Ping! malloc bug, and PIC and flat shared library support for m68k

Richard Sandiford richard at codesourcery.com
Wed Nov 29 09:15:49 UTC 2006


Ping!

Richard Sandiford <richard at codesourcery.com> writes:
> Mike Frysinger <vapier at gentoo.org> writes:
>> On Friday 17 November 2006 13:27, Richard Sandiford wrote:
>>> Are you asking instead whether normal .init_array and .fini_array
>>> sections are handled by the patch as-is?  If so, the answer's yes.
>>> It handles both .init/.fini sections and .init_array/.fini_array
>>> sections, much as __uClibc_main.c already does for "static"
>>> uClinux executables.
>>
>> i meant, we have all this code in place already to handle
>> __preinit_array_{end,start} in __uClibc_main.c for static binaries and the
>> pointers to these things are provided implicitly by the linker
>
> Oh, sorry, I hadn't realised it was specifically __preinit_array_*
> you're asking about.  The reason for that is:
>
>         /* .preinit_array is usually only supported for executables.
>          * However, the distinction between the executable and its
>          * shared libraries isn't as pronounced for flat files; a shared
>          * library is really just a part of an executable that can be
>          * shared with other executables.  We therefore allow
>          * .preinit_array to be used in libraries too.  */
>
> In other words, I wanted to handle an executable with shared flat libraries
> in the same way as a stand-alone executable would be handled.
>
>> in my mind, there should be no sep code to handle init/fini code with
>> shared flat ... so what am i missing ?  is it that since handling of
>> shared flat code is such a bastardize project (needed in order to stay
>> so damn small/lean compared to ELF), the init/fini arrays cannot be
>> provided by the linker and must be set up by the kernel and passed
>> down into userspace ?
>
> If you're asking about having no separate init/fini code at all,
> the reason is this:
>
> If you have a flat executable that uses shared libraries 0..n-1,
> the kernel's set-up procedure is as follows:
>
>   - populate the stack with argc, argc, envp etc.
>   - push the executable's entry point on the stack
>   - push shared library n-1's entry point on the stack
>   - ...
>   - push shared library 1's entry point on the stack
>   - transfer control to shared library 0's entry point
>
> Thus each shared library gets a chance to do some initialisation,
> then "returns" to the next startup routine.  This can't really
> be changed without changes to the file format.
>
> It's therefore not possible for each shared library's entry point to
> call a local __uClibc_main (which would have linker-resolved references
> to the library's initialisation and finalisation data).  If we did, one
> library would be finalised before the next is initialised.
>
> I think the only way to get the existing __uClibc_main to do all
> initialisation and finalisation is to call it once in the executable
> and have it somehow do the library's initialisation and finalisation
> too.  (This is not what would happen for ELF shared libraries,
> of course.)  And the only ways I can see of doing that are:
>
>   (1) For each shared library, insert functions into the executable's
>       .init and .fini that call the library's .init and .fini functions.
>       Create functions that iterate over the library's .preinit_array,
>       .init_array and .fini_array arrays, and insert pointers to these
>       functions in the executable's arrays.
>
>   (2) Link the contents of each library's initialisation and finalisation
>       sections into the executable rather than the library itself.
>
> (1) doesn't really achieve the goal of keeping everything in __uClibc_main
> because you still have separate array walking routines.  (2) doesn't really
> achieve the goal of shared libraries because you're duplicating information
> in each executable.
>
> Also, I think the only practical way of implementing (1) and (2) is to
> have two separate ELF objects for each shared library: the library itself
> (which is linked into the executable with -R), and a file containing the
> init/fini data that should be linked normally into the executable.
>
> Richard
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