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

[Alan Maguire]

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 at oracle.com>
Signed-off-by: Alan Maguire <alan.maguire at 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