linux-kernel - Re: Description of open_to_namei

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Message-ID: <20090212032821.GD28946@ZenIV.linux.org.uk>
Date:	Thu, 12 Feb 2009 03:28:21 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Description of open_to_namei_flags()?

On Thu, Feb 12, 2009 at 12:10:58PM +0900, Tetsuo Handa wrote:
> > static inline int open_to_namei_flags(int flag)
> > {
> > 	if ((flag+1) & O_ACCMODE)
> > 		flag++;
> > 	return flag;
> > }
> 
> I noticed that open_to_namei_flags() can't yield
> "00 - no permissions needed" output for "11 - special" input.
> To yield "00 - no permissions needed" output for "11 - special" input,
> I think
> 
>   static inline int open_to_namei_flags(int flag)
>   {
>   	return (flag + 1) & O_ACCMODE;
>   }
> 
> is needed.

No.  Comments are rather misleading; the code is correct.

> sys_open(path, 0) is open for reading.
> sys_open(path, 1) is open for writing.
> sys_open(path, 2) is open for reading and writing.
> What is sys_open(path, 3) for?

open for ioctls only; checks for rw permissions, doesn't allow read or
write calls.  Note that the latter is controlled by ->f_mode, which does
*not* come from open_to_namei_flags() (and is calculated as you suggested).
Results of open_to_namei_flags() are used for permission checks, where
3 -> read|write is correct.
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