[DTrace-devel] [PATCH] Fix dt_bvar_probedesc() for late USDT processes

[eugene.loh at oracle.com]

From: Eugene Loh <eugene.loh at oracle.com>

With commit 8bd26415b
("bpf: separate bvar implementation into separate functions"),
test/unittest/usdt/tst.nusdtprobes.sh started failing reproducibly
on all platforms.

In that patch, the get_bvar() function is factored into separate
functions.  It includes a change that looks basically like this:

    uint32_t    key = mst->prid;

    if (key < ((uint64_t)&NPROBES)) {
        [...]
    } else {
        char *s = bpf_map_lookup_elem(&usdt_names, &key);
        switch (idx) {
  -     case DIF_VAR_PROBENAME: s += DTRACE_FUNCNAMELEN;
  +     case DIF_VAR_PROBEPROV: s += DTRACE_FUNCNAMELEN;
  -     case DIF_VAR_PROBEFUNC: s += DTRACE_MODNAMELEN;
  +     case DIF_VAR_PROBEMOD : s += DTRACE_MODNAMELEN;
  -     case DIF_VAR_PROBEMOD : s += DTRACE_PROVNAMELEN;
  +     case DIF_VAR_PROBEFUNC: s += DTRACE_PROVNAMELEN;
  -     case DIF_VAR_PROBEPROV:
  +     case DIF_VAR_PROBENAME:
        }
        return (uint64_t)s;
    }

That is, for the case of key>=NPROBES (that is, for USDT probes that
were added after the dtrace session was started), the meanings of
prov, mod, func, and name were exchanged.

Restore the correct meanings.

Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
---
 bpf/get_bvar.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/bpf/get_bvar.c b/bpf/get_bvar.c
index c760126da..fadb06c00 100644
--- a/bpf/get_bvar.c
+++ b/bpf/get_bvar.c
@@ -185,13 +185,13 @@ noinline uint64_t dt_bvar_probedesc(const dt_dctx_t *dctx, uint32_t idx)
 			return (uint64_t)dctx->strtab;
 
 		switch (idx) {
-		case DIF_VAR_PROBEPROV:
+		case DIF_VAR_PROBENAME:
 			s += DTRACE_FUNCNAMELEN;
-		case DIF_VAR_PROBEMOD:
-			s += DTRACE_MODNAMELEN;
 		case DIF_VAR_PROBEFUNC:
+			s += DTRACE_MODNAMELEN;
+		case DIF_VAR_PROBEMOD:
 			s += DTRACE_PROVNAMELEN;
-		case DIF_VAR_PROBENAME:
+		case DIF_VAR_PROBEPROV:
 		}
 
 		return (uint64_t)s;
-- 
2.43.5