linux-ext4 - [Bug 205567] New: potential (possibly benign) data race on ext4_dir_entry_2->inode when getdents64 and rename happens on the same directory

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Date:   Mon, 18 Nov 2019 20:15:09 +0000
From:   bugzilla-daemon@...zilla.kernel.org
To:     linux-ext4@...r.kernel.org
Subject: [Bug 205567] New: potential (possibly benign) data race on
 ext4_dir_entry_2->inode when getdents64 and rename happens on the same
 directory

https://bugzilla.kernel.org/show_bug.cgi?id=205567

            Bug ID: 205567
           Summary: potential (possibly benign) data race on
                    ext4_dir_entry_2->inode when getdents64 and rename
                    happens on the same directory
           Product: File System
           Version: 2.5
    Kernel Version: 5.4-rc5
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: ext4
          Assignee: fs_ext4@...nel-bugs.osdl.org
          Reporter: mengxu.gatech@...il.com
        Regression: No

I am reporting a potential data race (maybe benign) in the ext4 layer on 
ext4_dir_entry_2->inode when getdents64 and rename happens on the same
directory.

[Setup]
mkdir(dir_foo, 0777);
open(dir_foo, 0x10000, 0777) = 0;
dup2(0, 199) = 199;

[Thread 1] getdents64(199, <some buffer>, 4469) = 48;
[Thread 2] rename(dir_foo, aaaaa) = 0;

The function call trace is shown below:

[Thread 1: SYS_getdents64]
__do_sys_getdents64
  ksys_getdents64
    iterate_dir
      ext4_readdir
        ext4_dx_readdir
          ext4_htree_fill_tree
            htree_dirblock_to_tree
              [READ] if (de->inode == 0)

[Thread 2: SYS_rename]
__do_renameat2
  do_renameat2
    vfs_rename
      ext4_rename2
        ext4_rename
          ext4_rename_dir_finish
            [WRITE] ent->parent_de->inode = cpu_to_le32(dir_ino);


I could confirm that the WRITE may happen before and after the READ operation
by controlling the timing of the two threads, i.e., by setting breakpoints
before the WRITE statement.

However, I am not very sure about the implication of such a data race (e.g.,
causing violations of assumptions). I would appreciate if you could help check
on this potential bug and advise whether this is a harmful data race or it
is intended. Thank you!

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