Re: Help needed to get Rust dtrace USDT working on/with FreeBSD linker
Date: Sun, 05 Jun 2022 19:44:32 UTC
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.