----- On Jun 14, 2019, at 3:24 PM, Florian Weimer fweimer@xxxxxxxxxx wrote:
> * Mathieu Desnoyers:
>
>> ----- On Jun 14, 2019, at 3:09 PM, Florian Weimer fweimer@xxxxxxxxxx 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.
I printed the address within rseq_init (), which happened to be invoked
by the linker startup waaaay too early. I followed your advice and moved
the rseq_init () invocation after linker re-relocation:
diff --git a/elf/rtld.c b/elf/rtld.c
index f29f284a7c..66b0894f9d 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -1410,9 +1410,6 @@ ERROR: '%s': cannot process note segment.\n", _dl_argv[0]);
/* Assign a module ID. Do this before loading any audit modules. */
GL(dl_rtld_map).l_tls_modid = _dl_next_tls_modid ();
- /* Publicize rseq registration ownership. */
- rseq_init ();
-
/* If we have auditing DSOs to load, do it now. */
bool need_security_init = true;
if (__glibc_unlikely (audit_list != NULL)
@@ -2284,6 +2281,11 @@ ERROR: ld.so: object '%s' cannot be loaded as audit interface: %s; ignored.\n",
HP_TIMING_ACCUM_NT (relocate_time, add);
}
+ /* Publicize rseq registration ownership. This must be performed
+ after rtld re-relocation, before invoking constructors of
+ preloaded libraries. */
+ rseq_init ();
+
/* Do any necessary cleanups for the startup OS interface code.
We do these now so that no calls are made after rtld re-relocation
which might be resolved to different functions than we expect.
It works fine now!
LD_PRELOAD=./s.so ./a
elf: __rseq_handled addr: 56300f0a402c
__rseq_handled s.so: 1 0x56300f0a402c
__rseq_abi.cpu_id s.so: -1 0x7fad2ff58760
__rseq_handled main: 1 0x56300f0a402c
__rseq_abi.cpu_id main: 27 0x7fad2ff58760
Thanks!
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
