linux-kernel - Re: [PATCH] kernel/printk: add kmsg SEEK

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Date:   Tue, 17 Mar 2020 05:53:58 -0300
From:   Bruno Meneguele <bmeneg@...hat.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        pmladek@...e.com, sergey.senozhatsky@...il.com, rostedt@...dmis.org
Subject: Re: [PATCH] kernel/printk: add kmsg SEEK_CUR handling

On Tue, Mar 17, 2020 at 11:03:26AM +0900, Sergey Senozhatsky wrote:
> On (20/03/13 08:02), Bruno Meneguele wrote:
> > Ok, I poorly expressed the notion of "documentantion". The userspace
> > doesn't tell about returning -ESPIPE, but to the functions work properly
> > they watch for -ESPIPE returning from the syscall. For instance, gblic
> > dprintf() implementation:
> > 
> > dprintf:
> >   __vdprintf_internal:
> >     _IO_new_file_attach:
> > 
> >   if (_IO_SEEKOFF (fp, (off64_t)0, _IO_seek_cur, _IOS_INPUT|_IOS_OUTPUT)
> >       == _IO_pos_BAD && errno != ESPIPE)
> >     return NULL;
> > 
> > With that, if the seek fails, but return anything other than ESPIPE the
> > dprintf() will also fail returning -EINVAL to dprintf() caller. While if
> > ESPIPE is returned, it's "ignored" and the call still works. The way we
> > have today make kmsg an exception case among the rest of the system
> > files where you can open with dprintf.
> > 
> > One of the things I could agree with is removing the SEEK call from
> > dprintf, since fprintf basically follows the same steps, but doesn't
> > seek anything.  But at the same time, IMO it makes sense to make kmsg
> > interface complaint with the errno return values.
> 
> The code in questions is very old. So let's add the missing bit to the
> kernel. At the same time, we probably can have a slightly more detailed
> documentation / code comment.
> 
> 	-ss
> 

Sure thing.

I'm going to send a v2 of this patch with more details still today or
tomorrow.

Thanks!

-- 
bmeneg 
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