[PATCH] cg: fix offset for > 8 bit bitfields in dt_cg_ctf_offsetof()

Alan Maguire
2025-08-26T08:12:29Z

The tcp provider uses dt_cg_tramp_get_member() to retrieve the
offset of the sk_protocol field in struct sock. However it
returns the wrong value on UEK6 since it is an 8-bit bitfield.
From pahole we see:

unsigned int __sk_flags_offset[0]; /* 560 0 */
unsigned int sk_padding:1; /* 560: 0 4 */
unsigned int sk_kern_sock:1; /* 560: 1 4 */
unsigned int sk_no_check_tx:1; /* 560: 2 4 */
unsigned int sk_no_check_rx:1; /* 560: 3 4 */
unsigned int sk_userlocks:4; /* 560: 4 4 */
unsigned int sk_protocol:8; /* 560: 8 4 */

In other words it is really at offset 561 but because we just
lookup the member offset and not the member type offset we get the
wrong value for the sk_protoocol.

This in turn causes tcp state-change probes (and in-progress UDP
probes) to not fire since we verify that sk_protocol == IPPROTO_TCP.

The fix is to look up the member _type_ offset and add it to the
bit offset we get for the member itself. With this in place the
state-change probes fire, but the local tcp tests still fail due
to separate issues with the tcp:::accept-established probe.

This issue is not seen on more recent kernels because sk_protocol
becomes a __u16 as the number of protocols exceeds 256.

Reported-by: Eugene Loh <eugene.loh@oracle.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
libdtrace/dt_cg.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index cd9e7f4e..7abb5bc6 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -1959,6 +1959,8 @@ dt_cg_ctf_offsetof(const char *structname, const char *membername,
dtrace_typeinfo_t sym;
ctf_file_t *ctfp;
ctf_membinfo_t ctm;
+ ctf_encoding_t cte;
+ int offset;

if (dtrace_lookup_by_type(yypcb->pcb_hdl, DTRACE_OBJ_EVERY, structname,
&sym))
@@ -1973,6 +1975,13 @@ dt_cg_ctf_offsetof(const char *structname, const char *membername,

longjmp(yypcb->pcb_jmpbuf, EDT_NOCTF);
}
+ offset = ctm.ctm_offset;
+
+ /* a bitfield may have an additional > 8 bit offset which means we need
+ * to adjust the reported byte offset.
+ */
+ if (ctf_type_encoding(ctfp, ctm.ctm_type, &cte) != CTF_ERR)
+ offset += cte.cte_offset;

if (sizep || ldopp) {
uint_t ldop;
@@ -1982,7 +1991,7 @@ dt_cg_ctf_offsetof(const char *structname, const char *membername,
*ldopp = ldop;
}

- return (ctm.ctm_offset / NBBY);
+ return (offset / NBBY);
}
static void
dt_cg_act_breakpoint(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind)
--
2.43.5