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Date: Tue, 9 Nov 2021 09:34:39 -0800
From: Kees Cook <keescook@...omium.org>
To: Yafang Shao <laoar.shao@...il.com>
Cc: akpm@...ux-foundation.org, netdev@...r.kernel.org,
bpf@...r.kernel.org, linux-perf-users@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, oliver.sang@...el.com, lkp@...el.com,
Petr Mladek <pmladek@...e.com>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
Andrii Nakryiko <andrii.nakryiko@...il.com>,
Michal Miroslaw <mirq-linux@...e.qmqm.pl>,
Peter Zijlstra <peterz@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
David Hildenbrand <david@...hat.com>,
Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] kthread: dynamically allocate memory to store kthread's
full name
On Mon, Nov 08, 2021 at 08:41:42AM +0000, Yafang Shao wrote:
> When I was implementing a new per-cpu kthread cfs_migration, I found the
> comm of it "cfs_migration/%u" is truncated due to the limitation of
> TASK_COMM_LEN. For example, the comm of the percpu thread on CPU10~19 are
> all with the same name "cfs_migration/1", which will confuse the user. This
> issue is not critical, because we can get the corresponding CPU from the
> task's Cpus_allowed. But for kthreads correspoinding to other hardware
> devices, it is not easy to get the detailed device info from task comm,
> for example,
>
> jbd2/nvme0n1p2-
> xfs-reclaim/sdf
>
> Currently there are so many truncated kthreads:
>
> rcu_tasks_kthre
> rcu_tasks_rude_
> rcu_tasks_trace
> poll_mpt3sas0_s
> ext4-rsv-conver
> xfs-reclaim/sd{a, b, c, ...}
> xfs-blockgc/sd{a, b, c, ...}
> xfs-inodegc/sd{a, b, c, ...}
> audit_send_repl
> ecryptfs-kthrea
> vfio-irqfd-clea
> jbd2/nvme0n1p2-
> ...
>
> We can shorten these names to work around this problem, but it may be
> not applied to all of the truncated kthreads. Take 'jbd2/nvme0n1p2-' for
> example, it is a nice name, and it is not a good idea to shorten it.
>
> One possible way to fix this issue is extending the task comm size, but
> as task->comm is used in lots of places, that may cause some potential
> buffer overflows. Another more conservative approach is introducing a new
> pointer to store kthread's full name if it is truncated, which won't
> introduce too much overhead as it is in the non-critical path. Finally we
> make a dicision to use the second approach. See also the discussions in
> this thread:
> https://lore.kernel.org/lkml/20211101060419.4682-1-laoar.shao@gmail.com/
>
> After this change, the full name of these truncated kthreads will be
> displayed via /proc/[pid]/comm:
>
> rcu_tasks_kthread
> rcu_tasks_rude_kthread
> rcu_tasks_trace_kthread
> poll_mpt3sas0_statu
> ext4-rsv-conversion
> xfs-reclaim/sdf1
> xfs-blockgc/sdf1
> xfs-inodegc/sdf1
> audit_send_reply
> ecryptfs-kthread
> vfio-irqfd-cleanup
> jbd2/nvme0n1p2-8
>
> Suggested-by: Petr Mladek <pmladek@...e.com>
> Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
> Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
> Cc: Michal Miroslaw <mirq-linux@...e.qmqm.pl>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Petr Mladek <pmladek@...e.com>
> ---
> TODO: will cleanup worker comm in the next step.
>
> ---
> fs/proc/array.c | 3 +++
> include/linux/kthread.h | 1 +
> kernel/kthread.c | 32 +++++++++++++++++++++++++++++++-
> 3 files changed, 35 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 49be8c8ef555..860e4deafa65 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -92,6 +92,7 @@
> #include <linux/string_helpers.h>
> #include <linux/user_namespace.h>
> #include <linux/fs_struct.h>
> +#include <linux/kthread.h>
>
> #include <asm/processor.h>
> #include "internal.h"
> @@ -102,6 +103,8 @@ void proc_task_name(struct seq_file *m, struct task_struct *p, bool escape)
>
> if (p->flags & PF_WQ_WORKER)
> wq_worker_comm(tcomm, sizeof(tcomm), p);
> + else if (p->flags & PF_KTHREAD)
> + get_kthread_comm(tcomm, sizeof(tcomm), p);
> else
> __get_task_comm(tcomm, sizeof(tcomm), p);
>
> diff --git a/include/linux/kthread.h b/include/linux/kthread.h
> index 346b0f269161..2a5c04494663 100644
> --- a/include/linux/kthread.h
> +++ b/include/linux/kthread.h
> @@ -33,6 +33,7 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
> unsigned int cpu,
> const char *namefmt);
>
> +void get_kthread_comm(char *buf, size_t buf_size, struct task_struct *tsk);
> void set_kthread_struct(struct task_struct *p);
>
> void kthread_set_per_cpu(struct task_struct *k, int cpu);
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 5b37a8567168..ce8258231eea 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -60,6 +60,8 @@ struct kthread {
> #ifdef CONFIG_BLK_CGROUP
> struct cgroup_subsys_state *blkcg_css;
> #endif
> + /* To store the full name if task comm is truncated. */
> + char *full_name;
> };
>
> enum KTHREAD_BITS {
> @@ -93,6 +95,18 @@ static inline struct kthread *__to_kthread(struct task_struct *p)
> return kthread;
> }
>
> +void get_kthread_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> +{
> + struct kthread *kthread = to_kthread(tsk);
> +
> + if (!kthread || !kthread->full_name) {
> + __get_task_comm(buf, buf_size, tsk);
> + return;
> + }
> +
> + strscpy_pad(buf, kthread->full_name, buf_size);
> +}
> +
> void set_kthread_struct(struct task_struct *p)
> {
> struct kthread *kthread;
> @@ -121,6 +135,7 @@ void free_kthread_struct(struct task_struct *k)
> #ifdef CONFIG_BLK_CGROUP
> WARN_ON_ONCE(kthread && kthread->blkcg_css);
> #endif
> + kfree(kthread->full_name);
> kfree(kthread);
> }
>
> @@ -399,12 +414,27 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
> if (!IS_ERR(task)) {
> static const struct sched_param param = { .sched_priority = 0 };
> char name[TASK_COMM_LEN];
> + va_list aq;
> + int len;
>
> /*
> * task is already visible to other tasks, so updating
> * COMM must be protected.
> */
> - vsnprintf(name, sizeof(name), namefmt, args);
> + va_copy(aq, args);
> + len = vsnprintf(name, sizeof(name), namefmt, aq);
> + va_end(aq);
> + if (len >= TASK_COMM_LEN) {
> + struct kthread *kthread = to_kthread(task);
> + char *full_name;
> +
> + full_name = kvasprintf(GFP_KERNEL, namefmt, args);
> + if (!full_name) {
> + kfree(create);
> + return ERR_PTR(-ENOMEM);
I'm not a fan of this out-of-line free/return. Why not just leave it
truncated when out of memory? For example just do:
struct kthread *kthread = to_kthread(task);
kthread->full_name = kvasprintf(GFP_KERNEL, namefmt, args);
> + }
> + kthread->full_name = full_name;
> + }
> set_task_comm(task, name);
> /*
> * root may have changed our (kthreadd's) priority or CPU mask.
> --
> 2.17.1
>
--
Kees Cook
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