and affect other files

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=218830

Theodore Tso (tytso@mit.edu) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |tytso@mit.edu
         Resolution|---                         |INVALID

--- Comment #1 from Theodore Tso (tytso@mit.edu) ---
This is a test/programming bug.   If you change reproduce.c so that it prints
fd_a  and fd_b, you'll see that they have the same value.   So the reason why
lseek didn't fail is because fd_a has the same integer value as fd_b --- and so
lseek didn't fail and affected the current position of fd_b.

This is a documented feature of the Linux/Unix/Posix interface.   File
descriptors are small integers, and if you close a file descriptor, it releases
that integer ---- think of it as an index in an array, i.e., struct file
*fd_array[MAX_FDS].   When you call open, it will find the first NULL pointer
in fd_array, and installs a pointer to the struct file, and returns that
integer as the file descriptor.

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[Bug 218830] lseek on closed file does not trigger an error and affect other files

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--- Comment #2 from Chi (zhangchi_seg@smail.nju.edu.cn) ---
Hi Theodore Tso,

Thank you for your quick response, I'm very sorry for making this incorrect bug
report, I will read the documentation carefully.

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