Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10)

From: Florian Weimer
Date: Fri Jun 14 2019 - 09:14:32 EST

* Mathieu Desnoyers:

> But my original issue remains: if I define a variable called __rseq_handled
> within either the main executable or the preloaded library, it overshadows
> the libc one:
> efficios@compudjdev:~/test/libc-sym$ ./a
> __rseq_handled main: 0 0x56135fd5102c
> __rseq_abi.cpu_id main: 29 0x7fcbeca6d5a0
> efficios@compudjdev:~/test/libc-sym$ LD_PRELOAD=./ ./a
> __rseq_handled 0 0x558f70aeb02c
> __rseq_abi.cpu_id -1 0x7fdca78b7760
> __rseq_handled main: 0 0x558f70aeb02c
> __rseq_abi.cpu_id main: 27 0x7fdca78b7760
> Which is unexpected.

Why is this unexpected? It has to be this way if the main program uses
a copy relocation of __rseq_handled. As long as there is just one
address across the entire program and initializes the copy of the
variable that is actually used, everything will be fine.