linux-kernel - Re: [RFC PATCH 1/3] fs: define new read

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Date:   Thu, 28 Sep 2017 10:33:23 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-integrity@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [RFC PATCH 1/3] fs: define new read_iter rwf flag

On Thu, 2017-09-28 at 06:54 -0700, Matthew Wilcox wrote:
> On Thu, Sep 28, 2017 at 08:39:31AM -0400, Mimi Zohar wrote:
> > Writing extended attributes requires exclusively taking the i_rwsem
> > lock.  To synchronize the file hash calculation and writing the file
> > hash as security.ima xattr, IMA-appraisal takes the i_rwsem lock
> > exclusively before calculating the file hash.  (Once the file hash
> > is calculated, the result is cached.  Taking the lock exclusively
> > prevents calculating the file hash multiple times.)
> > 
> > Some filesystems have recently replaced their filesystem dependent
> > lock with the global i_rwsem to read a file.  As a result, when IMA
> > attempts to calculate the file hash, reading the file attempts to
> > take the i_rwsem again.
> > 
> > To resolve this problem, this patch defines a new read_iter flag
> > named "rwf" to indicate that the i_rwsem has already been taken
> > exclusively.  Subsequent patches will set or test the "rwf" flag.
> 
> I don't like adding a bool parameter everywhere.

Me either!

> Why not add a flag
> to the kiocb ki_flags?
> 
> #define IOCB_RWSEM_HELD		(1 << 8)

Thank you for the suggestion.

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