Re: [AUCTeX-devel] [gs-devel] Asking help for preview-latex

[Ikumi Keita]

Hi Ken, thank you for your reply.

>>>>> Ken Sharp <address@hidden> writes:
> Hi Keita,
> (or should that be Ikumi ? Please accept an apology if I'm wrong)

No problem, call me by whichever name you like.  My given name is Keita
and the family name is Ikumi, but I don't care in English conversation.

> At 18:47 20/06/2019 +0900, Ikumi Keita wrote:
> So essentially what you are saying is that you want the rendered
> result of the PDF file to be in some colour other than the colour
> which it should be.

> For example, if the PDF file uses black, and the Emacs current colour
> is (eg) green, then you want the PDF rendered as green.

That's right.

> So I'm going to assume this is some highly limited application of PDF
> and Ghostscript where you can guarantee that the PDF file only uses
> the default colour space and colour and never sets any other colours.

Thanks for summarising, it looks sensible to me.

> That started us down a long road of hardening the PostScript and PDF
> interpreters to prevent any recurrence. Its quite clear that
> permitting unfettered access to the PDF interpreter allowed malicious
> software to bypass the security in Ghostscript with varying degrees of
> ease. Obviously we don't want to undo any of that work and expose
> ourselves to potential vulnerabilities again.

I see.  I understand the importance of security concerns.

> In order to investigate properly I need a file that I can run through
> Ghostscript, and a Ghostscript command line which will show the
> difference with and without the code above. I can probably invent a
> command line, but not the PDF file.

The files involved are attached with this message.  Let me explain how
these files are processed by preview-latex.
(0) The user types some key sequence in Emacs buffer of prv-tst.tex to
enable preview-latex.  The rest of the procedure are automated, and the
user does not have to intervene.

(1) First, preview-latex invokes pdflatex with the following command to
generate tst-prv.pdf:
pdflatex -file-line-error 
"\nonstopmode\nofiles\PassOptionsToPackage{active,tightpage,auctex}{preview}\AtBeginDocument{\ifx\ifPreview\undefined\RequirePackage[displaymath,floats,graphics,textmath,sections,footnotes]{preview}[2004/11/05]\fi}"
 "\input" "{\detokenize{" tst-prv.tex "}}"

(2) Then, preview-latex invokes pdf2dsc with the following command:
pdf2dsc tst-prv.pdf tst-prv.prv/tmp3GpVbI/preview.dsc
The 3rd argument specifies a DSC file generated by preview-latex and
saved in a temporal directory.  The content of the DSC file is attached
with this message as well.

(3) Next, preview-latex invokes gs binary with the following command.
"rungs" is just a wrapper for gs binary and equivalent to plain gs:
/usr/local/texlive/2019/bin/amd64-freebsd/rungs 
-dOutputFile\=\(tst-prv.prv/tmp3GpVbI/pr1-\%d.png\) -q -dDELAYSAFER -dNOPAUSE 
-DNOPLATFONTS -dPrinted -dTextAlphaBits\=4 -dGraphicsAlphaBits\=4 
-sDEVICE\=png16m -r108.606x108.587
If I understand correctly, this command lets gs to show a prompt "GS>"
and wait for subsequent input.  Preview-latex provides the following
inputs to the prompt inside Emacs:
{<</PermitFileReading[(tst-prv.pdf)(tst-prv.prv/tmp3GpVbI/preview.dsc)]>> 
setuserparams .locksafe} stopped pop {DELAYSAFER{.setsafe}if}stopped 
pop/.preview-BP currentpagedevice/BeginPage get dup null eq{pop{pop}bind}if 
def<</BeginPage{currentpagedevice/PageSize get dup 0 get 1 ne exch 1 get 1 ne 
or{.preview-BP 0.0937514 0.101564 0.14844 setrgbcolor clippath fill 0.8 0.8 0.8 
setrgbcolor}{pop}ifelse}bind/PageSize[1 
1]>>setpagedevice/preview-do{/.preview-ST[count 4 roll save]def dup length 0 
eq{pop}{setpagedevice}{ifelse exec}stopped{handleerror quit}if .preview-ST 
aload pop restore}bind def /GS_PDF_ProcSet GS_PDF_ProcSet dup maxlength dict 
copy dup begin/graphicsbeginpage{//graphicsbeginpage exec 0.8 0.8 0.8 3 copy rg 
RG}bind store end readonly store  (tst-prv.prv/tmp3GpVbI/preview.dsc)(r)file 
/.preview-ST 1 index def dup 0 setfileposition 415()/SubFileDecode filter cvx 
exec .preview-ST dup dup 415 setfileposition 53()/SubFileDecode filter 
cvx<<>>preview-do

(4) Then gs stops with this error message:
Error: /undefined in GS_PDF_ProcSet
Operand stack:
   GS_PDF_ProcSet
Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   
--nostringval--   2   %stopped_push   --nostringval--   --nostringval--   
%loop_continue   --nostringval--   --nostringval--   false   1   %stopped_push  
 .runexec2   --nostringval--   --nostringval--   --nostringval--   2   
%stopped_push   --nostringval--
Dictionary stack:
   --dict:739/1123(ro)(G)--   --dict:0/20(G)--   --dict:77/200(L)--
Current allocation mode is local
Last OS error: No such file or directory
Current file position is 627
GS<1>

> I believe that if you send me a PDF file which shows a difference,
> and the values substituted by the line '(mapconcat
> #'preview-gs-color-value fg " ")' above, I can probably simply render
> the file with and without that code to see what difference it makes.

The PDF files are just the same, with no difference.  The only
difference is in the Postscript code given at the procedure (3) above.
When Emacs have just standard foreground color, preview-latex omits
the following part and the rests are the same:
/GS_PDF_ProcSet GS_PDF_ProcSet dup maxlength dict copy dup 
begin/graphicsbeginpage{//graphicsbeginpage exec 0.8 0.8 0.8 3 copy rg RG}bind 
store end readonly store 

I hope these information are of help for your analysis.

Best regards,
Ikumi Keita

tst-prv.tex
Description: sample LaTeX file

tst-prv.pdf
Description: sample PDF file

preview.dsc
Description: sample DSC file

prev001.png
Description: generated PNG file