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.com
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Suggested-by: Petr Mladek <pmladek@suse.com>
Suggested-by: Steven 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/proc/array.c b/fs/proc/array.c
index ff869a6..4321aa6 100644 (file)
--- 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 346b0f2..2a5c044 100644 (file)
--- 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 7113003..a70cd5d 100644 (file)
--- 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);