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  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]
Date:   Wed, 15 Jan 2020 09:38:54 +0100
From:   Christoph Hellwig <>
To:     David Howells <>
Subject: Re: Problems with determining data presence by examining extents?

On Tue, Jan 14, 2020 at 04:48:29PM +0000, David Howells wrote:
> Again with regard to my rewrite of fscache and cachefiles:
> I've got rid of my use of bmap()!  Hooray!
> However, I'm informed that I can't trust the extent map of a backing file to
> tell me accurately whether content exists in a file because:
>  (a) Not-quite-contiguous extents may be joined by insertion of blocks of
>      zeros by the filesystem optimising itself.  This would give me a false
>      positive when trying to detect the presence of data.
>  (b) Blocks of zeros that I write into the file may get punched out by
>      filesystem optimisation since a read back would be expected to read zeros
>      there anyway, provided it's below the EOF.  This would give me a false
>      negative.

The whole idea of an out of band interface is going to be racy and suffer
from implementation loss.  I think what you want is something similar to
the NFSv4.2 READ_PLUS operation - give me that if there is any and
otherwise tell me that there is a hole.  I think this could be a new
RWF_NOHOLE or similar flag, just how to return the hole size would be
a little awkward.  Maybe return a specific negative error code (ENODATA?)
and advance the iov anyway.

Powered by blists - more mailing lists