diff mbox series

[v6,12/12] kernel/kthread: show a warning if kthread's comm is truncated

Message ID 20211025083315.4752-13-laoar.shao@gmail.com (mailing list archive)
State Superseded
Headers show
Series extend task comm from 16 to 24 | expand

Checks

Context Check Description
netdev/tree_selection success Not a local patch
bpf/vmtest-bpf-next success VM_Test
bpf/vmtest-bpf-next-PR success PR summary
bpf/vmtest-bpf success VM_Test
bpf/vmtest-bpf-PR success PR summary

Commit Message

Yafang Shao Oct. 25, 2021, 8:33 a.m. UTC
Show a warning if task comm is truncated. Below is the result
of my test case:

truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <keescook@chromium.org>
Cc: Petr Mladek <pmladek@suse.com>
---
 kernel/kthread.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Comments

Kees Cook Oct. 25, 2021, 9:35 p.m. UTC | #1
On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:
> Show a warning if task comm is truncated. Below is the result
> of my test case:
> 
> truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters
> 
> Suggested-by: Petr Mladek <pmladek@suse.com>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Petr Mladek <pmladek@suse.com>
> ---
>  kernel/kthread.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 5b37a8567168..46b924c92078 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -399,12 +399,17 @@ 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];
> +		int len;
>  
>  		/*
>  		 * task is already visible to other tasks, so updating
>  		 * COMM must be protected.
>  		 */
> -		vsnprintf(name, sizeof(name), namefmt, args);
> +		len = vsnprintf(name, sizeof(name), namefmt, args);
> +		if (len >= TASK_COMM_LEN) {

And since this failure case is slow-path, we could improve the warning
as other had kind of suggested earlier with something like this instead:

			char *full_comm;

			full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
			pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
				full_comm, name);

			kfree(full_comm);
		}
>  		set_task_comm(task, name);
>  		/*
>  		 * root may have changed our (kthreadd's) priority or CPU mask.
> -- 
> 2.17.1
>
Yafang Shao Oct. 26, 2021, 2:23 a.m. UTC | #2
On Tue, Oct 26, 2021 at 5:35 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:
> > Show a warning if task comm is truncated. Below is the result
> > of my test case:
> >
> > truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters
> >
> > Suggested-by: Petr Mladek <pmladek@suse.com>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Petr Mladek <pmladek@suse.com>
> > ---
> >  kernel/kthread.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index 5b37a8567168..46b924c92078 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -399,12 +399,17 @@ 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];
> > +             int len;
> >
> >               /*
> >                * task is already visible to other tasks, so updating
> >                * COMM must be protected.
> >                */
> > -             vsnprintf(name, sizeof(name), namefmt, args);
> > +             len = vsnprintf(name, sizeof(name), namefmt, args);
> > +             if (len >= TASK_COMM_LEN) {
>
> And since this failure case is slow-path, we could improve the warning
> as other had kind of suggested earlier with something like this instead:
>

It Makes sense to me.  I will do it as you suggested.

>                         char *full_comm;
>
>                         full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
>                         pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
>                                 full_comm, name);
>
>                         kfree(full_comm);
>                 }
> >               set_task_comm(task, name);
> >               /*
> >                * root may have changed our (kthreadd's) priority or CPU mask.
> > --
> > 2.17.1
> >
>
> --
> Kees Cook
Petr Mladek Oct. 27, 2021, 8:10 p.m. UTC | #3
On Mon 2021-10-25 14:35:42, Kees Cook wrote:
> On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:
> > Show a warning if task comm is truncated. Below is the result
> > of my test case:
> > 
> > truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters
> > 
> > Suggested-by: Petr Mladek <pmladek@suse.com>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Petr Mladek <pmladek@suse.com>
> > ---
> >  kernel/kthread.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index 5b37a8567168..46b924c92078 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -399,12 +399,17 @@ 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];
> > +		int len;
> >  
> >  		/*
> >  		 * task is already visible to other tasks, so updating
> >  		 * COMM must be protected.
> >  		 */
> > -		vsnprintf(name, sizeof(name), namefmt, args);
> > +		len = vsnprintf(name, sizeof(name), namefmt, args);
> > +		if (len >= TASK_COMM_LEN) {
> 
> And since this failure case is slow-path, we could improve the warning
> as other had kind of suggested earlier with something like this instead:
> 
> 			char *full_comm;
> 
> 			full_comm = kvasprintf(GFP_KERNEL, namefmt, args);

You need to use va_copy()/va_end() if you want to use the same va_args
twice.

For example, see how kvasprintf() is implemented. It calls
vsnprintf() twice and it uses va_copy()/va_end() around the the first call.

kvasprintf() could also return NULL if there is not enough memory.

> 			pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
> 				full_comm, name);

BTW: Is this message printed during normal boot? I did not tried the
patchset myself.

We should add this warning only if there is a good solution how to
avoid the truncated names. And we should me sure that the most common
kthreads/workqueues do not trigger it. It would be ugly to print many
warnings during boot if people could not get rid of them easily.

> 			kfree(full_comm);
> 		}
> >  		set_task_comm(task, name);
> >  		/*
> >  		 * root may have changed our (kthreadd's) priority or CPU mask.

Best Regards,
Petr
Yafang Shao Oct. 28, 2021, 1:42 a.m. UTC | #4
On Thu, Oct 28, 2021 at 4:10 AM Petr Mladek <pmladek@suse.com> wrote:
>
> On Mon 2021-10-25 14:35:42, Kees Cook wrote:
> > On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:
> > > Show a warning if task comm is truncated. Below is the result
> > > of my test case:
> > >
> > > truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters
> > >
> > > Suggested-by: Petr Mladek <pmladek@suse.com>
> > > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> > > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: Petr Mladek <pmladek@suse.com>
> > > ---
> > >  kernel/kthread.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > > index 5b37a8567168..46b924c92078 100644
> > > --- a/kernel/kthread.c
> > > +++ b/kernel/kthread.c
> > > @@ -399,12 +399,17 @@ 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];
> > > +           int len;
> > >
> > >             /*
> > >              * task is already visible to other tasks, so updating
> > >              * COMM must be protected.
> > >              */
> > > -           vsnprintf(name, sizeof(name), namefmt, args);
> > > +           len = vsnprintf(name, sizeof(name), namefmt, args);
> > > +           if (len >= TASK_COMM_LEN) {
> >
> > And since this failure case is slow-path, we could improve the warning
> > as other had kind of suggested earlier with something like this instead:
> >
> >                       char *full_comm;
> >
> >                       full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
>
> You need to use va_copy()/va_end() if you want to use the same va_args
> twice.
>
> For example, see how kvasprintf() is implemented. It calls
> vsnprintf() twice and it uses va_copy()/va_end() around the the first call.
>

Does it mean that if we want to call vsnprintf() three times, we must
use va_copy()/va_end() around the first call and the second call ?
IOW, if we call vsnprintf() multiple times, all the calls except the
last call should be protected by va_copy()/va_end().
Actually I don't quite understand why we should do it like this. I
will try to understand it, and appreciate it if you could explain it
in detail.

BTW,  can we use va_copy()/va_end() in vsnprintf(), then the caller
doesn't need to care how many times it will call vsnprintf().

> kvasprintf() could also return NULL if there is not enough memory.

Right. We need to do the NULL check.

>
> >                       pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
> >                               full_comm, name);
>
> BTW: Is this message printed during normal boot? I did not tried the
> patchset myself.
>

Yes, it will be printed at boot time.

> We should add this warning only if there is a good solution how to
> avoid the truncated names. And we should me sure that the most common
> kthreads/workqueues do not trigger it. It would be ugly to print many
> warnings during boot if people could not get rid of them easily.
>

As we have extended task comm to 24, there's no such warning printed
for the existing kthreads/workqueues.
IOW, it will only print for the newly introduced one if it has a long name.
That means this printing is under control.

> >                       kfree(full_comm);
> >               }
> > >             set_task_comm(task, name);
> > >             /*
> > >              * root may have changed our (kthreadd's) priority or CPU mask.
>
> Best Regards,
> Petr
Yafang Shao Oct. 29, 2021, 7:44 a.m. UTC | #5
On Thu, Oct 28, 2021 at 4:10 AM Petr Mladek <pmladek@suse.com> wrote:
>
> On Mon 2021-10-25 14:35:42, Kees Cook wrote:
> > On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:
> > > Show a warning if task comm is truncated. Below is the result
> > > of my test case:
> > >
> > > truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters
> > >
> > > Suggested-by: Petr Mladek <pmladek@suse.com>
> > > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > > Reviewed-by: Kees Cook <keescook@chromium.org>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> > > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: Petr Mladek <pmladek@suse.com>
> > > ---
> > >  kernel/kthread.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > > index 5b37a8567168..46b924c92078 100644
> > > --- a/kernel/kthread.c
> > > +++ b/kernel/kthread.c
> > > @@ -399,12 +399,17 @@ 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];
> > > +           int len;
> > >
> > >             /*
> > >              * task is already visible to other tasks, so updating
> > >              * COMM must be protected.
> > >              */
> > > -           vsnprintf(name, sizeof(name), namefmt, args);
> > > +           len = vsnprintf(name, sizeof(name), namefmt, args);
> > > +           if (len >= TASK_COMM_LEN) {
> >
> > And since this failure case is slow-path, we could improve the warning
> > as other had kind of suggested earlier with something like this instead:
> >
> >                       char *full_comm;
> >
> >                       full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
>
> You need to use va_copy()/va_end() if you want to use the same va_args
> twice.
>

Now I understand it.
So the patch will be:

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 5b37a8567168..c1ff67283725 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -399,12 +399,29 @@ 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];
+               char *full_comm;
+               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) {
+                       full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
+                       if (full_comm) {
+                               pr_warn("truncated kthread comm '%s'
to '%s' (pid:%d)\n",
+                                       full_comm, name, task->pid);
+                               kfree(full_comm);
+                       } else {
+                               pr_warn("truncated kthread comm '%s'
(pid:%d) by %d characters\n",
+                                       name, task->pid, len -
TASK_COMM_LEN + 1);
+
+                       }
+               }
                set_task_comm(task, name);
                /*
                 * root may have changed our (kthreadd's) priority or CPU mask.

That seems a little overkill to me.
I prefer to keep the v6 as-is.

> For example, see how kvasprintf() is implemented. It calls
> vsnprintf() twice and it uses va_copy()/va_end() around the the first call.
>
> kvasprintf() could also return NULL if there is not enough memory.
>
> >                       pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
> >                               full_comm, name);
>
> BTW: Is this message printed during normal boot? I did not tried the
> patchset myself.
>
> We should add this warning only if there is a good solution how to
> avoid the truncated names. And we should me sure that the most common
> kthreads/workqueues do not trigger it. It would be ugly to print many
> warnings during boot if people could not get rid of them easily.
>
> >                       kfree(full_comm);
> >               }
> > >             set_task_comm(task, name);
> > >             /*
> > >              * root may have changed our (kthreadd's) priority or CPU mask.
>
> Best Regards,
> Petr
diff mbox series

Patch

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 5b37a8567168..46b924c92078 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -399,12 +399,17 @@  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];
+		int len;
 
 		/*
 		 * task is already visible to other tasks, so updating
 		 * COMM must be protected.
 		 */
-		vsnprintf(name, sizeof(name), namefmt, args);
+		len = vsnprintf(name, sizeof(name), namefmt, args);
+		if (len >= TASK_COMM_LEN) {
+			pr_warn("truncated kthread comm:%s, pid:%d by %d characters\n",
+				name, task->pid, len - TASK_COMM_LEN + 1);
+		}
 		set_task_comm(task, name);
 		/*
 		 * root may have changed our (kthreadd's) priority or CPU mask.