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Authored by: BitOBear on Wednesday, May 16 2012 @ 04:08 AM EDT |
I was under the (obviously false, now that I think on it) impression that Dalvic
and Dexopt where largely written in java. But of course that's clearly wrong
because they do not run themselves.
The example then is the case of MS Office which is written in C.
The three types of code are the same. One could write the two trivial main
functions for the two "separate programs" as...
int main(int argc, char * argv[]) { return StartDalvic(argc,argv);}
and
int main(int argc, char * argv[]) { return StartDexopt(argc,argv);}
and an application that installs another application could have a call to
"StartDexopt(a,b);" where b contained the name of the dex file among
other things. This C++ call would have been forwared by a jini-style
"thunk". ("thunk" is a term of art for a tiny procedure that
promotes the execution of a program from one contex or environment (like Dalvik
code) to another (like the C++ code context that dexopt needs)).
So the application that just downloaded the file (say the Google Market a.k.a.
Google Play, or the browser) while running under Dalvic, passes the
"new_app.dex" file back into the C++ context of Dalvic, so that it, in
turn, can invoke the static linking dexopt pass, and then tell the running
application that the installation was successful.
So an Dalvic is running the fetching application (market or browser) and that is
then re-invoking the same body of code as a separate process/task/program to do
the target installation time work. At this moment the "target" of the
installation is "just data" to the installer. The target is not
running at this time because it -hasn't- been dexopt(ed) and so it isn't a real
application program yet.
This distinction is why you can tell Market to fetch an update and it can say
that the thing it downloaded was "not valid" and so it was not
installed.
A program cannot install itself because it is not yet a program. Only viruses
can break this rule because they are "program fragments" that steal
away and execute themselves as parasites on a running program. This one
exception is the antithesis of "installation time linking" and is not
part of the Android process etc.
Changing the language just changes the names, but it doesn't alter the
technique.[ Reply to This | Parent | # ]
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