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

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Date:   Fri, 13 Mar 2020 11:06:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bruno Meneguele' <bmeneg@...hat.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "pmladek@...e.com" <pmladek@...e.com>,
        "sergey.senozhatsky@...il.com" <sergey.senozhatsky@...il.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>
Subject: RE: [PATCH] kernel/printk: add kmsg SEEK_CUR handling

From: Bruno Meneguele
> Sent: 13 March 2020 11:02
> On Fri, Mar 13, 2020 at 04:34:25PM +0900, Sergey Senozhatsky wrote:
> > On (20/03/12 21:35), Bruno Meneguele wrote:
> > >
> > > Userspace libraries, e.g. glibc's dprintf(), expect the default return value
> > > for invalid seek situations: -ESPIPE, but when the IO was over /dev/kmsg the
> > > current state of kernel code was returning the generic case of an -EINVAL.
> > > Hence, userspace programs were not behaving as expected or documented.
> > >
> >
> > Hmm. I don't think I see ESPIPE in documentation [0], [1], [2]
> >
> > [0] https://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html
> > [1] http://man7.org/linux/man-pages/man3/dprintf.3p.html
> > [2] http://man7.org/linux/man-pages/man3/fprintf.3p.html
> >
> > 	-ss
> >
> 
> 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;

Someone explain why it is doing an explicit seek to the current position?
The only reason to do that is to get the current offset.

	David

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