[DTrace-devel] [PATCH] cg: allow providers to specify a skip count for stack retrieval

[Kris Van Hees]

Some probes cause extra frames to be reported with bpf_get_stack() that
need to be skipped to ensure we report the correct stack trace to the
consumer.  FBT probes based on fentry/fexit probes need this.

Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 libdtrace/dt_cg.c       | 6 ++++--
 libdtrace/dt_prov_fbt.c | 1 +
 libdtrace/dt_provider.h | 1 +
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index 27246a40..36975406 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -2595,7 +2595,8 @@ dt_cg_act_stack_sub(dt_pcb_t *pcb, dt_node_t *dnp, int reg, int off, dtrace_actk
 	dt_irlist_t	*dlp = &pcb->pcb_ir;
 	dt_regset_t	*drp = pcb->pcb_regs;
 	uint64_t	arg;
-	int		nframes, stacksize, prefsz, align = sizeof(uint64_t);
+	int		nframes, stacksize, prefsz, skip;
+	int		align = sizeof(uint64_t);
 	uint_t		lbl_valid = dt_irlist_label(dlp);
 
 	/* Get sizing information from dnp->dn_arg. */
@@ -2603,6 +2604,7 @@ dt_cg_act_stack_sub(dt_pcb_t *pcb, dt_node_t *dnp, int reg, int off, dtrace_actk
 	prefsz = kind == DTRACEACT_USTACK ? sizeof(uint64_t) : 0;
 	nframes = DTRACE_USTACK_NFRAMES(arg);
 	stacksize = nframes * sizeof(uint64_t);
+	skip = yypcb->pcb_probe->prov->impl->skip_frames;
 
 	/* Handle alignment and reserve space in the output buffer. */
 	if (reg >= 0) {
@@ -2644,7 +2646,7 @@ dt_cg_act_stack_sub(dt_pcb_t *pcb, dt_node_t *dnp, int reg, int off, dtrace_actk
 		emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, off + prefsz));
 	}
 	emit(dlp,  BPF_MOV_IMM(BPF_REG_3, stacksize));
-	emit(dlp,  BPF_MOV_IMM(BPF_REG_4, (0 & BPF_F_SKIP_FIELD_MASK)
+	emit(dlp,  BPF_MOV_IMM(BPF_REG_4, (skip & BPF_F_SKIP_FIELD_MASK)
 					  | (kind == DTRACEACT_USTACK ? BPF_F_USER_STACK : 0)));
 	dt_regset_xalloc(drp, BPF_REG_0);
 	emit(dlp,  BPF_CALL_HELPER(BPF_FUNC_get_stack));
diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c
index 0ddffa20..dfeecfd1 100644
--- a/libdtrace/dt_prov_fbt.c
+++ b/libdtrace/dt_prov_fbt.c
@@ -431,6 +431,7 @@ static void kprobe_detach(dtrace_hdl_t *dtp, const dt_probe_t *prp)
 dt_provimpl_t	dt_fbt_fprobe = {
 	.name		= prvname,
 	.prog_type	= BPF_PROG_TYPE_TRACING,
+	.skip_frames	= 4,
 	.populate	= &populate,
 	.load_prog	= &fprobe_prog_load,
 	.trampoline	= &fprobe_trampoline,
diff --git a/libdtrace/dt_provider.h b/libdtrace/dt_provider.h
index a24b1d00..bf3af0be 100644
--- a/libdtrace/dt_provider.h
+++ b/libdtrace/dt_provider.h
@@ -44,6 +44,7 @@ typedef struct dt_argdesc {
 typedef struct dt_provimpl {
 	const char *name;			/* provider generic name */
 	int prog_type;				/* BPF program type */
+	uint32_t skip_frames;			/* # of stack frames to skip */
 	int (*populate)(dtrace_hdl_t *dtp);	/* register probes */
 	int (*provide)(dtrace_hdl_t *dtp,	/* provide probes */
 		       const dtrace_probedesc_t *pdp);
-- 
2.42.0