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

[Michał Mirosław]

On Fri, Nov 05, 2021 at 02:34:58PM +0800, Yafang Shao wrote:
> On Thu, Nov 4, 2021 at 9:37 AM Michał Mirosław <mirq-linux@xxxxxxxxxxxx> wrote:
> >
> > On Mon, Nov 01, 2021 at 06:04:08AM +0000, Yafang Shao wrote:
> > > There're many truncated kthreads in the kernel, which may make trouble
> > > for the user, for example, the user can't get detailed device
> > > information from the task comm.
> > >
> > > This patchset tries to improve this problem fundamentally by extending
> > > the task comm size from 16 to 24, which is a very simple way.
> > [...]
> >
> > Hi,
> >
> > I've tried something like this a few years back. My attempt got mostly
> > lost in the mailing lists, but I'm still carrying the patches in my
> > tree [1]. My target was userspace thread names, and it turned out more
> > involved than I had time for.
> >
> > [1] https://rere.qmqm.pl/git/?p=linux;a=commit;h=2c3814268caf2b1fee6d1a0b61fd1730ce135d4a
> >     and its parents
> >
> 
> Hi Michal,
> 
> Thanks for the information.
> 
> I have looked through your patches.  It seems to contain six patches
> now and can be divided into three parts per my understanding.
> 
> 1. extend task comm len
> This parts contains below 4 patches:
> [prctl: prepare for bigger
> TASK_COMM_LEN](https://rere.qmqm.pl/git/?p=linux;a=commit;h=cfd99db9cf911bb4d106889aeba1dfe89b6527d0)
> [bluetooth: prepare for bigger
> TASK_COMM_LEN](https://rere.qmqm.pl/git/?p=linux;a=commit;h=ba2805f5196865b81cc6fc938ea53af2c7c2c892)
> [taskstats: prepare for bigger
> TASK_COMM_LEN](https://rere.qmqm.pl/git/?p=linux;a=commit;h=4d29bfedc57b36607915a0171f4864ec504908ca)
> [mm: make TASK_COMM_LEN
> configurable](https://rere.qmqm.pl/git/?p=linux;a=commit;h=362acc35582445174589184c738c4d86ec7d174b)
> 
> What kind of userspace issues makes you extend the task comm length ?
> Why not just use /proc/[pid]/cmdline ?

This was to enable longer thread names (as set by pthread_setname_np()).
Currently its 16 bytes, and that's too short for e.g. Chrome's or Firefox'es
threads. I believe that FreeBSD has 32-byte limit and so I expect that
major portable code is already prepared for bigger thread names.

> 2.  A fix
> Below patch:
> [procfs: signal /proc/PID/comm write
> truncation](https://rere.qmqm.pl/git/?p=linux;a=commit;h=d72027388d4d95db5438a7a574e0a03ae4b5d6d7)
> 
> It seems this patch is incomplete ?   I don't know what it means to do.

Currently writes to /proc/PID/comm are silently truncated. This patch
makes the write() call return the actual number of bytes actually written
and on subsequent calls return -ENOSPC. glibc checks the length in
pthread_setname_np() before write(), so the change is not currently
relevant for it. I don't know/remember what other runtimes do, though.

> 3. A feature provided for pthread_getname_np
> Below patch:
> [procfs: lseek(/proc/PID/comm, 0,
> SEEK_END)](https://rere.qmqm.pl/git/?p=linux;a=commit;h=2c3814268caf2b1fee6d1a0b61fd1730ce135d4a)
> 
> It seems this patch is useful. With this patch the userspace can
> directly get the TASK_COMM_LEN through the API.

This one I'm not really fond of because it abuses lseek() in that it
doesn't move the write pointer. But in case of /proc files this normally
would return EINVAL anyway.

Best Regards
Michał Mirosław