Re: NULL pointer dereference when access /proc/net

[haosdent]

> Not really - the crucial part is ->d_count == -128, i.e. it's already past
> __dentry_kill().

Is it possible that dentry is garbage collected due to memory usage,
but it still is stored in the dentry cache.

Available memory is 5% when this crash happens, not sure if this helps.
```
crash> kmem -i
                 PAGES        TOTAL      PERCENTAGE
    TOTAL MEM  32795194     125.1 GB         ----
         FREE  1870573       7.1 GB    5% of TOTAL MEM
         USED  30924621       118 GB   94% of TOTAL MEM
       SHARED  14145523        54 GB   43% of TOTAL MEM
      BUFFERS   112953     441.2 MB    0% of TOTAL MEM
       CACHED  14362325      54.8 GB   43% of TOTAL MEM
         SLAB   664531       2.5 GB    2% of TOTAL MEM

   TOTAL HUGE        0            0         ----
    HUGE FREE        0            0    0% of TOTAL HUGE

   TOTAL SWAP        0            0         ----
    SWAP USED        0            0    0% of TOTAL SWAP
    SWAP FREE        0            0    0% of TOTAL SWAP

 COMMIT LIMIT  16397597      62.6 GB         ----
    COMMITTED  27786060       106 GB  169% of TOTAL LIMIT
```

On Mon, Apr 26, 2021 at 1:04 AM haosdent <haosdent@xxxxxxxxx> wrote:
>
> Hi, Alexander, thanks a lot for your quick reply.
>
> > Not really - the crucial part is ->d_count == -128, i.e. it's already past
> > __dentry_kill().
>
> Thanks a lot for your information, we would check this.
>
> > Which tree is that?
> > If you have some patches applied on top of that...
>
> We use Ubuntu Linux Kernel "4.15.0-42.45~16.04.1" from launchpad directly
> without any modification,  the mapping Linux Kernel should be
> "4.15.18" according
> to https://people.canonical.com/~kernel/info/kernel-version-map.html
>
> On Mon, Apr 26, 2021 at 12:50 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Sun, Apr 25, 2021 at 11:22:15PM +0800, haosdent wrote:
> > > Hi, Alexander Viro and dear Linux Filesystems maintainers, recently we
> > > encounter a NULL pointer dereference Oops in our production.
> > >
> > > We have attempted to analyze the core dump and compare it with source code
> > > in the past few weeks, currently still could not understand why
> > > `dentry->d_inode` become NULL while other fields look normal.
> >
> > Not really - the crucial part is ->d_count == -128, i.e. it's already past
> > __dentry_kill().
> >
> > > [19521409.514784] RIP: 0010:__atime_needs_update+0x5/0x190
> >
> > Which tree is that?  __atime_needs_update() had been introduced in
> > 4.8 and disappeared in 4.18; anything of that age straight on mainline
> > would have a plenty of interesting problems.  If you have some patches
> > applied on top of that...  Depends on what those are, obviously.
>
>
>
> --
> Best Regards,
> Haosdent Huang



-- 
Best Regards,
Haosdent Huang