Bug#1002711: linux-image-5.14.0-4-amd64: poll syscall does not work on some /proc files

[Salvatore Bonaccorso <carnil@debian.org>]

Hi,

On Thu, Dec 30, 2021 at 07:19:06PM +0100, Vincent Danjean wrote:
> On 30/12/2021 14:52, Bastian Blank wrote:
> > On Mon, Dec 27, 2021 at 11:31:21PM +0100, Vincent Danjean wrote:
> > > On a plain (with more than two bytes) file, the second poll succeed.
> > > On /proc/bus/input/devices, the second poll hangs.
> > > Note: this is an old behavior. I initially observe it on an embeded system with
> > > a 4.1 kernel.
> > 
> > /proc is no real filesystem.  It simply does not support poll, because
> > the output is generated while you are reading it, so it does not know
> > when anything changes.  See also
> > https://unix.stackexchange.com/questions/74713/how-frequently-is-the-proc-file-system-updated-on-linux?rq=1
> > 
> > If you want to know about hardware changes, use udev or listen to kernel
> > netlink messages.
> 
> I understand that /proc is different. But I'm not monitoring it.
> My goal was only to read it totally from a busybox shell with a
> "while read" loop to find the eventX associated with the
> touchscreen of the e-reader.
> And the first read from busybox shell never complete (instead of
> returning the first line of the file) due to the fact that
> busybox use poll, then read a byte, then use poll again.
> 
> 
>   Do you think I should reassign this bug to busybox?
> 
>   Or do you think I'm wrong trying to read the /proc file
> from a shell script without to copy it elsewhere before?
> 
> Stalling:
> busybox sh -c 'while read l ; do echo $l ; done  < /proc/bus/input/devices'
> Working (but bash is not present on the initial target system):
> bash -c 'while read l ; do echo $l ; done  < /proc/bus/input/devices'
> 
>   As workaround, I'm currently using a copy of the file
> ("busybox cp" works). It should probably also be possible
> to get the same kind of info from /sys files but this seems
> less easy.
> 
>   I just saw that other files seems to work:
> Ok:
> busybox sh -c 'read l < /proc/bus/input/handlers ; echo $l'
> Stale:
> busybox sh -c 'read l < /proc/bus/input/devices ; echo $l'
> 
>   Regards,
>     Vincent

Do we still consider this a valid src:linux bug or should it be
closed, reassigned?

Regards,
Salvatore