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

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Message-ID: <20200313130634.GJ13406@glitch>
Date:   Fri, 13 Mar 2020 10:06:34 -0300
From:   Bruno Meneguele <bmeneg@...hat.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        "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

On Fri, Mar 13, 2020 at 11:06:42AM +0000, David Laight wrote:
> 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
> 

dprintf gets a fd as input and convert it to a FILE structure, with that
it can't garantuee the previous state of that fd: was it already
manipulated? Thus they check the current position to make sure it's not
junk.

But that's me guessing things about a code from 1996 :).

-- 
bmeneg 
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