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Date:   Thu, 28 Nov 2019 23:43:14 -0500
From:   Meng Xu <mengxu.gatech@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: potential data race on ext_inode_hdr(inode)->eh_depth,
 ext_inode_hdr(inode)->eh_max between a creat and unlink syscall

Hi Ted,

First, thank you for checking this out.

I hook every memory access in the kernel so I know that the [READ] and
[WRITE] are accessing to the exact same memory address. Plus, this
access cannot be from two malloc-ed inode because we replaced kfree
with a quarantine scheme like KASan so they two inodes will have to
have two different addresses. This is what confused me too.

In addition, just in case it may make a difference, there is an fsync
happening on another thread too. The three threads are like:

[Setup]
mkdir("foo", 511) = 0;
open("foo", 65536, 511) = 3;
creat("bar", 511) = 4;
symlink("foo", "sym_foo") = 0;
open("sym_foo", 65536, 511) = 5;
dup2(5, 195) = 195;

[Thread 0: fsync(195)]
[Thread 1: creat("bar", 438)]
[Thread 2: unlink("sym_foo")]

Or in orders observed at runtime:
Enter fsync(195);
Enter unlink("sym_foo");
Enter creat("bar", 438);

Exit unlink("sym_foo");
Exit creat("bar", 438);
Exit fsync(195);

I can provide more information (eg, other function calls on the trace
or memory access logs), if that would help in checking this case. And
I am sorry for wasting your time if this case does not make sense.

Best regards,
Meng

On Thu, Nov 28, 2019 at 6:19 PM Theodore Y. Ts'o <tytso@....edu> wrote:
>
> On Thu, Nov 28, 2019 at 12:03:04PM -0500, Meng Xu wrote:
> > I notice a potential data race on ext_inode_hdr(inode)->eh_depth,
> > ext_inode_hdr(inode)->eh_max between a create and unlink syscall.
> > Following is the trace:
> >
> > [Setup]
> > mkdir("foo", 511) = 0;
> > open("foo", 65536, 511) = 3;
> > create("bar", 511) = 4;
> > symlink("foo", "sym_foo") = 0;
> > open("sym_foo", 65536, 511) = 5;
> >
> > [Thread 1]
> > create("bar", 438);
> >
> > __do_sys_creat
> >   ksys_open
> >     do_filp_open
> >       path_openat
> >         do_last
> >           handle_truncate
> >             do_truncate
> >               notify_change
> >                 ext4_setattr
> >                   ext4_truncate
> >                     ext4_ext_truncate
> >                       ext4_ext _remove_space
> >                         [WRITE, 2 bytes] ext_inode_hdr(inode)->eh_depth = 0;
> >                         [WRITE, 2 bytes] ext_inode_hdr(inode)->eh_max
> > = cpu_to_le16(ext4_ext_space_root(inode, 0));
> >
> > [Thread 2]
> > unlink("sym_foo");
> >
> > __do_sys_unlink
> >   do_unlinkat
> >     iput
> >       iput_final
> >         evict
> >           ext4_evict_inode
> >             ext4_orphan_del
> >               ext4_mark_iloc_dirty
> >                 ext4_do_update_inode
> >                   [READ, 4 bytes] raw_inode->i_block[block] = ei->i_data[block];
> >
> >
> > I could observe that the order between the READ and WRITE is not
> > deterministic and I was curious what will happen if the READ takes
> > place in the middle of the two WRITES? Does it cause any damages or
> > violations?
>
> This makes no sense.  The inodes corresponding to "sym_foo" and "bar"
> are completely differenth.  So why would there be a data race?
>
> How are you concluding that that there is, in fact, a data race?
>
>                                        - Ted

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