Re: Help needed to get Rust dtrace USDT working on/with FreeBSD linker

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.