linux-kernel - Re: Question about TTY_DO_WRITE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Pine.LNX.4.64.0811201700320.1274@lxtpfaff.pcs.ditec.de>
Date:	Thu, 20 Nov 2008 17:39:38 +0100 (CET)
From:	Thomas Pfaff <tpfaff@....com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	linux-kernel@...r.kernel.org
Subject: Re: Question about TTY_DO_WRITE_WAKEUP

On Thu, 20 Nov 2008, Alan Cox wrote:

> > But n_tty_write_wakeup is only called from tty_wakeup when this bit is already 
> > set, therefore it makes no sense to set this bit in n_tty_write_wakeup again.
> 
> The base code should probably really use test_and_clear_bit() when
> calling that method.

Why should you test it, clear it and set it again in n_tty_write_wakeup ?

> > 
> > The flow looks to me as
> > 
> > If the tty driver sets TTY_DO_WRITE_WAKEUP a SIGIO is generated on every 
> > tty_wakeup.
> > 
> > If it is not set then n_tty_write_wakeup is never called and a SIGIO is not 
> > generated.
> 
> Which isn't perfect (excess SIGIO cases) but doesn't seem incorrect. If
> you've not blocked the tty output buffer then write() has not returned a
> short write and no SIGIO is due.
> 

Of course this is not incorrect, but this does not solve my problem with the 
TTY_DO_WRITE_WAKEUP flag.

IMHO a SIGIO on write possible should always be generated if the user wants it, 
currently it is generated when the user wants it and the tty driver enables the 
TTY_DO_WRITE_WAKEUP flag. Unfortunately most drivers don't set it.

Regarding excess SIGIO cases:

Once a write fails with EAGAIN a flag can be set and only in that case a SIGIO is 
generated, afterwards the bit is cleared. Maybe that is what TTY_DO_WRITE_WAKEUP 
was intended for. 

Thomas
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/