[NTG-pdftex] Patch for longstanding pdftex font subsetting bug

Melissa O'Neill oneill at cs.hmc.edu
Sat Mar 21 20:45:12 CET 2009


The enclosed patch fixes a longstanding bug in font subsetting for  
pdflatex that has existed since 1.20a-rc1 (March 2004).  This bug has  
been causing problems for Mac users for some time because OS X does  
not like the subtly invalid fonts that pdflatex can output (but it may  
have affected others as well).

The technical details and patch are below...

The issue relates to CharString encryption for Type 1 fonts.

In particular, the lenIV parameter of a font controls how much  
(encrypted) padding to put in front of the (encrypted) data.  The  
default is four, so four bytes of encrypted padding are produced.

In one part of the pdftex font-subsetting code (the part that writes  
out empty subroutines that have been dropped in the process of  
subsetting), pdftex incorrectly treats a padding of zero as meaning  
that it should abandon charstring encryption altogether.  Not so.   
(FWIW, by undocumented convention, a padding of -1 does, in fact, mean  
no encryption.)

By not encrypting data that is supposed to be encrypted, the  
subroutine ends up containing operator 0x1b, the hhcurveto operator,  
rather than 0x0b, return.  This is an error in two ways, first,  
subroutines should return, and, second, the hhcurveto operator expects  
some arguments.

Note that the subroutines that are affected by this bug are exactly  
those that are *never called*, so we might expect the bug to be  
harmless.  It would appear, however, that Apple's font technology does  
process these "uncalled" subroutines and does not react well to their  
invalid content.

Thus, to have been affected by this bug, you need to

    a) Use a font where lenIV = 0, and

    b) Use a Mac (or anything else that needs uncalled subroutines to  
nevertheless be valid),

which explains why so many people have said, "I see no problem; it  
works for me" in response to the anguished cries of Mac users with  
PDFTeX-generated documents everywhere.

FWIW, the buggy code was checked into the repository for version 1.20a- 
rc1.  Prior to that point, this aspect of font subsetting worked  
correctly.

The patch is, thankfully, trivial, changing (t1_lenIV > 0) to  
(t1_lenIV >= 0).

Fonts known to be affected by this bug include cm-super, sanskrit, and  
fonts processed with cfftot1 (i.e., OpenType fonts used with LaTeX),  
as well as many Adobe Type 1 fonts.

To assist Mac users, I have put a precompiled version of pdftex-1.40.9  
on my website at http://www.cs.hmc.edu/~oneill/dist/pdftex-1.40.9-patched.zip

If you have enjoyed this patch, share it with your friends.  I'm told  
it goes well with a glass of fine Merlot.

     Melissa.

P.S.  Sadly, I don't know if this is the *only* bug in fonts and  
subsetting that causes fonts to be subtly invalid, but it is at least  
*a* bug that's been squished.

Enc.
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