From: Yafang Shao <laoar.shao@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Kees Cook <keescook@chromium.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
Peter Zijlstra <peterz@infradead.org>,
Al Viro <viro@zeniv.linux.org.uk>,
Valentin Schneider <valentin.schneider@arm.com>,
Qiang Zhang <qiang.zhang@windriver.com>,
robdclark <robdclark@chromium.org>,
christian <christian@brauner.io>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
David Miller <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
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Subject: Re: [PATCH v7 00/11] extend task comm from 16 to 24
Date: Tue, 2 Nov 2021 09:09:50 +0800 [thread overview]
Message-ID: <CALOAHbAx55AUo3bm8ZepZSZnw7A08cvKPdPyNTf=E_tPqmw5hw@mail.gmail.com> (raw)
In-Reply-To: <YYAPhE9uX7OYTlpv@alley>
On Tue, Nov 2, 2021 at 12:02 AM Petr Mladek <pmladek@suse.com> wrote:
>
> On Mon 2021-11-01 22:34:30, Yafang Shao wrote:
> > On Mon, Nov 1, 2021 at 10:07 PM Petr Mladek <pmladek@suse.com> wrote:
> > > On Mon 2021-11-01 06:04:08, Yafang Shao wrote:
> > > > 4. Print a warning if the kthread comm is still truncated.
> > > >
> > > > 5. What will happen to the out-of-tree tools after this change?
> > > > If the tool get task comm through kernel API, for example prctl(2),
> > > > bpf_get_current_comm() and etc, then it doesn't matter how large the
> > > > user buffer is, because it will always get a string with a nul
> > > > terminator. While if it gets the task comm through direct string copy,
> > > > the user tool must make sure the copied string has a nul terminator
> > > > itself. As TASK_COMM_LEN is not exposed to userspace, there's no
> > > > reason that it must require a fixed-size task comm.
> > >
> > > The amount of code that has to be updated is really high. I am pretty
> > > sure that there are more potential buffer overflows left.
> > >
> > > You did not commented on the concerns in the thread
> > > https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@mail.gmail.com/
> > >
> > I thought Steven[1] and Kees[2] have already clearly explained why we
> > do it like that, so I didn't give any more words on it.
> >
> > [1]. https://lore.kernel.org/all/20211025170503.59830a43@gandalf.local.home/
>
> Steven was against switching task->comm[16] into a dynamically
> allocated pointer. But he was not against storing longer names
> separately.
>
> > [2]. https://lore.kernel.org/all/202110251406.56F87A3522@keescook/
>
> Honestly, I am a bit confused by Kees' answer. IMHO, he agreed that
> switching task->comm[16] into a pointer was not worth it.
>
> But I am not sure what he meant by "Agreed -- this is a small change
> for what is already an "uncommon" corner case."
>
>
> > > Several people suggested to use a more conservative approach.
> >
> > Yes, they are Al[3] and Alexei[4].
> >
> > [3]. https://lore.kernel.org/lkml/YVkmaSUxbg%2FJtBHb@zeniv-ca.linux.org.uk/
>
> IMHO, Al suggested to store the long name separately and return it
> by proc_task_name() when available.
>
>
> > [4]. https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@mail.gmail.com/
>
> Alexei used dentry->d_iname as an exaxmple. struct dentry uses
> d_iname[DNAME_INLINE_LEN] for short names. And dynamically
> allocated d_name for long names, see *__d_alloc() implementation.
>
Thanks for the summary.
So with Stenven's new reply[1], the opinion in common is storing long
names into a separate place. And no one is against it now.
[1]. https://lore.kernel.org/lkml/20211101120636.3cfc5afa@gandalf.local.home/
> > > I mean
> > > to keep comm[16] as is and add a new pointer to the full name. The buffer
> > > for the long name might be dynamically allocated only when needed.
> > >
> >
> > That would add a new allocation in the fork() for the threads with a long name.
> > I'm not sure if it is worth it.
>
> The allocation will be done only when needed. IMHO, the performance is
> important only for userspace processes. I am not aware of any kernel
> subsystem that would heavily create and destroy kthreads.
>
XFS may create many kthreads with longer names, especially if there're
many partitions in the disk.
For example,
xfs-reclaim/sd{a, b, c, ...}
xfs-blockgc/sd{a, b, c, ...}
xfs-inodegc/sd{a, b, c, ...}
They are supposed to be created at boot time, and shouldn't be heavily
created and destroyed.
>
> > > The pointer might be either in task_struct or struct kthread. It might
> > > be used the same way as the full name stored by workqueue kthreads.
> > >
> >
> > If we decide to do it like that, I think we'd better add it in
> > task_struct, then it will work for all tasks.
>
> Is it really needed for userspace processes? For example, ps shows
> the information from /proc/*/cmdline instead.
>
Right. The userspace processes can be obtained from /proc/*/cmdline.
>
> > > The advantage of the separate pointer:
> > >
> > > + would work for names longer than 32
> > > + will not open security holes in code
> > >
> >
> > Yes, those are the advantages. And the disadvantage of it is:
> >
> > - new allocation in fork()
>
> It should not be a problem if we do it only when necessary and only
> for kthreads.
>
So if no one against, I will do it in two steps,
1. Send the task comm cleanups in a separate patchset named "task comm cleanups"
This patchset includes patch #1, #2, #4, #5, #6, #7 and #9.
Cleaning them up can make it less error prone, and it will be
helpful if we want to extend task comm in the future :)
2. Keep the current comm[16] as-is and introduce a separate pointer
to store kthread's long name
Now we only care about kthread, so we can put the pointer into a
kthread specific struct.
For example in the struct kthread, or in kthread->data (which may
conflict with workqueue).
And then dynamically allocate a longer name if it is truncated,
for example,
__kthread_create_on_node
len = vsnprintf(name, sizeof(name), namefmt, args);
if (len >= TASK_COMM_LEN) {
/* create a longer name */
}
And then we modify proc_task_name(), so the user can get
kthread's longer name via /proc/[pid]/comm.
And then free the allocated memory when the kthread is destroyed.
--
Thanks
Yafang
next prev parent reply other threads:[~2021-11-02 1:10 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-01 6:04 [PATCH v7 00/11] extend task comm from 16 to 24 Yafang Shao
2021-11-01 6:04 ` [PATCH v7 01/11] fs/exec: make __set_task_comm always set a nul terminated string Yafang Shao
2021-11-01 6:04 ` [PATCH v7 02/11] fs/exec: make __get_task_comm always get " Yafang Shao
2021-11-01 6:04 ` [PATCH v7 03/11] sched.h: use __must_be_array instead of BUILD_BUG_ON in get_task_comm Yafang Shao
2021-11-01 6:04 ` [PATCH v7 04/11] drivers/infiniband: make setup_ctxt always get a nul terminated task comm Yafang Shao
2021-11-01 6:04 ` [PATCH v7 05/11] fs/binfmt_elf: make prpsinfo " Yafang Shao
2021-11-01 6:04 ` [PATCH v7 06/11] samples/bpf/test_overhead_kprobe_kern: make it adopt to task comm size change Yafang Shao
2021-11-01 6:04 ` [PATCH v7 07/11] tools/bpf/bpftool/skeleton: " Yafang Shao
2021-11-01 23:47 ` Andrii Nakryiko
2021-11-01 6:04 ` [PATCH v7 08/11] tools/perf/test: make perf test " Yafang Shao
2021-11-17 14:31 ` Arnaldo Carvalho de Melo
2021-11-18 14:18 ` Yafang Shao
2021-11-01 6:04 ` [PATCH v7 09/11] tools/testing/selftests/bpf: make it " Yafang Shao
2021-11-01 23:47 ` Andrii Nakryiko
2021-11-01 6:04 ` [PATCH v7 10/11] sched.h: extend task comm from 16 to 24 Yafang Shao
2021-11-01 6:04 ` [PATCH v7 11/11] kernel/kthread: show a warning if kthread's comm is truncated Yafang Shao
2021-11-01 12:44 ` [PATCH v7 00/11] extend task comm from 16 to 24 Matthew Wilcox
2021-11-01 13:12 ` Yafang Shao
2021-11-01 14:07 ` Petr Mladek
2021-11-01 14:34 ` Yafang Shao
2021-11-01 16:02 ` Petr Mladek
2021-11-01 16:06 ` Steven Rostedt
2021-11-02 1:09 ` Yafang Shao [this message]
2021-11-02 1:18 ` Steven Rostedt
2021-11-02 1:26 ` Yafang Shao
2021-11-02 7:56 ` Petr Mladek
2021-11-02 13:48 ` Yafang Shao
2021-11-02 9:26 ` David Hildenbrand
2021-11-04 1:37 ` Michał Mirosław
2021-11-05 6:34 ` Yafang Shao
2021-11-05 23:57 ` Michał Mirosław
2021-11-06 9:12 ` Yafang Shao
2021-11-06 11:29 ` Michał Mirosław
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