From nobody Mon Jun 6 19:40:05 2022 X-Original-To: freebsd-hackers@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 006A41BD03F0 for ; Mon, 6 Jun 2022 19:40:14 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4LH3jn0YlWz54QS for ; Mon, 6 Jun 2022 19:40:12 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.16.1/8.16.1) with ESMTPS id 256Je6NN075538 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Mon, 6 Jun 2022 22:40:09 +0300 (EEST) (envelope-from kostikbel@gmail.com) Received: (from kostik@localhost) by tom.home (8.16.1/8.16.1/Submit) id 256Je5Xk075510; Mon, 6 Jun 2022 22:40:05 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Mon, 6 Jun 2022 22:40:05 +0300 From: Konstantin Belousov To: Michael Yan Ka Chiu Cc: freebsd-hackers@freebsd.org Subject: Re: Help needed to get Rust dtrace USDT working on/with FreeBSD linker Message-ID: References: <51c74c5b-731a-445f-be96-0baf6cd0ebe2@www.fastmail.com> List-Id: Technical discussions relating to FreeBSD List-Archive: https://lists.freebsd.org/archives/freebsd-hackers List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-freebsd-hackers@freebsd.org MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <51c74c5b-731a-445f-be96-0baf6cd0ebe2@www.fastmail.com> X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FROM, NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.5 X-Spam-Checker-Version: SpamAssassin 3.4.5 (2021-03-20) on tom.home X-Rspamd-Queue-Id: 4LH3jn0YlWz54QS X-Spamd-Bar: / Authentication-Results: mx1.freebsd.org; dkim=none; dmarc=fail reason="No valid SPF, No valid DKIM" header.from=gmail.com (policy=none); spf=softfail (mx1.freebsd.org: 2001:470:d5e7:1::1 is neither permitted nor denied by domain of kostikbel@gmail.com) smtp.mailfrom=kostikbel@gmail.com X-Spamd-Result: default: False [0.36 / 15.00]; ARC_NA(0.00)[]; RCVD_TLS_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FREEMAIL_FROM(0.00)[gmail.com]; NEURAL_HAM_LONG(-1.00)[-0.999]; MIME_GOOD(-0.10)[text/plain]; HAS_XAW(0.00)[]; R_SPF_SOFTFAIL(0.00)[~all:c]; NEURAL_SPAM_MEDIUM(0.81)[0.813]; TO_MATCH_ENVRCPT_SOME(0.00)[]; NEURAL_SPAM_SHORT(0.54)[0.544]; RCPT_COUNT_TWO(0.00)[2]; MLMMJ_DEST(0.00)[freebsd-hackers]; FROM_EQ_ENVFROM(0.00)[]; R_DKIM_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; ASN(0.00)[asn:6939, ipnet:2001:470::/32, country:US]; RCVD_COUNT_TWO(0.00)[2]; FREEMAIL_ENVFROM(0.00)[gmail.com]; DMARC_POLICY_SOFTFAIL(0.10)[gmail.com : No valid SPF, No valid DKIM,none] X-ThisMailContainsUnwantedMimeParts: N On Mon, Jun 06, 2022 at 03:44:32AM +0800, Michael Yan Ka Chiu wrote: > On Mon, Jun 6, 2022, at 3:01 AM, Konstantin Belousov wrote: > > On Sun, Jun 05, 2022 at 04:26:37PM +0800, Michael Yan Ka Chiu wrote: > > > Hi everyone, > > > > > > I’m working on a PR to get the Rust Usdt crate working on FreeBSD. This crate > > > basically allow adding DTrace probes to rust sources by compiling the probe during > > > macro invocation and embed to a custom section (set_dtrace_probe) using inline > > > assembly. > > > > > > The problem I am encountering is that the linker will optimize the compiled probes > > > out in the custom section, the only workaround I have found is to force the linker to > > > link all the dead code by invoking `-C link-dead-code=yes`. On Illumos, the workaround > > > is to reference another section such that the Illumos linker will not throw the probes > > > away; however the same workaround does not work on FreeBSD. > > > > > > I wonder if there're any tricks similar to the Illumos fix, by putting some inline asm there > > > to trick the linker and not throw out the probes. > > > > > > Thanks In advance, > > > Michael > > > > > > References: > > > The PR: https://github.com/oxidecomputer/usdt/pull/63 > > > The illumos fix: https://github.com/oxidecomputer/usdt/blob/eac0fe5f03c3fbf23468ead5cb140f62d51ac3f3/usdt-impl/src/record.rs#L251 > > > > > > > > > > GNU as seems to gain support for the "R" flag for sections, which should > > prevent them from linker GC. Not sure if llvm toolchain has this, it > > requires both as and lld to recognize the flag. > > > > Anyway, try it? See GNU as documentation for the .section directive, > > ELF type flags. > > Thanks! This seems to solve a big part of the issue. > > If i pass “cargo:rustc-link-arg=-Xlinker” and “cargo:rustc-link-arg=—no-gc-sections” it does prevent the linker from removing the probes. > > I am still looking for solutions that does not involve explicit involvement of the flags by the crate consumer, and maybe something not turning gc off entirely but this is a great progress. Did you tried the "R" section flag, as I noted above?