Re: how does ld.so call ELF's entry?

[Sofiane Akermoun <akersof@xxxxxxxxx>]

The address is passed by the elf loader in something called "auxillary
vectors" structure.
In elf.h you can investigate on a_type/entry type and find that there
is defininition of AT_ENTRY.
As i wrote previsouly if you check in
http://lxr.linux.no/#linux+v3.8.6/fs/binfmt_elf.c you will find how
the elf loader prepare the execution of the interpreter.
To describe the step 4) i mentioned previously, with more accuracy...
the interpreter is executed
by the elf loader as a program loader with known parameters passed on the
stack through Elf32_auxv_t structure. For an easy to understand
explanation you can read an article from phrack 58:
http://www.phrack.org/issues.html?issue=58&id=5#article

regards,

Sofiane Akermoun
alersof@xxxxxxxxx

2013/4/7 ishare <june.tune.sea@xxxxxxxxx>:
> On Sun, Apr 07, 2013 at 06:45:58AM +0200, Sofiane Akermoun wrote:
>> EntryPoint is set in the ELF Header at assembling/linking time.
>
>   how does the interpreter know this address?
>
>>
>> 2013/4/7 ishare <june.tune.sea@xxxxxxxxx>:
>> > On Sat, Apr 06, 2013 at 08:43:16PM -0700, Chris Evans wrote:
>> >> With a far jump to entry offset
>> >
>> >   How does it know the entry point's address?
>> >
>> >>
>> >>
>> >> Sent from my iPad
>> >>
>> >> On Apr 6, 2013, at 8:39 PM, ishare <june.tune.sea@xxxxxxxxx> wrote:
>> >>
>> >> > On Sun, Apr 07, 2013 at 05:11:28AM +0200, Sofiane Akermoun wrote:
>> >> >> Hello,
>> >> >>
>> >> >>
>> >> >> The interpreter is specified during linking process and the program header
>> >> >> of your binary is filled with good values.
>> >> >> Then when the operating system load your binary, he finds next the
>> >> >> interpreter to use.
>> >> >> The linker ld sets the good values by default but you can overwrite it or
>> >> >> specify other values if you want.
>> >> >> The steps are:
>> >> >> 1)The operating system loads your binary
>> >> >> 2)The program loader system execute the Interpreter specifiy in the binary
>> >> >> 3)The interpreter gather all the dynamic libraries needed in memory
>> >> >
>> >> >
>> >> >> 4)The Control is passed to the entry point of your program
>> >> >
>> >> >  How does  the step 4 been  done ?
>> >> >
>> >> >  thanks!
>> >> >>
>> >> >> The entry point is specified in your code source as a "global". And could
>> >> >> be find in your object file by the linker.
>> >> >> In theory there are some default tag to specify entry point, like "..start"
>> >> >> for nasm, but you can also passed it to the linker.
>> >> >>
>> >> >> regards,
>> >> >>
>> >> >> Sofiane Akermoun
>> >> >> akersof@xxxxxxxxx
>> >> >>
>> >> >>
>> >> >>
>> >> >>
>> >> >>
>> >> >> 2013/4/7 ishare <june.tune.sea@xxxxxxxxx>
>> >> >>
>> >> >>>
>> >> >>>  For an ELF ,which needs a interpreter , how is it  been called by the
>> >> >>> interpreter ?
>> >> >>>
>> >> >>>  As I know the interpreter is loaded first and do something essential
>> >> >>> ,then call the main routine of ELF .
>> >> >>>  How is this procedure implemented ?
>> >> >>>
>> >> >>>  Thanks!
>> >> >>> --
>> >> >>> To unsubscribe from this list: send the line "unsubscribe linux-assembly"
>> >> >>> in
>> >> >>> the body of a message to majordomo@xxxxxxxxxxxxxxx
>> >> >>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> >> >>
>> >> >>
>> >> >>
>> >> >> --
>> >> >> Sofiane AKERMOUN
>> >> >> akersof@xxxxxxxxx
>> >> > --
>> >> > To unsubscribe from this list: send the line "unsubscribe linux-assembly" in
>> >> > the body of a message to majordomo@xxxxxxxxxxxxxxx
>> >> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> > --
>> > To unsubscribe from this list: send the line "unsubscribe linux-assembly" in
>> > the body of a message to majordomo@xxxxxxxxxxxxxxx
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>>
>>
>>
>> --
>> Sofiane AKERMOUN
>> akersof@xxxxxxxxx



-- 
Sofiane AKERMOUN
akersof@xxxxxxxxx
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