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

* Mathieu Desnoyers:

----- On Jun 14, 2019, at 3:09 PM, Florian Weimer fweimer@redhat.com wrote:

quoted

* Mathieu Desnoyers:

quoted

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=./s.so ./a
__rseq_handled s.so: 0 0x558f70aeb02c
__rseq_abi.cpu_id s.so: -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 ld.so initializes the copy of the
variable that is actually used, everything will be fine.

Here is a printout of the __rseq_handled address observed by ld.so, it
does not match:

LD_PRELOAD=./s.so ./a
elf: __rseq_handled addr: 7f501c98a140
__rseq_handled s.so: 0 0x55817a88d02c
__rseq_abi.cpu_id s.so: -1 0x7f501c983760
__rseq_handled main: 0 0x55817a88d02c
__rseq_abi.cpu_id main: 27 0x7f501c983760

Where do you print the address?  Before or after the self-relocation of
the dynamic loader?  The address is only correct after self-relocation.

Thanks,
Florian