[tor-bugs] #8770 [Firefox Patch Issues]: Verify that @font-face fallback fonts can't be probed

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#8770: Verify that @font-face fallback fonts can't be probed
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 Reporter:  mikeperry             |          Owner:  mikeperry
     Type:  task                  |         Status:  new      
 Priority:  major                 |      Milestone:           
Component:  Firefox Patch Issues  |        Version:           
 Keywords:  tbb-fingerprinting    |         Parent:           
   Points:                        |   Actualpoints:           
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Comment(by mikeperry):

 Quoting myself from #8455, in case this is a problem:

 > It looks like the fallback might be governed by callers of
 gfxUserFontSet::FindFontEntry(). Two possiblitities for further
 investigation are gfxFontGroup::FindPlatformFont() and
 gfxFontGroup::ForEachFontInternal() and their subsequnt calls to
 gfxPlatform::ResolveFontName() and
 gfxPlatformFontList::FindFontForFamily(). Both set
 gfxFontGroup->mSkipDrawing if it's not yet time to use the fallback
 font... The bad news is it looks like you can still compute the width of
 the fallback font even before it is actually drawn (see
 gfxTextRun::Draw()).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/8770#comment:2>
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