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

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Date:   Thu, 21 Oct 2021 21:00:51 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Yafang Shao <laoar.shao@...il.com>, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, arnaldo.melo@...il.com,
        pmladek@...e.com, peterz@...radead.org, viro@...iv.linux.org.uk,
        valentin.schneider@....com, qiang.zhang@...driver.com,
        robdclark@...omium.org, christian@...uner.io,
        dietmar.eggemann@....com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-perf-users@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, oliver.sang@...el.com, lkp@...el.com
Subject: Re: [PATCH v5 00/15] extend task comm from 16 to 24 for
 CONFIG_BASE_FULL

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)


-- 
Kees Cook