[Bug malloc/25533] dlopen with RTLD_DEEPBIND crashes when using custom memory manager

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=25533

--- Comment #8 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Simon Richter from comment #7)
> Rereading the documentation of RTLD_DEEPBIND, I'm even slightly confused why
> applying RTLD_DEEPBIND to an object will also pull the symbols of its
> dependencies to the front (so glibc's "realloc" wins over libtbbmalloc's, as
> libudev is only linked against glibc). I would have expected libudev's
> symbols to take precedence for internal resolution, and then the same order
> as in the rest of the program for the other symbols.

We discussed this during today's patch review call. The issue here is that some
(many?) uses of RTLD_DEEPBIND depend on this altered symbol resolution
behavior: the newly loaded object is supposed to bind against the symbols of
its dependencies as if there was nothing else in the process image, to avoid
unwanted preemption. In some cases, this is even done to prevent preemption
coming from LD_PRELOAD. For malloc, this is pretty much never going to work,
but it might be exactly what's needed for other libc symbols. It's hard to tell
where to draw the line.

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