Date: Mon, 6 Jun 2022 22:40:05 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Michael Yan Ka Chiu <nyan@myuji.xyz> Cc: freebsd-hackers@freebsd.org Subject: Re: Help needed to get Rust dtrace USDT working on/with FreeBSD linker Message-ID: <Yp5YFVBB/UFnvnZM@kib.kiev.ua> In-Reply-To: <51c74c5b-731a-445f-be96-0baf6cd0ebe2@www.fastmail.com> References: <f5c27ce0-0dc7-467f-b293-857c74125648@www.fastmail.com> <Ypz9gaGQ/Nqja5kF@kib.kiev.ua> <51c74c5b-731a-445f-be96-0baf6cd0ebe2@www.fastmail.com>
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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?
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