linux-kernel - Re: [BUG] drivers/tty: read() on a noncanonical blocking tty randomly fails when VMIN

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Date:	Wed, 4 May 2016 16:07:44 -0700
From:	Peter Hurley <peter@...leysoftware.com>
To:	Julio Guerra <julio@...jump.io>,
	"Peter Hurley <peter@...leysoftware.com>; Greg Kroah-Hartman" 
	<gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [BUG] drivers/tty: read() on a noncanonical blocking tty randomly
 fails when VMIN > received >= buf

Hi Julio,

On 05/04/2016 04:00 PM, Julio Guerra wrote:
> Hi,
> 
> When a tty (here a slave pty) is set in noncanonical input and blocking read modes, a read() randomly blocks when:
> "VMIN > kernel received >= user buffer size > 0".
> 
> The standard says that read() should block until VMIN bytes are received [1][2]. Whether this is an implementation defined case not really specified by POSIX or not, it should not behave randomly (otherwise it really should be documented in termios manpage).

This is not a bug.

>From the termios(3) man page:

       * MIN > 0; TIME == 0: read(2) blocks until the lesser of MIN bytes or the number of bytes requested are avail‐
         able, and returns the lesser of these two values.

Regards,
Peter Hurley


> I isolated it in the following example (with VMIN = 5, received = 4, user buffer = 3):
> https://gist.github.com/Julio-Guerra/b3fdefab281403073607d81cabcea04a
> 
> Since it is random, you will need run it several times to observe both cases. When correctly behaving, it should block in the read() for ever (C-c to kill it). When incorrectly behaving, it does not block and reads into the user buffer what is present in the kernel receive buffer (string "any").
> 
> Example where it does not block 3 times out of 4:
>  > $ ./a.out
>  > res=3 buf=any
>  > $ ./a.out
>  > res=3 buf=any
>  > $ ./a.out
>  > ^C⏎
> 
> Linux version 4.5.1-1-ARCH (builduser@...ias) (gcc version 5.3.0 (GCC) ) #1 SMP PREEMPT Thu Apr 14 19:19:32 CEST 2016
> 
> [1] "Canonical and noncanonical mode", man termios
> [2] http://www.gnu.org/software/libc/manual/html_node/Noncanonical-Input.html
>