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@xxxxxxxxx/
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@xxxxxxxx>
Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Cc: Michal Miroslaw <mirq-linux@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
---
Changes since v1:
1. leave it turncated when out of memory (Kees & Petr)
2. do null check in free_kthread_struct (Petr)
---
fs/proc/array.c | 3 +++
include/linux/kthread.h | 1 +
kernel/kthread.c | 32 ++++++++++++++++++++++++++++++--
3 files changed, 34 insertions(+), 2 deletions(-)
diff --git a/fs/proc/array.c b/fs/proc/array.c
index ff869a66b34e..4321aa63835d 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 7113003fab63..a70cd5dc94e3 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;
@@ -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);
--
2.17.1
