Re: GSoC ShlibMemLoad

2008/7/5 Andreas Kupries <akupries@...>:
>
>> > While walking to the office I managed to come up with a number of additional
>> > ideas ...
>> >
>> > printf is different from the other functions in two respects.
>> >
>> >        It takes a variable number of arguments,
>> > and     it operates at a higher IO level than write and consorts.
>> >
>> >        Both might be a possible source of the problem I guess.
>> >        Although I have no idea of how that could be.
>> >
>> > Things to consider:
>> >
>> >        - Does using 'sprintf()' (+write) work ?
>> >                sprintf is also varargs, but doesn't do any I/O
>> sprintf does not work
>> >
>> >        - Does 'fprintf()' work ?
>> >                It is like printf, i.e. varargs and highlevel I/O
>> >                However it explicitly takes an output argument.
>> >                printf (...) is like fprintf (stdout, ...).
>> fprintf does not work
>> >        - Does fflush() work?
>> >                Highlevel I/O, but no varargs. It is also outside
>> >                of the printf family of functions in general.
>> fflush surprisingly works...
>> Anyway, the problem is not in varargs, because I also tried with
>> functions like putchar, puts, etc and they all does not work.
>
> Hm ... All these functions have lots of state ...
>
>
> Some leading questions ...
>
> You are using 'printf' in rtld itself to print log/debug messages, and
> it works there. What is the address of this 'printf' function ?
>
> Is the address of the 'printf' function seen by the loaded
> test-library the same ? (**)
>
> If not, why?
These addresses differ. I am not sure why, but I think that printf in
log/debug messages uses printf from libc.so which was loaded at the
beginning of rtld execution by the regular dynamic loader and printf
in libprintf.so uses printf from libc.so loaded by
load_needed_objects.
> Do you remember my comments about missing symbols ?
>
>        I.e.: The list of libraries we have in our rtld is the list of
>        memory-loaded libraries (ML/lib) and not of all libraries
>        (ALL/lib).
>
>        And a symbol not found in ML/lib has to be looked for in
>        ALL/lib too ?
>
> What does that same distinction mean for 'load_needed_objects' ?
I think that load_needed_objects does not know anything about ALL/lib
and if our library needs another one it is loaded for the second time.
Anyway, all libraries I have tested needed just libc.so.
> And (especially) a library like 'libc.so' which is loaded by the OS
> and thus in ALL/lib?
>
>
> (**) In (one of) the other test libraries you are using a for-loop and
>     write to print the pid. If you encapsulate that into a function
>     FOO to generally print unsigned numbers and then put FOO into the
>     libprintf code you can print any number you need, like function
>     addresses.  Inside of rtld itself it is of course possible to use
>     the working printf to print its address.
>
Because I have already studied this whole code for quite a long time
and have not came up with any solution I am thinking of writing an
email to John Polstra who is an author of the rtld for FreeBSD. Maybe
he will be able to help out. Do you think it is a good idea?

Today I also wrote a simple program which generates defs.h file.

Greetings,
Daniel

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