bug#38045: IceCat: some codecs don't work without workaround

[Mark H Weaver <mhw@netris.org>]

Hi Jakub,

Jakub Kądziołka <kuba@kadziolka.net> wrote:
> I had some problems with video codecs in IceCat 68.3.0-guix0-preview1.
> For example, consider this page: http://demo.nimius.net/video_test/. By
> default, the videos under the headings H.264 / AAC and MPEG4 don't work
> ("No video with supported format and MIME type found.").
> 
> The following steps make the first of these videos work:
> 1. Open about:config
> 2. Click "I accept the risk!"
> 3. Set security.sandbox.content.read_path_whitelist to /gnu/store/
>    (the trailing / is important).
> 
> The instructions were originally sketched out in this help-guix
> message:
> https://lists.gnu.org/archive/html/help-guix/2019-12/msg00150.html
> 
> I believe it would be beneficial to make this a default.
> 
> On IRC, bandali suggested that it would be better to only whitelist the
> necessary store subdirectories. I don't know how to gather such a list,
> but it it seems like a good idea.

Thank you for bringing this to my attention.  I agree with Amin Bandali
that a more precise whitelist is preferable.  Moreover, I was not
comfortable whitelisting all of /gnu/store.

I'm glad to report that it appears to be sufficient to whitelist the
RUNPATH of libavcodec.so, plus the /share/mime/ directory from
shared-mime-info.  I've implemented this in commit
429c8284d232c3f9fbe3dc87a3da323f3a864c03 and pushed it to 'master'.

> I don't know how about:config entries modified by the user behave when
> IceCat is updated, but in some of the behaviors I can imagine, the
> config entry stops updating,

As currently implemented, we now arrange to set the *default* value of
'security.sandbox.content.read_path_whitelist' to an appropriate
whitelist.

Users who have customized 'security.sandbox.content.read_path_whitelist'
to work around this issue should now erase that customization, by
right-clicking on its entry in <about:config>, and clicking on "Reset".
It might also be necessary to restart IceCat after doing so.

> in which case it would be better to add the paths to some internal
> whitelist (I reckon such a whitelist already exists and contains
> something like /usr/lib).

I agree that it would be preferable, but I wasn't sufficiently motivated
to implement it.  Feel free to propose a patch.  I'm not sure it would
make much of a difference in practice though, because the net result for
anyone who has customized it to /gnu/store/ will be the same: until they
reset their customization, their effective whitelist will be all of
/gnu/store/*.

What do you think?

Anyway, thanks to everyone who contributed to this fix!  I'm closing
both the older bug (38045) and the more recent duplicate (38831), but
feel free to reopen if appropriate.

       Mark