linux-kernel - Re: [PATCH v5 00/15] extend task comm from 16 to 24 for CONFIG_BASE

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Date:   Fri, 22 Oct 2021 14:20:55 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Valentin Schneider <valentin.schneider@....com>,
        Qiang Zhang <qiang.zhang@...driver.com>,
        robdclark <robdclark@...omium.org>,
        christian <christian@...uner.io>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        David Miller <davem@...emloft.net>, kuba@...nel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, andrii@...nel.org,
        kafai@...com, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, john.fastabend@...il.com,
        kpsingh@...nel.org, netdev <netdev@...r.kernel.org>,
        bpf@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        kernel test robot <oliver.sang@...el.com>,
        kbuild test robot <lkp@...el.com>
Subject: Re: [PATCH v5 00/15] extend task comm from 16 to 24 for CONFIG_BASE_FULL

On Fri, Oct 22, 2021 at 12:00 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Oct 21, 2021 at 08:52:22PM -0700, Andrew Morton wrote:
> > On Thu, 21 Oct 2021 03:45:07 +0000 Yafang Shao <laoar.shao@...il.com> wrote:
> >
> > > This patchset changes files among many subsystems. I don't know which
> > > tree it should be applied to, so I just base it on Linus's tree.
> >
> > I can do that ;)
> >
> > > There're many truncated kthreads in the kernel, which may make trouble
> > > for the user, for example, the user can't get detailed device
> > > information from the task comm.
> >
> > That sucked of us.
> >
> > > This patchset tries to improve this problem fundamentally by extending
> > > the task comm size from 16 to 24. In order to do that, we have to do
> > > some cleanups first.
> >
> > It's at v5 and there's no evidence of review activity?  C'mon, folks!
>
> It's on my list! :) It's a pretty subtle area that rarely changes, so I
> want to make sure I'm a full coffee to do the review. :)
>
> > > 1. Make the copy of task comm always safe no matter what the task
> > > comm size is. For example,
> > >
> > >   Unsafe                 Safe
> > >   strlcpy                strscpy_pad
> > >   strncpy                strscpy_pad
> > >   bpf_probe_read_kernel  bpf_probe_read_kernel_str
> > >                          bpf_core_read_str
> > >                          bpf_get_current_comm
> > >                          perf_event__prepare_comm
> > >                          prctl(2)
> > >
> > > 2. Replace the old hard-coded 16 with a new macro TASK_COMM_LEN_16 to
> > > make it more grepable.
> > >
> > > 3. Extend the task comm size to 24 for CONFIG_BASE_FULL case and keep it
> > > as 16 for CONFIG_BASE_SMALL.
> >
> > Is this justified?  How much simpler/more reliable/more maintainable/
> > would the code be if we were to make CONFIG_BASE_SMALL suffer with the
> > extra 8 bytes?
>
> Does anyone "own" CONFIG_BASE_SMALL? Gonna go with "no":
>
> $ git ann init/Kconfig| grep 'config BASE_SMALL'
> 1da177e4c3f41   (Linus Torvalds 2005-04-16 15:20:36 -0700 2054)config BASE_SMALL
>
> And it looks mostly unused:
>
> $ git grep CONFIG_BASE_SMALL | cut -d: -f1 | sort -u | xargs -n1 git ann -f | grep 'CONFIG_BASE_SMALL'
> b2af018ff26f1   (Ingo Molnar    2009-01-28 17:36:56 +0100       18)#if CONFIG_BASE_SMALL == 0
> fcdba07ee390d   ( Jiri Olsa     2011-02-07 19:31:25 +0100       54)#define CON_BUF_SIZE (CONFIG_BASE_SMALL ? 256 : PAGE_SIZE)
> Blaming lines: 100% (46/46), done.
> 1da177e4c3f41   (Linus Torvalds 2005-04-16 15:20:36 -0700       28)#define PID_MAX_DEFAULT (CONFIG_BASE_SMALL ? 0x1000 : 0x8000)
> 1da177e4c3f41   (Linus Torvalds 2005-04-16 15:20:36 -0700       34)#define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
> Blaming lines: 100% (162/162), done.
> f86dcc5aa8c79   (Eric Dumazet   2009-10-07 00:37:59 +0000       31)#define UDP_HTABLE_SIZE_MIN     (CONFIG_BASE_SMALL ? 128 : 256)
> 02c02bf12c5d8   (Matthew Wilcox 2017-11-03 23:09:45 -0400       1110)#define XA_CHUNK_SHIFT        (CONFIG_BASE_SMALL ? 4 : 6)
> a52b89ebb6d44   (Davidlohr Bueso        2014-01-12 15:31:23 -0800       4249)#if CONFIG_BASE_SMALL
> 7b44ab978b77a   (Eric W. Biederman      2011-11-16 23:20:58 -0800       78)#define UIDHASH_BITS (CONFIG_BASE_SMALL ? 3 : 7)
>
>
> --

Right. CONFIG_BASE_SMALL is seldomly used in the kernel.
As you have already removed 64 bytes from task_struct, I think we can
extend the 8 bytes for CONFIG_BASE_SMALL as well.

-- 
Thanks
Yafang