linux-kernel - Re: RFC: reject unknown open flags

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Date:   Thu, 30 Mar 2017 23:05:59 +0300
From:   Boaz Harrosh <boaz@...xistor.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        libc-alpha <libc-alpha@...rceware.org>
Subject: Re: RFC: reject unknown open flags

On 03/30/2017 09:45 PM, Linus Torvalds wrote:
> On Thu, Mar 30, 2017 at 11:26 AM, Christoph Hellwig <hch@....de> wrote:
>>
>> That would be nice, but still won't work as we blindly copy f_flags
>> into F_GETFL, not even masking our internal FMODE_ bits.
> 
> Ok, *that* is just silly of us, and we could try to just fix, and even backport.
> 
> There's no possible valid use I could see where that should break
> (famous last words - user code does some damn odd things at times).
> 
> Of course, that won't fix old kernels that are out there, but then
> neither would your original patch...
> 
> Side note: I think you *can* detect the O_ATOMIC support by using
> F_SETFL, because F_SETFL only allows you to change flags that we
> recognize. So somebody who really wants to *guarantee* that O_ATOMIC
> is there and honored even with old kernels could presumable do
> something like
> 
>    fd = open(..); // *no* O_ATOMIC
>    fcnt(fd, F_SETFL, O_ATOMIC);
>    if (fcnt(fd, F_GETFL, NULL) & O_ATOMIC)
>         // Yay! We actually got it
>    else
>         // I guess we need to fall back on old behavior
> 
> although I agree that that is ridiculously inconvenient and not a
> great thing, and it's worth trying to aim for some better model.
> 

Perhaps in that case it is time for an F_GETFL2 an F_GET_REAL_FL
that gives you the nice simple user code Linus wanted for new applications.
and solves forward and backwords for applications and Kernels?

Just my $0.017
Boaz

>                     Linus
>