Re: [DTrace-devel] [PATCH] bpf: force generating code that all verifiers accept

On Thu, Aug 14, 2025 at 08:51:21PM +0100, Sam James wrote:
> Kris Van Hees via DTrace-devel <[email protected]> writes:
> 
> > The compiler could optimize val = *valp in a way where the verifier on
> > older kernels would complain.  We use inline assembler to force the
> > not optimize this expression and instead to always read the value as a
> > scalar.
> 
> I'd mention a known-bad kernel version and some testcase which
> definitely hit it, so we know when we can remove it in future.

Thanks for the feedback - good idea.

> > Signed-off-by: Kris Van Hees <[email protected]>
> > ---
> >  bpf/get_dvar.c | 19 ++++++++++++++++++-
> >  1 file changed, 18 insertions(+), 1 deletion(-)
> >
> > diff --git a/bpf/get_dvar.c b/bpf/get_dvar.c
> > index 073cca57c..aa14eca58 100644
> > --- a/bpf/get_dvar.c
> > +++ b/bpf/get_dvar.c
> > @@ -150,7 +150,24 @@ noinline void *dt_get_assoc(uint32_t id, const char *tuple, uint64_t store,
> >  		if (valp == 0)
> >  			return dt_no_dvar();
> >  		*valp = (uint64_t)valp;
> > -		val = *valp;
> > +		/*
> > +		 * We used to do:
> > +		 *	val = *valp;
> > +		 * but the compiler could use knowledge that *valp is valp from
> > +		 * the assignment above, and use that same value (whith is a
> > +		 * map_value address).  Older kernels do not allow a map_value
> > +		 * address to be used as map key, and a verifier failure would
> > +		 * be triggered by this code optimization.
> > +		 *
> > +		 * We use inline assembler to force reading the value from the
> > +		 * map value rather than allowing the compiler to optimize this
> > +		 * code.  This works for all kernels.
> > +		 */
> > +		asm ("ldxdw %0, %1" \
> > +			: "=r" (val) \
> > +			: "m" (*valp) \
> > +			: /* no clobber */
> > +		);
> >  	} else {
> >  		/*
> >  		 * Record the value (used as key into the dvars map), and if we