linux-kernel - Re: NULL pointer dereference when access /proc/net

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Date:   Sun, 25 Apr 2021 16:50:54 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     haosdent <haosdent@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        zhengyu.duan@...pee.com, Haosong Huang <huangh@....com>
Subject: Re: NULL pointer dereference when access /proc/net

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.