From patchwork Fri Nov 4 09:42:23 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Renlin Li X-Patchwork-Id: 17180 Received: (qmail 107005 invoked by alias); 4 Nov 2016 09:42:38 -0000 Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org Delivered-To: mailing list libc-alpha@sourceware.org Received: (qmail 106986 invoked by uid 89); 4 Nov 2016 09:42:37 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-3.2 required=5.0 tests=BAYES_00, KAM_LAZY_DOMAIN_SECURITY, RP_MATCHES_RCVD autolearn=ham version=3.3.2 spammy=GOT, 4925, lp64 X-HELO: foss.arm.com From: Renlin Li To: libc-alpha@sourceware.org Cc: Marcus Shawcroft , Richard Earnshaw Subject: [GLIBC][AARCH64]Rewrite elf_machine_load_address using _DYNAMIC symbol Message-ID: <581C57FF.2090901@foss.arm.com> Date: Fri, 4 Nov 2016 09:42:23 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 Hi all, This patch rewrites aarch64 elf_machine_load_address to use special _DYNAMIC symbol instead of _dl_start. The static address of _DYNAMIC symbol is stored in the first GOT entry. Here is the change which makes this solution work. https://sourceware.org/ml/binutils/2013-06/msg00248.html i386, x86_64 targets use the same method to do this as well. The original implementation relies on a trick that R_AARCH64_ABS32 relocation being resolved at link time and the static address fits in the 32bits. However, in LP64, normally, the address is defined to be 64 bit. Additionally, the original inline assembly is not optimized. It uses 4 instructions including a jump. Optimally, the new implementation here is just two instructions: ldr %1, _GLOBAL_OFFSET_TABLE_ adr %2, _DYNAMIC The size of ld.so is around 130K, so it's save to use ldr, adr to get the address. The address range for those two instruction is +/-1MB. And by the way, this method is ILP32 safe as well. aarch64 linux toolchain regression test OK. OK to commit? Regards, Renlin Li ChangeLog: 2016-11-04 Renlin Li * sysdeps/aarch64/dl-machine.h (elf_machine_load_address): Use _DYNAMIC symbol to calculate load address. diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h index 217e179..b2f6618 100644 --- a/sysdeps/aarch64/dl-machine.h +++ b/sysdeps/aarch64/dl-machine.h @@ -49,25 +49,19 @@ elf_machine_load_address (void) /* To figure out the load address we use the definition that for any symbol: dynamic_addr(symbol) = static_addr(symbol) + load_addr - The choice of symbol is arbitrary. The static address we obtain - by constructing a non GOT reference to the symbol, the dynamic - address of the symbol we compute using adrp/add to compute the - symbol's address relative to the PC. - This depends on 32bit relocations being resolved at link time - and that the static address fits in the 32bits. */ - - ElfW(Addr) static_addr; - ElfW(Addr) dynamic_addr; + _DYNAMIC symbol is used here as its static address is stored in + the special unrelocated first GOT entry. */ + ElfW(Addr) static_addr, dynamic_addr; asm (" \n" -" adrp %1, _dl_start; \n" -" add %1, %1, #:lo12:_dl_start \n" -" ldr %w0, 1f \n" -" b 2f \n" -"1: \n" -" .word _dl_start \n" -"2: \n" - : "=r" (static_addr), "=r" (dynamic_addr)); + "adr %0, _DYNAMIC \n" +#ifdef __LP64__ + "ldr %1, _GLOBAL_OFFSET_TABLE_ \n" +#else + "ldr %w1, _GLOBAL_OFFSET_TABLE_ \n" +#endif + : "=r" (dynamic_addr), "=r" (static_addr)); + return dynamic_addr - static_addr; }