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

[Florian Weimer <fweimer at redhat dot com>]

* Mathieu Desnoyers:

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