linux-kernel - Re: [driver-core:debugfs_cleanup 4/5] fs/d_path.c:59 prepend() warn: unsigned 'p->len' is never less than zero.

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Message-ID: <Yc9ba7ur1iVhaJd5@zeniv-ca.linux.org.uk>
Date:   Fri, 31 Dec 2021 19:35:07 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     kernel test robot <lkp@...el.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kbuild-all@...ts.01.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [driver-core:debugfs_cleanup 4/5] fs/d_path.c:59 prepend() warn:
 unsigned 'p->len' is never less than zero.

On Sat, Jan 01, 2022 at 01:08:41AM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git debugfs_cleanup
> head:   a04bbe0a2c7e98669e11a47f94e53dd8228bbeba
> commit: e95d5bed5d58c2f5352969369827e7135fa2261e [4/5] fs: make d_path-like functions all have unsigned size
> config: i386-randconfig-m031-20211228 (https://download.01.org/0day-ci/archive/20220101/202201010156.bJvO7Gaw-lkp@intel.com/config)
> compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@...el.com>
> 
> smatch warnings:
> fs/d_path.c:59 prepend() warn: unsigned 'p->len' is never less than zero.

What do you mean, "unsigned p->len"?

->len really *can* be negative - that's how running out of buffer is indicated.

Greg, I stand by the comment I made back in July - this kind of "hardening" is
useless; there's no legitimate reason to pass a huge buffer length, especially
since there's a limit on the length of pathname any syscall would accept.
See https://www.spinics.net/lists/linux-fsdevel/msg200370.html for the
variant I would prefer.