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Re: [PATCH] tmpfs: add padding to tmpfs_dirent structure


From: Carl Fredrik Hammar
Subject: Re: [PATCH] tmpfs: add padding to tmpfs_dirent structure
Date: Thu, 3 Jun 2010 23:00:23 +0200
User-agent: Mutt/1.5.20 (2009-06-14)

On Thu, May 20, 2010 at 12:18:44AM +0200, Sergio Lopez wrote:
> El Wed, 19 May 2010 23:32:37 +0200
> Samuel Thibault <address@hidden> escribió:
> 
> > Sergio Lopez, le Wed 19 May 2010 17:33:39 +0200, a écrit :
> > > El Wed, 19 May 2010 16:05:27 +0200
> > > Carl Fredrik Hammar <address@hidden> escribió:
> > > 
> > > > On Tue, May 18, 2010 at 04:19:34PM +0200, Sergio Lopez wrote:
> > > > > 
> > > > > This patch adds some padding to tmpfs_dirent structure as it's
> > > > > suggested in the wiki
> > > > > (http://www.gnu.org/software/hurd/hurd/translator/tmpfs.html).
> > > > 
> > > > It seems Samuel beat you to it back in January: see commit 97c569.
> > > 
> > > Humm... I didn't see that. Anyway, I think that change should be
> > > reverted and the assertion in diskfs_get_directs removed.
> > 
> > Err, why removing it?  Isn't there a need for the whole struct dirent
> > structure to be allocated or something like that?  (Which probably was
> > the reason for the assertion).
> 
> diskfs_get_directs needs to allocate a buffer and fill it with dirent
> entries to describe the contents from the requested directory. The size
> of this buffer is dn_stat.st_size plus two dirent structures (to hold
> "." and "..").
> 
> The code seems to assume that dn_stat.st_size represents the size of a
> collection of tmpfs_dirent entries, so it needs to check that this
> structure is the same size or bigger than dirent, to ensure the buffer
> is big enough.
> 
> But, in diskfs_direnter_hard, dn_stat.st_size is actually increased
> with the value of dirent (and not tmpfs_dirent), so there's no need to
> worry about the sizes of those structures.

I looked into the code and came to the same conclusion.  I even checked
if dn_stat.st_size was modified anywhere else, and all the other cases
involved non-directories.

Regards,
  Fredrik



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