Commit d6986ce2 authored by Yafang Shao's avatar Yafang Shao Committed by Linus Torvalds

kthread: dynamically allocate memory to store kthread's full name

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
all have 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 corresponding 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

Link: https://lkml.kernel.org/r/20211120112850.46047-1-laoar.shao@gmail.comSigned-off-by: default avatarYafang Shao <laoar.shao@gmail.com>
Reviewed-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Suggested-by: default avatarPetr Mladek <pmladek@suse.com>
Suggested-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Michal Miroslaw <mirq-linux@rere.qmqm.pl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 3087c61e
......@@ -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);
......
......@@ -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);
......
......@@ -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;
......@@ -118,9 +132,13 @@ void free_kthread_struct(struct task_struct *k)
* or if kmalloc() in kthread() failed.
*/
kthread = to_kthread(k);
if (!kthread)
return;
#ifdef CONFIG_BLK_CGROUP
WARN_ON_ONCE(kthread && kthread->blkcg_css);
WARN_ON_ONCE(kthread->blkcg_css);
#endif
kfree(kthread->full_name);
kfree(kthread);
}
......@@ -406,12 +424,22 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
task = create->result;
if (!IS_ERR(task)) {
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);
/* leave it truncated when out of memory. */
kthread->full_name = kvasprintf(GFP_KERNEL, namefmt, args);
}
set_task_comm(task, name);
}
kfree(create);
......
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