[DTrace-devel] [PATCH 4/4] Set fbt:::return arg0=-1 to indicate we do not know the real value

[eugene.loh at oracle.com]

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

Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
---
 libdtrace/dt_prov_fbt.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c
index 88638bc6..32204201 100644
--- a/libdtrace/dt_prov_fbt.c
+++ b/libdtrace/dt_prov_fbt.c
@@ -173,6 +173,18 @@ static void trampoline(dt_pcb_t *pcb)
 
 		emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, PT_REGS_RET));
 		emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_0));
+
+		/*
+		 * fbt:::return arg0 should be the function offset for
+		 * return instruction.  Since we use kretprobes, however,
+		 * which do not fire until the function has returned to
+		 * its caller, information about the returning instruction
+		 * in the callee has been lost.
+		 *
+		 * Set arg0=-1 to indicate that we do not know the value.
+		 */
+		dt_cg_xsetx(dlp, NULL, DT_LBL_NONE, BPF_REG_0, -1);
+		emit(dlp,  BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(0), BPF_REG_0));
 	} else
 		dt_cg_tramp_copy_entry_args(pcb, BPF_REG_8);
 	dt_cg_tramp_epilogue(pcb);
-- 
2.18.4