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McKenney" , Boqun Feng , Will Deacon , Dave Watson , Paul Turner , Rich Felker , linux-kernel , linux-api Message-ID: <189377747.3315.1560519247118.JavaMail.zimbra@efficios.com> In-Reply-To: <87wohoti47.fsf@oldenburg2.str.redhat.com> References: <20190503184219.19266-1-mathieu.desnoyers@efficios.com> <1635690189.3049.1560507249693.JavaMail.zimbra@efficios.com> <87tvcsv1pk.fsf@oldenburg2.str.redhat.com> <1190407525.3131.1560516910936.JavaMail.zimbra@efficios.com> <1085273942.3137.1560517301721.JavaMail.zimbra@efficios.com> <87d0jguxdk.fsf@oldenburg2.str.redhat.com> <1779359826.3226.1560518318701.JavaMail.zimbra@efficios.com> <87wohoti47.fsf@oldenburg2.str.redhat.com> Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.138] X-Mailer: Zimbra 8.8.12_GA_3803 (ZimbraWebClient - FF67 (Linux)/8.8.12_GA_3794) Thread-Topic: glibc: Perform rseq(2) registration at C startup and thread creation (v10) Thread-Index: J/z3tq5R2TjIrEv1ISEFn2sEl/wrzw== Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Jun 14, 2019, at 3:24 PM, Florian Weimer fweimer@redhat.com wrote: > * 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. 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