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%%Page: 0 1
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0 SI
14 /Helvetica-Bold AF
18273 8221 MT
(Transparently Interposing User Code)SH
22939 10016 MT
(at the System Interface)SH
24922 14893 MT
(Michael B. Jones)SH
11 /Helvetica AF
24028 17623 MT
(Carnegie Mellon University)SH
23048 18860 MT
(Computer Science Department)SH
25492 20097 MT
(5000 Forbes Avenue)SH
25400 21334 MT
(Pittsburgh, PA 15217)SH
29469 22571 MT
(USA)SH
26364 23808 MT
(+1-412-268-3069)SH
24844 25045 MT
(mbj@mach.cs.cmu.edu)SH
14 /Helvetica-Bold AF
27761 29799 MT
(Abstract)SH
11 /Helvetica AF
7200 32529 MT
(Many contemporary operating systems utilize a system call interface)
213 W( between the operating)212 W
7200 33766 MT
(system and its clients.)
67 W( Increasing)
441 W( numbers of systems are providing low-level mechanisms for)68 W
7200 35003 MT
(intercepting and handling system calls in user code.  Nonetheless, they typically)
194 W( provide no)193 W
7200 36240 MT
(higher-level tools or abstractions for effectively utilizing these mechanisms.  Using them has)180 W
7200 37477 MT
(typically required reimplementation of a substantial portion of the system interface from scratch,)32 W
7200 38714 MT
(making the use of such facilities unwieldy at best.)SH
7200 41092 MT
(I am currently constructing an object-oriented toolkit which substantially)
113 W( increases the ease of)114 W
7200 42329 MT
(interposing user code between clients and instances)
119 W( of the system interface by allowing such)118 W
7200 43566 MT
(code to be written in)
149 W( terms of the high-level objects provided by this interface, rather than in)150 W
7200 44803 MT
(terms of the intercepted system calls themselves.  This toolkit)
138 W( helps enable new interposition)137 W
7200 46040 MT
(agents to be written which otherwise would not have been attempted.)SH
7200 48418 MT
(I am also currently)
466 W( using this toolkit to construct several agents including:  protected)467 W
7200 49655 MT
(environments for running untrusted binaries, modified filesystem)
145 W( namespaces, logical devices)144 W
7200 50892 MT
(implemented entirely in user space,)
166 W( transparent network data compression and/or encryption)167 W
7200 52129 MT
(agents, and system call)
26 W( tracing tools.  Examples of other interesting agents which could be built)25 W
7200 53366 MT
(include: transactional)
1128 W( software environments, and emulators for other)
411 W( operating system)412 W
7200 54603 MT
(environments.)SH
7200 56981 MT
(Having the flexibility to simultaneously support several variant sets of operating system services)22 W
7200 58218 MT
(may be particularly useful in a workstation environment; a diversity of software which)
119 W( expects)120 W
7200 59455 MT
(different operating system environments can be supported)
93 W( on a single operating system base.)92 W
7200 60692 MT
(Even just the ability to provide multiple configurable views of the)
1 W( filesystem namespace provides)2 W
7200 61929 MT
(a flexibility not found in traditional operating systems.)SH
9.5 SS 
8312 65493 MT
(This research)
3 W( was supported by the Defense Advanced Research Projects Agency \050DOD\051, and monitored by)2 W
7200 66589 MT
(the Avionics)
127 W( Laboratory, Air Force Wright Aeronautical Laboratories, Aeronautical Systems Division \050AFSC\051,)128 W
7200 67685 MT
(Wright-Patterson AFB, Ohio 45433-6543 under Contract F33615-87-C-1499, ARPA)
468 W( Order No. 4976,)467 W
7200 68781 MT
(Amendment 20.)SH
8312 71018 MT
(The views and)
291 W( conclusions contained in this document are those of the authors and should not be)292 W
7200 72114 MT
(interpreted as representing the official)
3 W( policies, either expressed or implied, of the Defense Advanced Research)2 W
7200 73210 MT
(Projects Agency or the U.S. government.)SH
ES
%%Page: 1 2
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0 SI
13 /Helvetica-Bold AF
7200 8148 MT
(1. Introduction)SH
12 SS 
7200 11903 MT
(1.1. Background)SH
11 /Helvetica AF
8312 13329 MT
(Many contemporary operating systems provide an interface between user code and)
289 W( the)290 W
7200 14755 MT
(operating system services based on special ``system)
37 W( calls''.  One can view the system interface)36 W
7200 16181 MT
(as simply a special form)
220 W( of structured communication channel, allowing such operations as)221 W
7200 17607 MT
(interposing programs which record or modify the communications taking place on this channel.)SH
8312 20174 MT
(The following)
114 W( figures should help clarify both the system interface and interposition.  Figure)113 W
7200 21600 MT
(1-1 depicts uses)
SH( of the system interface without interposition.  In this view the kernel provides all)1 W
7200 23026 MT
(instances of the operating system interface.  Figure 1-2 depicts uses)
154 W( of the system interface)153 W
7200 24452 MT
(with interposition.  In this view both the kernel)
49 W( and interposition agents provide instances of the)50 W
7200 25878 MT
(operating system interface.  Figure 1-3 depicts more uses of)
392 W( the system interface with)391 W
7200 27304 MT
(interposition. In)
340 W( this view, like the kernel, agents can share state and provide multiple)
17 W( instances)18 W
7200 28730 MT
(of the operating system interface.)SH
1 1 0 2600 57067 GB 
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 GE /Helvetica-Bold SF
11886 44299 MT
(Figure 1-1:)SH
/Helvetica SF
18122 XM
(The kernel provides instances of the operating system interface.)SH
12 /Helvetica-Bold AF
7200 48054 MT
(1.2. Motivation for Interposition)SH
11 /Helvetica AF
8312 49480 MT
(Interposition can be used to provide programming facilities which would not)
373 W( otherwise)372 W
7200 50906 MT
(available. In)
1614 W( particular, it can allow for a multiplicity of simultaneously coexisting)654 W
7200 52332 MT
(implementations of the system call services, which in turn)
237 W( may utilize one another, without)236 W
7200 53758 MT
(requiring changes to)
255 W( existing client binaries and without modifying the underlying kernel to)256 W
7200 55184 MT
(support each implementation.)SH
8312 57223 MT
(Alternate system call implementations can be used to provide)
29 W( a number of important services)28 W
7200 58649 MT
(not typically available on system call-based operating systems.  Some examples include:)SH
/Symbol SF
9448 60248 MT
(\267)SH
/Helvetica SF
10260 XM
(system call tracing and monitoring facilities.)
1 W( Debuggers)
309 W( and program trace facilities)2 W
10260 61485 MT
(can be constructed which allow monitoring of a program's use of system)
6 W( services in)5 W
10260 62722 MT
(a easily customizable manner.)SH
/Symbol SF
9448 64678 MT
(\267)SH
/Helvetica SF
10260 XM
(emulation of)
578 W( other operating system environments.  Alternate system call)579 W
10260 65915 MT
(implementations can be used to)
179 W( concurrently run binaries from variant operating)178 W
10260 67152 MT
(systems \050such)
212 W( as ULTRIX or System V binaries in a BSD environment\051 on the)213 W
10260 68389 MT
(same platform.)SH
ES
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0 begin
storexyn
newpath
n 1 gt {
0 0 0 0 0 0 1 1 true subspline
n 2 gt {
0 0 0 0 1 1 2 2 false subspline
1 1 n 3 sub {
/i exch def
i 1 sub dup i dup i 1 add dup i 2 add dup false subspline
} for
n 3 sub dup n 2 sub dup n 1 sub dup 2 copy false subspline
} if
n 2 sub dup n 1 sub dup 2 copy 2 copy false subspline
patternNone not brushLeftArrow not brushRightArrow not and and { ifill } if
brushNone not { istroke } if
0 0 1 1 leftarrow
n 2 sub dup n 1 sub dup rightarrow
} if
end
} dup 0 4 dict put def

/Circ {
newpath
0 360 arc
patternNone not { ifill } if
brushNone not { istroke } if
} def

/CBSpl {
0 begin
dup 2 gt {
storexyn
newpath
n 1 sub dup 0 0 1 1 2 2 true subspline
1 1 n 3 sub {
/i exch def
i 1 sub dup i dup i 1 add dup i 2 add dup false subspline
} for
n 3 sub dup n 2 sub dup n 1 sub dup 0 0 false subspline
n 2 sub dup n 1 sub dup 0 0 1 1 false subspline
patternNone not { ifill } if
brushNone not { istroke } if
} {
Poly
} ifelse
end
} dup 0 4 dict put def

/Elli {
0 begin
newpath
4 2 roll
translate
scale
0 0 1 0 360 arc
patternNone not { ifill } if
brushNone not { istroke } if
end
} dup 0 1 dict put def

/Line {
0 begin
2 storexyn
newpath
x 0 get y 0 get moveto
x 1 get y 1 get lineto
brushNone not { istroke } if
0 0 1 1 leftarrow
0 0 1 1 rightarrow
end
} dup 0 4 dict put def

/MLine {
0 begin
storexyn
newpath
n 1 gt {
x 0 get y 0 get moveto
1 1 n 1 sub {
/i exch def
x i get y i get lineto
} for
patternNone not brushLeftArrow not brushRightArrow not and and { ifill } if
brushNone not { istroke } if
0 0 1 1 leftarrow
n 2 sub dup n 1 sub dup rightarrow
} if
end
} dup 0 4 dict put def

/Poly {
3 1 roll
newpath
moveto
-1 add
{ lineto } repeat
closepath
patternNone not { ifill } if
brushNone not { istroke } if
} def

/Rect {
0 begin
/t exch def
/r exch def
/b exch def
/l exch def
newpath
l b moveto
l t lineto
r t lineto
r b lineto
closepath
patternNone not { ifill } if
brushNone not { istroke } if
end
} dup 0 4 dict put def

/Text {
ishow
} def

/idef {
dup where { pop pop pop } { exch def } ifelse
} def

/ifill {
0 begin
gsave
patternGrayLevel -1 ne {
fgred bgred fgred sub patternGrayLevel mul add
fggreen bggreen fggreen sub patternGrayLevel mul add
fgblue bgblue fgblue sub patternGrayLevel mul add setrgbcolor
eofill
} {
eoclip
originalCTM setmatrix
pathbbox /t exch def /r exch def /b exch def /l exch def
/w r l sub ceiling cvi def
/h t b sub ceiling cvi def
/imageByteWidth w 8 div ceiling cvi def
/imageHeight h def
bgred bggreen bgblue setrgbcolor
eofill
fgred fggreen fgblue setrgbcolor
w 0 gt h 0 gt and {
l b translate w h scale
w h true [w 0 0 h neg 0 h] { patternproc } imagemask
} if
} ifelse
grestore
end
} dup 0 8 dict put def

/istroke {
gsave
brushDashOffset -1 eq {
[] 0 setdash
1 setgray
} {
brushDashArray brushDashOffset setdash
fgred fggreen fgblue setrgbcolor
} ifelse
brushWidth setlinewidth
originalCTM setmatrix
stroke
grestore
} def

/ishow {
0 begin
gsave
fgred fggreen fgblue setrgbcolor
/fontDict printFont findfont printSize scalefont dup setfont def
/descender fontDict begin 0 [FontBBox] 1 get FontMatrix end
transform exch pop def
/vertoffset 0 descender sub printSize sub printFont /Courier ne
printFont /Courier-Bold ne and { 1 add } if def {
0 vertoffset moveto show
/vertoffset vertoffset printSize sub def
} forall
grestore
end
} dup 0 3 dict put def

/patternproc {
0 begin
/patternByteLength patternString length def
/patternHeight patternByteLength 8 mul sqrt cvi def
/patternWidth patternHeight def
/patternByteWidth patternWidth 8 idiv def
/imageByteMaxLength imageByteWidth imageHeight mul
stringLimit patternByteWidth sub min def
/imageMaxHeight imageByteMaxLength imageByteWidth idiv patternHeight idiv
patternHeight mul patternHeight max def
/imageHeight imageHeight imageMaxHeight sub store
/imageString imageByteWidth imageMaxHeight mul patternByteWidth add string def
0 1 imageMaxHeight 1 sub {
/y exch def
/patternRow y patternByteWidth mul patternByteLength mod def
/patternRowString patternString patternRow patternByteWidth getinterval def
/imageRow y imageByteWidth mul def
0 patternByteWidth imageByteWidth 1 sub {
/x exch def
imageString imageRow x add patternRowString putinterval
} for
} for
imageString
end
} dup 0 12 dict put def

/min {
dup 3 2 roll dup 4 3 roll lt { exch } if pop
} def

/max {
dup 3 2 roll dup 4 3 roll gt { exch } if pop
} def

/arrowhead {
0 begin
transform originalCTM itransform
/taily exch def
/tailx exch def
transform originalCTM itransform
/tipy exch def
/tipx exch def
/dy tipy taily sub def
/dx tipx tailx sub def
/angle dx 0 ne dy 0 ne or { dy dx atan } { 90 } ifelse def
gsave
originalCTM setmatrix
tipx tipy translate
angle rotate
newpath
0 0 moveto
arrowHeight neg arrowWidth 2 div lineto
arrowHeight neg arrowWidth 2 div neg lineto
closepath
patternNone not {
originalCTM setmatrix
/padtip arrowHeight 2 exp 0.25 arrowWidth 2 exp mul add sqrt brushWidth mul
arrowWidth div def
/padtail brushWidth 2 div def
tipx tipy translate
angle rotate
padtip 0 translate
arrowHeight padtip add padtail add arrowHeight div dup scale
arrowheadpath
ifill
} if
brushNone not {
originalCTM setmatrix
tipx tipy translate
angle rotate
arrowheadpath
istroke
} if
grestore
end
} dup 0 9 dict put def

/arrowheadpath {
newpath
0 0 moveto
arrowHeight neg arrowWidth 2 div lineto
arrowHeight neg arrowWidth 2 div neg lineto
closepath
} def

/leftarrow {
0 begin
y exch get /taily exch def
x exch get /tailx exch def
y exch get /tipy exch def
x exch get /tipx exch def
brushLeftArrow { tipx tipy tailx taily arrowhead } if
end
} dup 0 4 dict put def

/rightarrow {
0 begin
y exch get /tipy exch def
x exch get /tipx exch def
y exch get /taily exch def
x exch get /tailx exch def
brushRightArrow { tipx tipy tailx taily arrowhead } if
end
} dup 0 4 dict put def

/midpoint {
0 begin
/y1 exch def
/x1 exch def
/y0 exch def
/x0 exch def
x0 x1 add 2 div
y0 y1 add 2 div
end
} dup 0 4 dict put def

/thirdpoint {
0 begin
/y1 exch def
/x1 exch def
/y0 exch def
/x0 exch def
x0 2 mul x1 add 3 div
y0 2 mul y1 add 3 div
end
} dup 0 4 dict put def

/subspline {
0 begin
/movetoNeeded exch def
y exch get /y3 exch def
x exch get /x3 exch def
y exch get /y2 exch def
x exch get /x2 exch def
y exch get /y1 exch def
x exch get /x1 exch def
y exch get /y0 exch def
x exch get /x0 exch def
x1 y1 x2 y2 thirdpoint
/p1y exch def
/p1x exch def
x2 y2 x1 y1 thirdpoint
/p2y exch def
/p2x exch def
x1 y1 x0 y0 thirdpoint
p1x p1y midpoint
/p0y exch def
/p0x exch def
x2 y2 x3 y3 thirdpoint
p2x p2y midpoint
/p3y exch def
/p3x exch def
movetoNeeded { p0x p0y moveto } if
p1x p1y p2x p2y p3x p3y curveto
end
} dup 0 17 dict put def

/storexyn {
/n exch def
/y n array def
/x n array def
n 1 sub -1 0 {
/i exch def
y i 3 2 roll put
x i 3 2 roll put
} for
} def

%%EndProlog

%I Idraw 7 Grid 10 

%%%Page: 1 1

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%I b u
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[ 0.9 0 0 0.9 0 0 ] concat
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[
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Begin %I Rect
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[
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Begin %I Rect
%I b 65535
2 0 0 [] 0 SetB
%I cfg Black
0 0 0 SetCFg
%I cbg White
1 1 1 SetCBg
none SetP %I p n
%I t
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Begin %I Text
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%I t
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[
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Begin %I Rect
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End %I eop

End %I eop

showpage

%%Trailer

end

%%EndDocument
 GE /Helvetica-Bold SF
15829 35232 MT
(Figure 1-2:)SH
/Helvetica SF
22065 XM
(Both the kernel and interposition agents provide)SH
23106 36469 MT
(instances of the operating system interface.)SH
1 1 0 2500 78606 GB 
%%BeginDocument: full_interposition.ps
%!PS-Adobe-2.0 EPSF-1.2
%%DocumentFonts: Helvetica-Oblique
%%%Pages: 1
%%BoundingBox: 47 147 440 409
%%EndComments

50 dict begin

/arrowHeight 8 def
/arrowWidth 4 def
/none null def
/numGraphicParameters 17 def
/stringLimit 65535 def

/Begin {
save
numGraphicParameters dict begin
} def

/End {
end
restore
} def

/SetB {
dup type /nulltype eq {
pop
false /brushRightArrow idef
false /brushLeftArrow idef
true /brushNone idef
} {
/brushDashOffset idef
/brushDashArray idef
0 ne /brushRightArrow idef
0 ne /brushLeftArrow idef
/brushWidth idef
false /brushNone idef
} ifelse
} def

/SetCFg {
/fgblue idef
/fggreen idef
/fgred idef
} def

/SetCBg {
/bgblue idef
/bggreen idef
/bgred idef
} def

/SetF {
/printSize idef
/printFont idef
} def

/SetP {
dup type /nulltype eq {
pop true /patternNone idef
} {
/patternGrayLevel idef
patternGrayLevel -1 eq {
/patternString idef
} if
false /patternNone idef
} ifelse
} def

/BSpl {
0 begin
storexyn
newpath
n 1 gt {
0 0 0 0 0 0 1 1 true subspline
n 2 gt {
0 0 0 0 1 1 2 2 false subspline
1 1 n 3 sub {
/i exch def
i 1 sub dup i dup i 1 add dup i 2 add dup false subspline
} for
n 3 sub dup n 2 sub dup n 1 sub dup 2 copy false subspline
} if
n 2 sub dup n 1 sub dup 2 copy 2 copy false subspline
patternNone not brushLeftArrow not brushRightArrow not and and { ifill } if
brushNone not { istroke } if
0 0 1 1 leftarrow
n 2 sub dup n 1 sub dup rightarrow
} if
end
} dup 0 4 dict put def

/Circ {
newpath
0 360 arc
patternNone not { ifill } if
brushNone not { istroke } if
} def

/CBSpl {
0 begin
dup 2 gt {
storexyn
newpath
n 1 sub dup 0 0 1 1 2 2 true subspline
1 1 n 3 sub {
/i exch def
i 1 sub dup i dup i 1 add dup i 2 add dup false subspline
} for
n 3 sub dup n 2 sub dup n 1 sub dup 0 0 false subspline
n 2 sub dup n 1 sub dup 0 0 1 1 false subspline
patternNone not { ifill } if
brushNone not { istroke } if
} {
Poly
} ifelse
end
} dup 0 4 dict put def

/Elli {
0 begin
newpath
4 2 roll
translate
scale
0 0 1 0 360 arc
patternNone not { ifill } if
brushNone not { istroke } if
end
} dup 0 1 dict put def

/Line {
0 begin
2 storexyn
newpath
x 0 get y 0 get moveto
x 1 get y 1 get lineto
brushNone not { istroke } if
0 0 1 1 leftarrow
0 0 1 1 rightarrow
end
} dup 0 4 dict put def

/MLine {
0 begin
storexyn
newpath
n 1 gt {
x 0 get y 0 get moveto
1 1 n 1 sub {
/i exch def
x i get y i get lineto
} for
patternNone not brushLeftArrow not brushRightArrow not and and { ifill } if
brushNone not { istroke } if
0 0 1 1 leftarrow
n 2 sub dup n 1 sub dup rightarrow
} if
end
} dup 0 4 dict put def

/Poly {
3 1 roll
newpath
moveto
-1 add
{ lineto } repeat
closepath
patternNone not { ifill } if
brushNone not { istroke } if
} def

/Rect {
0 begin
/t exch def
/r exch def
/b exch def
/l exch def
newpath
l b moveto
l t lineto
r t lineto
r b lineto
closepath
patternNone not { ifill } if
brushNone not { istroke } if
end
} dup 0 4 dict put def

/Text {
ishow
} def

/idef {
dup where { pop pop pop } { exch def } ifelse
} def

/ifill {
0 begin
gsave
patternGrayLevel -1 ne {
fgred bgred fgred sub patternGrayLevel mul add
fggreen bggreen fggreen sub patternGrayLevel mul add
fgblue bgblue fgblue sub patternGrayLevel mul add setrgbcolor
eofill
} {
eoclip
originalCTM setmatrix
pathbbox /t exch def /r exch def /b exch def /l exch def
/w r l sub ceiling cvi def
/h t b sub ceiling cvi def
/imageByteWidth w 8 div ceiling cvi def
/imageHeight h def
bgred bggreen bgblue setrgbcolor
eofill
fgred fggreen fgblue setrgbcolor
w 0 gt h 0 gt and {
l b translate w h scale
w h true [w 0 0 h neg 0 h] { patternproc } imagemask
} if
} ifelse
grestore
end
} dup 0 8 dict put def

/istroke {
gsave
brushDashOffset -1 eq {
[] 0 setdash
1 setgray
} {
brushDashArray brushDashOffset setdash
fgred fggreen fgblue setrgbcolor
} ifelse
brushWidth setlinewidth
originalCTM setmatrix
stroke
grestore
} def

/ishow {
0 begin
gsave
fgred fggreen fgblue setrgbcolor
/fontDict printFont findfont printSize scalefont dup setfont def
/descender fontDict begin 0 [FontBBox] 1 get FontMatrix end
transform exch pop def
/vertoffset 0 descender sub printSize sub printFont /Courier ne
printFont /Courier-Bold ne and { 1 add } if def {
0 vertoffset moveto show
/vertoffset vertoffset printSize sub def
} forall
grestore
end
} dup 0 3 dict put def

/patternproc {
0 begin
/patternByteLength patternString length def
/patternHeight patternByteLength 8 mul sqrt cvi def
/patternWidth patternHeight def
/patternByteWidth patternWidth 8 idiv def
/imageByteMaxLength imageByteWidth imageHeight mul
stringLimit patternByteWidth sub min def
/imageMaxHeight imageByteMaxLength imageByteWidth idiv patternHeight idiv
patternHeight mul patternHeight max def
/imageHeight imageHeight imageMaxHeight sub store
/imageString imageByteWidth imageMaxHeight mul patternByteWidth add string def
0 1 imageMaxHeight 1 sub {
/y exch def
/patternRow y patternByteWidth mul patternByteLength mod def
/patternRowString patternString patternRow patternByteWidth getinterval def
/imageRow y imageByteWidth mul def
0 patternByteWidth imageByteWidth 1 sub {
/x exch def
imageString imageRow x add patternRowString putinterval
} for
} for
imageString
end
} dup 0 12 dict put def

/min {
dup 3 2 roll dup 4 3 roll lt { exch } if pop
} def

/max {
dup 3 2 roll dup 4 3 roll gt { exch } if pop
} def

/arrowhead {
0 begin
transform originalCTM itransform
/taily exch def
/tailx exch def
transform originalCTM itransform
/tipy exch def
/tipx exch def
/dy tipy taily sub def
/dx tipx tailx sub def
/angle dx 0 ne dy 0 ne or { dy dx atan } { 90 } ifelse def
gsave
originalCTM setmatrix
tipx tipy translate
angle rotate
newpath
0 0 moveto
arrowHeight neg arrowWidth 2 div lineto
arrowHeight neg arrowWidth 2 div neg lineto
closepath
patternNone not {
originalCTM setmatrix
/padtip arrowHeight 2 exp 0.25 arrowWidth 2 exp mul add sqrt brushWidth mul
arrowWidth div def
/padtail brushWidth 2 div def
tipx tipy translate
angle rotate
padtip 0 translate
arrowHeight padtip add padtail add arrowHeight div dup scale
arrowheadpath
ifill
} if
brushNone not {
originalCTM setmatrix
tipx tipy translate
angle rotate
arrowheadpath
istroke
} if
grestore
end
} dup 0 9 dict put def

/arrowheadpath {
newpath
0 0 moveto
arrowHeight neg arrowWidth 2 div lineto
arrowHeight neg arrowWidth 2 div neg lineto
closepath
} def

/leftarrow {
0 begin
y exch get /taily exch def
x exch get /tailx exch def
y exch get /tipy exch def
x exch get /tipx exch def
brushLeftArrow { tipx tipy tailx taily arrowhead } if
end
} dup 0 4 dict put def

/rightarrow {
0 begin
y exch get /tipy exch def
x exch get /tipx exch def
y exch get /taily exch def
x exch get /tailx exch def
brushRightArrow { tipx tipy tailx taily arrowhead } if
end
} dup 0 4 dict put def

/midpoint {
0 begin
/y1 exch def
/x1 exch def
/y0 exch def
/x0 exch def
x0 x1 add 2 div
y0 y1 add 2 div
end
} dup 0 4 dict put def

/thirdpoint {
0 begin
/y1 exch def
/x1 exch def
/y0 exch def
/x0 exch def
x0 2 mul x1 add 3 div
y0 2 mul y1 add 3 div
end
} dup 0 4 dict put def

/subspline {
0 begin
/movetoNeeded exch def
y exch get /y3 exch def
x exch get /x3 exch def
y exch get /y2 exch def
x exch get /x2 exch def
y exch get /y1 exch def
x exch get /x1 exch def
y exch get /y0 exch def
x exch get /x0 exch def
x1 y1 x2 y2 thirdpoint
/p1y exch def
/p1x exch def
x2 y2 x1 y1 thirdpoint
/p2y exch def
/p2x exch def
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 GE /Helvetica-Bold SF
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(Figure 1-3:)SH
/Helvetica SF
21239 XM
(Like the kernel, agents can share state and provide)SH
21058 66975 MT
(multiple instances of the operating system interface.)SH
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11 /Symbol AF
9448 8080 MT
(\267)SH
/Helvetica SF
10260 XM
(protected environments for)
20 W( running untrusted binaries.  A wrapper environment can)19 W
10260 9317 MT
(be constructed which allows untrusted, possibly malicious, binaries)
212 W( to be run in)213 W
10260 10554 MT
(such a way that actions)
53 W( which they take may be monitored and emulated, possibly)52 W
10260 11791 MT
(without actually being performed, and resources which they use can be limited in)118 W
10260 13028 MT
(such a way that the untrusted binaries can't tell.  A wide variety of monitoring and)82 W
10260 14265 MT
(emulating schemes are)
529 W( possible from simple automatic resource restriction)530 W
10260 15502 MT
(environments to heuristic)
233 W( evaluations of the target program's behavior, possibly)232 W
10260 16739 MT
(including interactive decisions made by human)
476 W( beings during the protected)477 W
10260 17976 MT
(execution. This)
388 W( is particularly)
41 W( timely in today's environments of increased software)40 W
10260 19213 MT
(sharing with the potential for viruses and Trojan horses.)SH
/Symbol SF
9448 21169 MT
(\267)SH
/Helvetica SF
10260 XM
(transactional software environments.  Applications can be constructed which)437 W
10260 22406 MT
(provide an environment in which)
246 W( persistent state changes made by unmodified)245 W
10260 23643 MT
(programs can be emulated and performed transactionally.  For instance, a)
101 W( simple)102 W
10260 24880 MT
(``run transaction'' command could be constructed)
138 W( which runs arbitrary unmodified)137 W
10260 26117 MT
(programs \050e.g.,)231 W
/Courier SF
18488 XM
(/bin/csh)SH
/Helvetica SF
(\051 such that all persistent execution side)
231 W( effects \050e.g.,)232 W
10260 27354 MT
(filesystem writes\051 are remembered and)
9 W( appear within the transactional environment)8 W
10260 28591 MT
(to have been performed normally, but where in actuality the user is presented)
57 W( with)58 W
10260 29828 MT
(a ``commit'')
242 W( or ``abort'' choice at the end of such a session.  Indeed one such)241 W
10260 31065 MT
(transactional program invocations)
595 W( could occur within another, transparently)596 W
10260 32302 MT
(providing nested transactions.)SH
/Symbol SF
9448 34258 MT
(\267)SH
/Helvetica SF
10260 XM
(alternate or enhanced semantics.  Environments can)
44 W( be constructed which provide)43 W
10260 35495 MT
(alternate or enhanced)
67 W( semantics for unmodified binaries.  One such enhancement)68 W
10260 36732 MT
(in which people have expressed)
150 W( interest is the ability to ``mount'' a search list of)149 W
10260 37969 MT
(directories in the filesystem namespace such that the union of)
334 W( their contents)335 W
10260 39206 MT
(appears to reside in a single directory.  This could be used in)
407 W( a software)406 W
10260 40443 MT
(development environment to allow distinct)
92 W( source and object directories to appear)93 W
10260 41680 MT
(as a single directory when running)SH
/Courier SF
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(make)SH
/Helvetica SF
(.)SH
12 /Helvetica-Bold AF
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(1.3. Problems with Existing Systems)SH
11 /Helvetica AF
8312 46861 MT
(Increasing numbers of operating systems, e.g., Mach, SunOS version 4, System V.4, are)204 W
7200 48287 MT
(providing low-level mechanisms)
344 W( for intercepting system calls.  Nonetheless, they typically)345 W
7200 49713 MT
(provide no higher-level tools or abstractions for effectively utilizing these)
102 W( mechanisms, making)101 W
7200 51139 MT
(the use of such facilities unwieldy at best.)SH
8312 53706 MT
(Part of the difficulty with)
68 W( writing system call interposition agents in the past has been that no)69 W
7200 55132 MT
(one set of interfaces is)
112 W( appropriate across a range of such agents other than the lowest level)111 W
7200 56558 MT
(system call interception services.)
393 W( Different)
1093 W( agents interact with different subsets of the)394 W
7200 57984 MT
(operating system interface in widely different ways.  Thus, only the)
91 W( bare minimum interception)90 W
7200 59410 MT
(facilities have been provided \320)
230 W( the lowest common denominator which is generally useful.)231 W
7200 60836 MT
(Consequently, each agent has typically)
17 W( been constructed completely from scratch.  No leverage)16 W
7200 62262 MT
(was gained from the work done on other agents.  Not surprisingly, experience with past)
5 W( systems)6 W
7200 63688 MT
(has shown that such facilities are rarely used.)SH
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13 /Helvetica-Bold AF
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(2. Position Statement)SH
11 /Helvetica AF
8312 9574 MT
(An object-oriented)
340 W( toolkit can be constructed which substantially increases the ease of)339 W
7200 11000 MT
(interposing user code between clients and instances of)
118 W( the system interface by allowing such)119 W
7200 12426 MT
(code to be written in terms of the high-level objects provided by this)
150 W( interface, rather than in)149 W
7200 13852 MT
(terms of the intercepted system)
217 W( calls themselves.  Providing a toolkit exposing the multiple)218 W
7200 15278 MT
(layers of abstraction present in the system interface provides a useful set of tools and)
13 W( interfaces)12 W
7200 16704 MT
(at each level.  Different)
38 W( agents may then exploit the toolkit objects best suited to their individual)39 W
7200 18130 MT
(needs. Consequently,)
1080 W( substantial amounts of toolkit code are able to be reused when)386 W
7200 19556 MT
(constructing different)
334 W( system call interception agents.  Furthermore, having such a toolkit)335 W
7200 20982 MT
(enables new system)
129 W( call implementations to be written which otherwise would not have been)128 W
7200 22408 MT
(attempted.)SH
8312 24975 MT
(The ability to simultaneously support)
863 W( multiple customized and special purpose)864 W
7200 26401 MT
(implementations of the operating system)
99 W( services provides a flexibility in the operating system)98 W
7200 27827 MT
(environment not generally available today.  This flexibility may be)
194 W( particularly important in a)195 W
7200 29253 MT
(workstation environment, where the user may, for instance, want the ability to run)
307 W( binary)306 W
7200 30679 MT
(applications which expect somewhat different operating system)
160 W( environments \050e.g., BSD 4.3,)161 W
7200 32105 MT
(OSF/1, ULTRIX, System V, etc.\051)
91 W( without having to have copies of each, and without having to)90 W
7200 33531 MT
(reboot to switch worlds.  Even just the ability to dynamically rearrange the appearance of)
123 W( the)124 W
7200 34957 MT
(filesystem namespace will allow software to be used)
480 W( which has different configuration)479 W
7200 36383 MT
(assumptions than a given workstation's default environment.)SH
13 /Helvetica-Bold AF
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(3. Motivation for an Object-Oriented Toolkit)SH
11 /Helvetica AF
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(An underlying premise behind object-oriented programming is)
25 W( that when implementing similar)26 W
7200 43063 MT
(functions it)
298 W( should be possible to extract the commonalities between them and share the)297 W
7200 44489 MT
(implementations of the shared functionality.  The incremental work of building another similar)139 W
7200 45915 MT
(function should then be proportional only to the differences.  This approach pays)
232 W( off when)231 W
7200 47341 MT
(substantial commonality exists between multiple logical functions.)SH
8312 49908 MT
(For wide classes)
12 W( of interesting emulation agents, a large portion of the functionality needed to)13 W
7200 51334 MT
(build each agent is also needed by other agents.  Nearly all)
164 W( need translation from machine-)163 W
7200 52760 MT
(specific system call numbers and argument formats to logical system call interfaces.)
265 W( Any)837 W
7200 54186 MT
(agents manipulating ``open''ed objects)
44 W( need support for file \050or socket\051 descriptors.  Any agents)43 W
7200 55612 MT
(manipulating pathnames need)
318 W( support for pathname component walking; many also need)319 W
7200 57038 MT
(support for translating pathnames to open objects.  Some facility for)
95 W( object reference counting)94 W
7200 58464 MT
(or garbage collection is needed by most agents.  Facilities for sharing objects between multiple)49 W
7200 59890 MT
(emulated processes is needed by)
119 W( many agents, particularly those which allow changes made)118 W
7200 61316 MT
(by one emulated process to be seen by another.)SH
8312 63883 MT
(Not every agent needs every kind of emulation support.  Indeed,)
83 W( to the extent that an agent)84 W
7200 65309 MT
(does not modify the behavior of a particular)
12 W( portion of the system interface, it is possible to pass)11 W
7200 66735 MT
(uses of that)
358 W( portion through to the next level of system interface largely unmodified for)359 W
7200 68161 MT
(execution. An)
754 W( object-oriented structure readily facilitates composing)
224 W( agents from just those)223 W
7200 69587 MT
(toolkit components)
156 W( which benefit them, while using inheritance to automatically access those)157 W
7200 71013 MT
(functions which the agent has custom built.)SH
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8312 8002 MT
(Finally, an object-oriented)
145 W( toolkit should both be easier to use and provide more long term)144 W
7200 9428 MT
(benefits than ad hoc)
28 W( approaches.  One alternate approach to writing agents would be to always)29 W
7200 10854 MT
(write one by copying another one and modifying it.  This has several drawbacks.  For)
26 W( one, such)25 W
7200 12280 MT
(modifications tend to be of)
325 W( an undisciplined ``whatever makes it work'' nature; the toolkit)326 W
7200 13706 MT
(approach helps maintain clean interfaces.)
63 W( Even)
430 W( for similar agents, any improvements made to)62 W
7200 15132 MT
(one are not reflected back)
61 W( into the other; any improvements made to toolkit objects are shared)62 W
7200 16558 MT
(by all clients.)
296 W( Likewise,)
896 W( as new agents are built with the toolkit, any new useful objects)295 W
7200 17984 MT
(developed can be added to the toolkit suite, improving its usability)
18 W( over time; such accumulation)19 W
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(of useful tools would be less likely with an ad hoc approach.)SH
13 /Helvetica-Bold AF
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(4. Design and Structure of the Toolkit)SH
11 /Helvetica AF
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(I am currently designing and building a)
229 W( toolkit on top of the Mach system call emulation)228 W
7200 26090 MT
(mechanism which can be used to interpose)
147 W( user code on the BSD 4.3 system call interface.)148 W
7200 27516 MT
(This toolkit is structured in an object-oriented manner, allowing)
63 W( programs to be written in terms)62 W
7200 28942 MT
(of several different layers of objects by utilizing inheritance.  Abstractions exposed at)
109 W( different)110 W
7200 30368 MT
(layers include)
8 W( such objects as pathnames, file descriptors, processes, signals, sockets, devices,)7 W
7200 31794 MT
(etc., as)
412 W( well as the system calls themselves.  The structure of the toolkit permits the)413 W
7200 33220 MT
(programmer to program at whatever levels of abstraction are appropriate for)
147 W( the agent being)146 W
7200 34646 MT
(constructed.)SH
8312 37213 MT
(The base)
352 W( layer of the toolkit handles intercepting the system calls themselves.  Such)353 W
7200 38639 MT
(operations as monitoring system call usage are done at this level.)SH
8312 41206 MT
(The second layer of the toolkit is)
254 W( structured around the primary objects provided by the)253 W
7200 42632 MT
(system call interface.  In BSD 4.3, such objects include pathnames, file descriptors,)
110 W( pids, and)111 W
7200 44058 MT
(process groups.  Such operations as pathname transformations, filesystem)
130 W( usage monitoring,)129 W
7200 45484 MT
(and process usage monitoring are done at this level.)SH
8312 48051 MT
(A third set of toolkit layers focuses on secondary objects)
12 W( provided by the system call interface)13 W
7200 49477 MT
(which are normally used through primary objects.  Such)
323 W( objects include files, directories,)322 W
7200 50903 MT
(symbolic links, devices, pipes, and sockets.  Operations which are specific)
81 W( to these secondary)82 W
7200 52329 MT
(objects such as file encryption, directory transformations, etc. are done by these layers.)SH
13 /Helvetica-Bold AF
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(5. Status)SH
11 /Helvetica AF
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(The bulk of the toolkit, providing for numeric and symbolic system call)
250 W( interception, plus)249 W
7200 59009 MT
(descriptor, open object, and pathname manipulation has been)
306 W( completed.  Several useful)307 W
7200 60435 MT
(agents have already been built.  I am in the midst of writing my)
224 W( Ph.D. dissertation on this)223 W
7200 61861 MT
(research.)SH
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(Table of Contents)SH
12 SS 
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(1. Introduction)SH
53333 XM
(1)SH
11 SS 
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(1.1. Background)SH
53388 XM
(1)SH
11484 11975 MT
(1.2. Motivation for Interposition)SH
53388 XM
(1)SH
11484 13219 MT
(1.3. Problems with Existing Systems)SH
53388 XM
(3)SH
12 SS 
9201 14558 MT
(2. Position Statement)SH
53333 XM
(4)SH
9201 15897 MT
(3. Motivation for an Object-Oriented Toolkit)SH
53333 XM
(4)SH
9201 17236 MT
(4. Design and Structure of the Toolkit)SH
53333 XM
(5)SH
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(5. Status)SH
53333 XM
(5)SH
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(List of Figures)SH
12 SS 
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(Figure 1-1:)
SH( The)
668 W( kernel provides instances of the)
464 W( operating system)465 W
53333 XM
(1)SH
16338 10826 MT
(interface.)SH
9201 12165 MT
(Figure 1-2:)
SH( Both)
668 W( the)
99 W( kernel and interposition agents provide instances)98 W
53333 XM
(2)SH
16338 13504 MT
(of the operating system interface.)SH
9201 14843 MT
(Figure 1-3:)
SH( Like)
668 W( the kernel, agents can)
24 W( share state and provide multiple)25 W
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(2)SH
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(instances of the operating system interface.)SH
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