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[Xen-devel] Re: [PATCH]: xl: introduce xlu_cfg_get_(string|long)_default

To: Gianni Tedesco <gianni.tedesco@xxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH]: xl: introduce xlu_cfg_get_(string|long)_default
From: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
Date: Tue, 31 Aug 2010 17:50:17 +0100
Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
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Gianni Tedesco writes ("[PATCH]: xl: introduce 
> The function init_dm_info() in xl_cmdimpl.c is used to initialise a
> libxl_device_model_info structure with some default values. After being
> called, some of those values are overridden. For the string values which
> are not overwritten we either end up with a double free or attempt to
> free a string literal resulting in a segfault. This type of usage model
> would be complex to fix by adding strdup()'s in the init function and
> then checking and freeing when over-writing.
> My proposed solution is to add default versions of xlu_cfg_get_string
> and xlu_cfg_get_long. This way both double-free's and leaks are avoided
> and arguably the intention of the code is clearer.

I think in general the approach of trying to do the defaults one
config setting at a time like this is insufficiently powerful.

If we're going to change it, it would be better to do all the default
setting at the end, after the config file has been completely read.
That means that the default for one config setting can depend on the
actual values for another.

It also gets rid of the memory management problem, because defaults
never need to be applied to anything that is a non-null pointer.


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