Re: [Qemu-devel] [PATCH 1/2] block: make bdrv_find_backing_image compare canonical filenames

[Jeff Cody]

On 10/10/2012 02:34 PM, Eric Blake wrote:
> On 10/10/2012 12:29 PM, Jeff Cody wrote:
> 
>>> That's a LOT of stack space, which risks stack overflow, will mostly be
>>> unused, and still doesn't work if you have super-deep hierarchies larger
>>> than PATH_MAX.  Would you be better off using realpath(,NULL) for its
>>> allocating semantics, and then free()ing the results?
>>>
>>
>> That is the main reason I changed it from being a recursive function, to
>> an iterative one.
>>
>> Do we know that realpath(,NULL) behaves the same on all platforms?
> 
> Gnulib lists the following platforms as mis-handling NULL:
> Mac OS X 10.5, FreeBSD 6.4, OpenBSD 4.4, Solaris 10.
>
>>
>> We had a thread back in April that touched on the use of realpath, and
>> concerns were raised then that realpath(,NULL) was not necessarily safe
>> across all OSes:
>>
>> https://lists.gnu.org/archive/html/qemu-devel/2012-04/msg01417.html
> 
> In fact, that message points out an even more insidious portability bug
> in your algorithm: on Solaris 10, realpath("relative", buffer) leaves
> buffer containing "relative" rather than an absolute name, but your
> algorithm depends on matching absolute names.  I don't know if we port
> qemu to Solaris 10, but it's worth considering my question back in that
> thread - does glib provide us a more portable function for converting a
> relative name into a canonical path that is guaranteed to work everywhere?
> 

We are already relying on realpath() in block.c.  So rather than making
this patch series much more complicated for a minor fix, I think that if
there are concerns about realpath() on other OSes such as Solaris 10,
then those concerns should be addressed in a separate patch series.

>>
>> That said, if there is concern over the stack usage, to be safe I can
>> manually g_malloc() each array.
> 
> g_malloc() would solve the stack size concern, but not the Solaris 10
> relative name bug.
>