From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.4 Received: from second.openwall.net (second.openwall.net [193.110.157.125]) by inbox.vuxu.org (Postfix) with SMTP id F0D9721773 for ; Thu, 9 May 2024 15:51:27 +0200 (CEST) Received: (qmail 32662 invoked by uid 550); 9 May 2024 13:51:22 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 32614 invoked from network); 9 May 2024 13:51:22 -0000 Date: Thu, 9 May 2024 09:51:35 -0400 From: Rich Felker To: Maxim Blinov Cc: musl@lists.openwall.com Message-ID: <20240509135135.GR10433@brightrain.aerifal.cx> References: <20240509123516.GQ10433@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] IFUNC support On Thu, May 09, 2024 at 02:16:44PM +0100, Maxim Blinov wrote: > Hi Rich, thanks for your reply, > > > It sounds like you have an XY problem: wanting target_clones to work. > > The way I got into the subject of relocs and IFUNCs, is that clang for > musl RISC-V outputs binaries that generate these relocs, and one of > the binaries was a test case with resolve_multiver in it. either the > compiler or musl was wrong, and i initially guessed (incorrectly) that > musl was at fault. > > > If GCC was built correctly targeting musl, it should not support ifunc > > generation at all; you shouldn't end up with unknown relocations in an > > output binary because the compiler should never have emitted them. > > That's my conclusion aswell. so far in my testing when building > something with target_clones, or resolve_multiver, I see: > > - gcc for musl, x86_64: errors out > - gcc for musl, riscv: generates binary with IFUNCs > - clang for musl, x86_64: generates binary with IFUNCs > - clang for musl, riscv: generates binary with IFUNCs > > For the LLVM side, I've opened an issue against LLVM about this > (although I'm still not 200% sure its not a misconfiguration on my > end.), link: https://github.com/llvm/llvm-project/issues/91313. LLVM > currently appears to generate IFUNCs regardless. i admit i haven't yet > properly dug around in a debugger to figure out why. > > For the gcc side, the reason i believe is as below: > > gcc x86_64 does the right thing: gcc/configure.ac imports > gcc/config.gcc, which has logic[1] that turns off IFUNC support if > we're targeting a triple that ends in `musl`. The resultant compiled > gcc will error out if you try to use the feature. This is applied to > all triples ending in `musl`, so in theory that should be the end of > discussion. > > but gcc for *RISC-V* doesn't, because the logic for checking whether > or not we have support for IFUNCS is overridden *after* gcc/config.gcc > has been parsed, in gcc/configure.ac [2], by: > - assembling a test assembly file with a `.type foo, %gnu_indirect_function`, > - linking, > - objdumping the binary and greping for `R_RISCV_IRELATIVE` > > well, i suppose outsourcing the logic to gnu ld is not unreasonable, > but does it mean that gnu ld targeting anything ending in `musl` > should throw up at the sight of `gnu_indirect_function`? > > [1]: https://github.com/gcc-mirror/gcc/blob/2790195500ec523cad9c7292816540e2fc19f456/gcc/config.gcc#L3670-L3684 > [2]: https://github.com/gcc-mirror/gcc/blob/2790195500ec523cad9c7292816540e2fc19f456/gcc/configure.ac#L3057-L3107 I suspect ld tries to avoid having too much of this kind of C implementation policy knowledge. Mechanically, an ELF-based target is able to represent IFUNC relocations even if they won't be usable at runtime; it's generally the compiler's responsibility to produce code that's compatible with the target libc. I think the order of the probes should be inverted, or the second probe should just be skipped if ifunc was already disabled by the first. What ld wants to do with this is a matter independent of the logic bug in gcc's configure. Rich