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Re: Dynamically wrap non dynamic symbol


On Thu, Sep 21, 2017 at 02:32:34PM +1200, Christian Gagneraud wrote:
> On 21 September 2017 at 13:54, R0b0t1 <r030t1@gmail.com> wrote:
> > Hello Chris,
> >
> > On Wed, Sep 20, 2017 at 7:28 PM, Christian Gagneraud <chgans@gmail.com> wrote:
> >> Hi there,
> >>
> >> I am trying to wrap function calls, at runtime, in a dynamic way and
> >> without having to modify the original binary.
> >>
> >
> > That makes me think of dyncall: http://www.dyncall.org/.
> >
> >> Basically i have a function "void *install_hook(const char
> >> *symbol_name, void *replacement)" that replace the function associated
> >> with the symbol "symbol_name" with the function pointer "replacement",
> >> and returns the original function pointer.
> >>
> >
> > But this makes me think you want something else, i.e. you want to
> > intercept calls made to a library. Is this right?
> 
> Yes, I can already intercept any calls to any dynamic libraries, by
> looking up the symbol name in the PT_DYNAMIC segment, and modifying
> the jmprel table.
> But i want to do the same for private symbol of the executable.
> 
> >
> >> Using dl_iterate_phdr it is possible to install wrapper around dynamic
> >> symbols, but i would like to do the same for symbols that are local to
> >> the executable.
> >>
> >> Objects are compiled with "gcc -fPIC" and executable is generate with "ld -pie".
> >>
> >> I thought that "ld -pie" will make all the local symbols show up in
> >> the dynamic program section header, but it doesn't.
> >>
> >
> > I think I understand the above but could personally use some
> > explanation of it if you have the time.
> 
> Basically, what i'm trying to do is to force the linker to treat
> private symbols as position independent exported symbol, so that my
> trick above works on private symbols too.
> 
> Actually, --export-dynamic will add all the private executable symbols
> to the PT_DYNAMIC dynsym tables, but won't generate relocation for
> them, so i cannot intercept them.
> 
> Currently the executable code contains eg "callq 0xc15" when calling
> one of this private symbol. I would prefer to have a call into the
> PLT.
> 
> Some context: I'm working on a Qt/C++ profiling tool, and the idea is
> that the user can at any moment requires traces for known overrides of
> a particular object of a given class.
> Since Qt provides introspection i can then deduce the list of symbols
> that i need to intercept in order to profile these calls along the
> class hierarchy (if virtual).

Hmm... I think maybe we are doing the similar thing here(i.e. library
call/syscall intercepts). But I don't really understand what the
"private symbol" means. Are they those hidden by something like
"-fvisibility=hidden" when using  gcc to compile?

Yubin


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