[DTrace-devel] [PATCH v2] Handle unlabeled BPF_NOP instructions

[eugene.loh at oracle.com]

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

In BPF code generation, we emit branches with a label id stored in
the offset field, to be resolved during assembly.  Since "jmp 0" is
used to encode a NOP in BPF, we need to exclude jumps with offset 0
from jump-target relocation.

Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 libdtrace/dt_as.c  | 8 ++++++--
 libdtrace/dt_dis.c | 2 +-
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/libdtrace/dt_as.c b/libdtrace/dt_as.c
index 437a2d86..5b6f5cf3 100644
--- a/libdtrace/dt_as.c
+++ b/libdtrace/dt_as.c
@@ -330,8 +330,8 @@ fail:
 
 	/*
 	 * Make a second pass through the instructions, relocating each branch
-	 * label to the index of the final instruction in the buffer and noting
-	 * any other instruction-specific DIFO flags such as dtdo_destructive.
+	 * target (a label ID) to the relative location of the label and noting
+	 * any instruction-specific DIFO flags such as dtdo_destructive.
 	 */
 	for (i = 0; i < dp->dtdo_len; i++) {
 		struct bpf_insn instr = dp->dtdo_buf[i];
@@ -341,6 +341,10 @@ fail:
 		if (BPF_CLASS(instr.code) != BPF_JMP)
 			continue;
 
+		/* We ignore NOP (jmp 0). */
+		if (BPF_IS_NOP(instr))
+			continue;
+
 		/* We ignore function calls and function exits. */
 		if (op == BPF_CALL || op == BPF_EXIT)
 			continue;
diff --git a/libdtrace/dt_dis.c b/libdtrace/dt_dis.c
index d047bde3..27e98c30 100644
--- a/libdtrace/dt_dis.c
+++ b/libdtrace/dt_dis.c
@@ -346,7 +346,7 @@ dt_dis_jump(const dtrace_difo_t *dp, const char *name, uint_t addr,
 	    const struct bpf_insn *in, const char *rname, FILE *fp)
 {
 	if (in->off == 0)
-		fprintf(fp, "nop");
+		fprintf(fp, "nop\n");
 	else {
 		int	n;
 
-- 
2.18.4