[cfe-dev] [llvm-dev] AST of whole program

Wed Oct 5 02:54:06 PDT 2016

There are plenty of "not a library" places where re-ordering/changing the
data structure will cause trouble. For example storing data in binary form
in a file (and plenty of simple applications just write the struct data
straight to a file, so order is important) - rebuilding the app, or having
multiple applications that are built using the same header.

Applications that share content using shared memory or memory mapped files
would be another case where re-ordering the data in any unpredictable way
will be disastrous. (I've written applications where there are several
instances of the actual application, and then a separate status
application, where the actual application is storing "what I'm doing right
now" in shared memory, and the status application reads a snapshot every
second or so, displaying any changes in what's going on in each of the
applications)

So for sure, this sort of thing needs to be done in a way that can either
be predictable over two different binaries, and/or be possible to turn off
for certain data structures.

--
Mats

On 5 October 2016 at 05:55, Vedant Kumar via cfe-dev <cfe-dev at lists.llvm.org
> wrote:

> > I am trying to change the layout of fields(randomize) in a struct in a c
> program.
>
> Have you taken a look at this paper?
>
>     https://www.utdallas.edu/~zxl111930/file/DIMVA09.pdf
>
> The authors describe their approach to data structure layout randomization
> in
> some detail, including pros/cons of implementing the feature at the AST
> level.
> There are some other interesting bits, like their decision to introduce an
> explicit obfuscation struct attribute.
>
> > ... there are structs that I should not touch like structs defined in
> libraries.
>
> Having an explicit "reorder" attribute is one way to work around this.
>
> > So if I randomize a struct in one compilation unit and then realize that
> actually, I shouldn't have randomized it when clang was working on another
> compilation unit, there is no way to go back and revert the layout of the
> struct that I already randomized in previous translation unit because it is
> already over.
>
> It sounds like you need some rule that tells you whether or not it's OK to
> randomize a struct, regardless of how many translation units you have
> already
> processed.
>
> > So what I am thinking is I should look all the translation units in AST
> level before they create llvm IR and decide which structs I should
> randomize, then randomize the structs I have decided to randomize, then let
> clang to create llvm IR using modified ASTs.
>
> Let's say you take this approach, and you collate the AST's for every
> source
> file in a project. What information do you plan on gathering that will
> help you
> determine the right structs to reorder? Can you guarantee that your
> decision
> procedure will never reorder a struct that is not meant to be reordered,
> and
> will always reorder all other structs?
>
> > I am trying to transform programs like apache.
>
> I suspect that you'd need to manually audit that codebase and apply
> "reorder"
> attributes to get good results. I could be wrong though :).
>
> > Also I am not sure about one thing. Can I make sure that a struct is
> defined in a library or in the source code of the program by looking only
> one translation unit without any false flag? If I can, then there is no
> need for what I am asking for.
>
> In the paper I linked to, the authors mention several other conditions
> under
> which it's inappropriate to randomize structs.
>
> vedant
>
> > On Oct 4, 2016, at 6:57 PM, Anil Altinay <aaltinay101 at gmail.com> wrote:
> >
> > Hi Vedant,
> >
> > What kind of transformation are you interested in, and what kind of
> programs
> > are you looking to transform?
> >
> > I am trying to change the layout of fields(randomize) in a struct in a c
> program. I already figured out how to change the layout of fields in a
> struct but there are structs that I should not touch like structs defined
> in libraries. So if I randomize a struct in one compilation unit and then
> realize that actually, I shouldn't have randomized it when clang was
> working on another compilation unit, there is no way to go back and revert
> the layout of the struct that I already randomized in previous translation
> unit because it is already over. So what I am thinking is I should look all
> the translation units in AST level before they create llvm IR and decide
> which structs I should randomize, then randomize the structs I have decided
> to randomize, then let clang to create llvm IR using modified ASTs.
> >
> > I am trying to transform programs like apache.
> >
> > By 'AST of whole program', do you mean AST's for the source from all
> libraries
> > linked into the program?
> >
> > I am not sure if I understand your question but I will try to explain
> what I meant. For each translation unit, AST gets created. The problem is I
> can only see AST of current translation unit. I cannot see AST of next
> translation unit because clang works on one translation unit at a time.
> Maybe I should dump AST of each translation unit to the disk, decide which
> structs can be randomized, change the AST on the disk and start compilation
> from modified ASTs. But this may be so slow and I do not really know how I
> can do this.
> >
> > Also I am not sure about one thing. Can I make sure that a struct is
> defined in a library or in the source code of the program by looking only
> one translation unit without any false flag? If I can, then there is no
> need for what I am asking for.
> >
> > I hope that I explained what I am trying to do clearly. If you have any
> suggestion how I should do this, I would really appreciate hearing your
> opinion. Thank you very much for your quick response.
> >
> > Anil
> >
> >
> >
> >
> > On Tue, Oct 4, 2016 at 5:58 PM, Vedant Kumar <vsk at apple.com> wrote:
> > - llvm-dev, + cfe-dev
> >
> > Hi,
> >
> > What kind of transformation are you interested in, and what kind of
> programs
> > are you looking to transform?
> >
> > By 'AST of whole program', do you mean AST's for the source from all
> libraries
> > linked into the program?
> >
> > vedant
> >
> >
> > > On Oct 4, 2016, at 5:50 PM, Anil Altinay via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > >
> > > Hello,
> > >
> > > I would like to do transformations on AST of a c program but I need to
> have access to all ASTs created for the program to do right changes. LLVM
> processes one translation unit at a time and because of it, I do not have
> access to AST of all the translation units at the same time. Do you have
> any suggestion how I can access all the ASTs created for a program, do
> analysis on the ASTs and do modifications on the ASTs?
> > >
> > > As a summary:
> > >
> > >       • I need to have access to ASTs of the program at the same time.
> > >       • Do analysis on ASTs.
> > >       • Modify ASTs based on my analysis and create llvm IR from
> modified ASTs.
> > > Thank you,
> > > Anil
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > llvm-dev at lists.llvm.org
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
>
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