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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 > > --- > > 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