[Bug dynamic-link/30007] rfe: dlopen to user buffer or to specified address

["stsp at users dot sourceforge.net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30007

--- Comment #4 from Stas Sergeev <stsp at users dot sourceforge.net> ---
(In reply to Adhemerval Zanella from comment #3)
> This seems a very specific usercase that I am not sure if it fits on usual
> case of dlopen usage.  Mapping to a preferred address basically defeats ASRL
> and has other issues: what this dlopen extension should do if the required
> mapping is not large enough, if the address is already occupied, or if the
> mapping does not have the correct permissions?

My current plan (which may of course
change) is to have the LD_AUDIT func
that will tell the length and get the
address back. The user have to make sure
there is no any mapping at that address
for a specified length (I am looking in
/proc/self/maps to find the needed hole).
If eventually it appears the hole is not
large enough, dlopen() should just fail.
That basically addresses the aforementioned
concern of yours.
After dlopen() succeeded, knowing the
length I will mmap(MAP_SHARED) another
buffer, memcpy() the solib there and
mmap() it back to its addr, but in a
shared buffer. So basically all the dirty
work is on my side, dlopen() should only
tell me the length and get the address back.

> In the case of passing mapping not sufficing

Passing mapping was just an option to
consider. I already realized it would
be a bad extension, so currently my plan
is to pass only address in response to
the length, and then, knowing length,
create that "other mapping" by hands,
then memcpy(), then remap back. More work
for me, but much smaller extension to glibc.


> We used to have hacks to force mmap to load executables to 32-bits, it was
> added to overcome some particular architecture limitations, and it has
> caused some issues and it was eventually removed (check
> ea5814467a02c9d2d7608b6445c5d60e2a81d3ee).

Wow, just recently removed!
Quite sad I haven't had a chance to try
it out... But I think we can come up with
something much more flexible. Which is why
I created that ticket before actually
prototyping the thing myself. Eg
LD_PREFER_MAP_32BIT_EXEC wouldn't give me
the length, so I'd have difficulties wrapping
that into a shared mapping.
I really aim for something very small and
simple on a glibc side, with more work on
a user's side. But if glibc isn't helpful,
I'll need to implement/use alternative
dynamic linker, which would be quite bad.

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