Re: [PATCH v7 00/11] extend task comm from 16 to 24

[Yafang Shao]

On Tue, Nov 2, 2021 at 12:02 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> On Mon 2021-11-01 22:34:30, Yafang Shao wrote:
> > On Mon, Nov 1, 2021 at 10:07 PM Petr Mladek <pmladek@xxxxxxxx> wrote:
> > > On Mon 2021-11-01 06:04:08, Yafang Shao wrote:
> > > > 4. Print a warning if the kthread comm is still truncated.
> > > >
> > > > 5. What will happen to the out-of-tree tools after this change?
> > > >    If the tool get task comm through kernel API, for example prctl(2),
> > > >    bpf_get_current_comm() and etc, then it doesn't matter how large the
> > > >    user buffer is, because it will always get a string with a nul
> > > >    terminator. While if it gets the task comm through direct string copy,
> > > >    the user tool must make sure the copied string has a nul terminator
> > > >    itself. As TASK_COMM_LEN is not exposed to userspace, there's no
> > > >    reason that it must require a fixed-size task comm.
> > >
> > > The amount of code that has to be updated is really high. I am pretty
> > > sure that there are more potential buffer overflows left.
> > >
> > > You did not commented on the concerns in the thread
> > > https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@xxxxxxxxxxxxxx/
> > >
> > I thought Steven[1] and  Kees[2] have already clearly explained why we
> > do it like that, so I didn't give any more words on it.
> >
> > [1]. https://lore.kernel.org/all/20211025170503.59830a43@xxxxxxxxxxxxxxxxxx/
>
> Steven was against switching task->comm[16] into a dynamically
> allocated pointer. But he was not against storing longer names
> separately.
>
> > [2]. https://lore.kernel.org/all/202110251406.56F87A3522@keescook/
>
> Honestly, I am a bit confused by Kees' answer. IMHO, he agreed that
> switching task->comm[16] into a pointer was not worth it.
>
> But I am not sure what he meant by "Agreed -- this is a small change
> for what is already an "uncommon" corner case."
>
>
> > > Several people suggested to use a more conservative approach.
> >
> > Yes, they are Al[3] and Alexei[4].
> >
> > [3]. https://lore.kernel.org/lkml/YVkmaSUxbg%2FJtBHb@xxxxxxxxxxxxxxxxxxxxx/
>
> IMHO, Al suggested to store the long name separately and return it
> by proc_task_name() when available.
>
>
> > [4]. https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@xxxxxxxxxxxxxx/
>
> Alexei used dentry->d_iname as an exaxmple. struct dentry uses
> d_iname[DNAME_INLINE_LEN] for short names. And dynamically
> allocated d_name for long names, see *__d_alloc() implementation.
>

Thanks for the summary.
So with Stenven's new reply[1], the opinion in common is storing long
names into a separate place. And no one is against it now.

[1]. https://lore.kernel.org/lkml/20211101120636.3cfc5afa@xxxxxxxxxxxxxxxxxx/

> > > I mean
> > > to keep comm[16] as is and add a new pointer to the full name. The buffer
> > > for the long name might be dynamically allocated only when needed.
> > >
> >
> > That would add a new allocation in the fork() for the threads with a long name.
> > I'm not sure if it is worth it.
>
> The allocation will be done only when needed. IMHO, the performance is
> important only for userspace processes. I am not aware of any kernel
> subsystem that would heavily create and destroy kthreads.
>

XFS may create many kthreads with longer names, especially if there're
many partitions in the disk.
For example,
    xfs-reclaim/sd{a, b, c, ...}
    xfs-blockgc/sd{a, b, c, ...}
    xfs-inodegc/sd{a, b, c, ...}

They are supposed to be created at boot time, and shouldn't be heavily
created and destroyed.

>
> > > The pointer might be either in task_struct or struct kthread. It might
> > > be used the same way as the full name stored by workqueue kthreads.
> > >
> >
> > If we decide to do it like that, I think we'd better add it in
> > task_struct, then it will work for all tasks.
>
> Is it really needed for userspace processes? For example, ps shows
> the information from /proc/*/cmdline instead.
>

Right. The userspace processes can be obtained from /proc/*/cmdline.

>
> > > The advantage of the separate pointer:
> > >
> > >    + would work for names longer than 32
> > >    + will not open security holes in code
> > >
> >
> > Yes, those are the advantages.  And the disadvantage of it is:
> >
> >  - new allocation in fork()
>
> It should not be a problem if we do it only when necessary and only
> for kthreads.
>

So if no one against, I will do it in two steps,

1. Send the task comm cleanups in a separate patchset named "task comm cleanups"
    This patchset includes patch #1, #2, #4,  #5, #6, #7 and #9.
    Cleaning them up can make it less error prone, and it will be
helpful if we want to extend task comm in the future :)

2.  Keep the current comm[16] as-is and introduce a separate pointer
to store kthread's long name
     Now we only care about kthread, so we can put the pointer into a
kthread specific struct.
     For example in the struct kthread, or in kthread->data (which may
conflict with workqueue).

     And then dynamically allocate a longer name if it is truncated,
for example,
     __kthread_create_on_node
         len = vsnprintf(name, sizeof(name), namefmt, args);
         if (len >= TASK_COMM_LEN) {
             /* create a longer name */
         }

     And then we modify proc_task_name(), so the user can get
kthread's longer name via /proc/[pid]/comm.

     And then free the allocated memory when the kthread is destroyed.

--
Thanks
Yafang