[DTrace-devel] [PATCH] Use highest bit to decide if PC is user space or kernel

[eugene.loh at oracle.com]

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

Currently, we decide if a PC is user space or kernel based on whether
the BPF helper function probe_read_kernel() can read the address.
On systems where this BPF helper function does not exist, there is a
back-up mechanism that simply checks the highest bit of the PC.

Apparently, on some systems, the helper function can read addresses
even in user space.  That is, the mechanism that uses the helper function
is not reliable.

So, just use the "back-up mechanism" all the time.

Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
---
 libdtrace/dt_cg.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index 4791536b..ac99d3dd 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -414,7 +414,6 @@ dt_cg_tramp_copy_args_from_regs(dt_pcb_t *pcb, int called)
 void
 dt_cg_tramp_copy_pc_from_regs(dt_pcb_t *pcb)
 {
-	dtrace_hdl_t	*dtp = pcb->pcb_hdl;
 	dt_regset_t	*drp = pcb->pcb_regs;
 	dt_irlist_t	*dlp = &pcb->pcb_ir;
 	uint_t		Luser = dt_irlist_label(dlp);
@@ -429,7 +428,18 @@ dt_cg_tramp_copy_pc_from_regs(dt_pcb_t *pcb)
         emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_3));
 
 	/* check if the PC is kernel or user space */
-	if (dtp->dt_bpfhelper[BPF_FUNC_probe_read_kernel] == BPF_FUNC_probe_read_kernel) {
+#if 0
+	if (pcb->pcb_hdl->dt_bpfhelper[BPF_FUNC_probe_read_kernel]
+	    == BPF_FUNC_probe_read_kernel) {
+#else
+	/*
+	 * We do not seem to be guaranteed for some kernels that
+	 * probe_read_kernel(), even if it exists, will signal an error
+	 * when reading user space addresses.  So, do not use the
+	 * probe_read_kernel() mechanism for now.
+	 */
+	if (0) {
+#endif
 		/*
 		 * Use probe_read_kernel() if it is really is probe_read_kernel().
 		 * On older kernels, it does not exist and is aliased to something else.
@@ -452,7 +462,6 @@ dt_cg_tramp_copy_pc_from_regs(dt_pcb_t *pcb)
 	} else {
 		/*
 		 * If no real probe_read_kernel() exists, just test the highest bit.
-		 * This is not as robust, but probably works just fine for us.
 		 */
 
 		/* if the highest bit is 0, assume it was user space */
-- 
2.18.4