Date: Mon, 06 Jun 2022 03:44:32 +0800 From: "Michael Yan Ka Chiu" <nyan@myuji.xyz> To: "Konstantin Belousov" <kostikbel@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: Help needed to get Rust dtrace USDT working on/with FreeBSD linker Message-ID: <51c74c5b-731a-445f-be96-0baf6cd0ebe2@www.fastmail.com> In-Reply-To: <Ypz9gaGQ/Nqja5kF@kib.kiev.ua> References: <f5c27ce0-0dc7-467f-b293-857c74125648@www.fastmail.com> <Ypz9gaGQ/Nqja5kF@kib.kiev.ua>
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[-- Attachment #1 --] 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. [-- Attachment #2 --] <!DOCTYPE html><html><head><title></title><style type="text/css">p.MsoNormal,p.MsoNoSpacing{margin:0}</style></head><body><div>On Mon, Jun 6, 2022, at 3:01 AM, Konstantin Belousov wrote:<br></div><blockquote type="cite" id="qt" style=""><div>On Sun, Jun 05, 2022 at 04:26:37PM +0800, Michael Yan Ka Chiu wrote:<br></div><div>> Hi everyone,<br></div><div>> <br></div><div>> I’m working on a PR to get the Rust Usdt crate working on FreeBSD. This crate<br></div><div>> basically allow adding DTrace probes to rust sources by compiling the probe during<br></div><div>> macro invocation and embed to a custom section (set_dtrace_probe) using inline <br></div><div>> assembly.<br></div><div>> <br></div><div>> The problem I am encountering is that the linker will optimize the compiled probes<br></div><div>> out in the custom section, the only workaround I have found is to force the linker to<br></div><div>> link all the dead code by invoking `-C link-dead-code=yes`. On Illumos, the workaround<br></div><div>> is to reference another section such that the Illumos linker will not throw the probes <br></div><div>> away; however the same workaround does not work on FreeBSD.<br></div><div>> <br></div><div>> I wonder if there're any tricks similar to the Illumos fix, by putting some inline asm there<br></div><div>> to trick the linker and not throw out the probes.<br></div><div>> <br></div><div>> Thanks In advance,<br></div><div>> Michael<br></div><div>> <br></div><div>> References:<br></div><div>> The PR: <a href="https://github.com/oxidecomputer/usdt/pull/63">https://github.com/oxidecomputer/usdt/pull/63</a><br></div><div>> The illumos fix: <a href="https://github.com/oxidecomputer/usdt/blob/eac0fe5f03c3fbf23468ead5cb140f62d51ac3f3/usdt-impl/src/record.rs#L251">https://github.com/oxidecomputer/usdt/blob/eac0fe5f03c3fbf23468ead5cb140f62d51ac3f3/usdt-impl/src/record.rs#L251</a><br></div><div>> <br></div><div>> <br></div><div><br></div><div>GNU as seems to gain support for the "R" flag for sections, which should<br></div><div>prevent them from linker GC. Not sure if llvm toolchain has this, it<br></div><div>requires both as and lld to recognize the flag.<br></div><div><br></div><div>Anyway, try it? See GNU as documentation for the .section directive,<br></div><div>ELF type flags.<br></div></blockquote><div><br></div><div>Thanks! This seems to solve a big part of the issue.<br></div><div><br></div><div>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.<br></div><div><br></div><div>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.<br></div><div><br></div></body></html>
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