[DTrace-devel] [PATCH] bpf: force generating code that all verifiers accept
Kris Van Hees
kris.van.hees at oracle.com
Wed Aug 13 21:18:26 UTC 2025
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.
Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
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
--
2.45.2
More information about the DTrace-devel
mailing list