From: Olivier Dion <odion@efficios.com>
Date: Wed, 22 Feb 2023 20:19:14 +0000 (-0500)
Subject: Fix: adding a user space probe fails on thumb functions
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=56a9a0b750db49640129cb9b13cf384e9e82284c;p=lttng-tools.git

Fix: adding a user space probe fails on thumb functions

On some architectures, calling convention details are embedded in the
symbol addresses. Uprobe requires a "clean" symbol offset (or at least,
an address where an instruction boundary would be legal) to add
instrumentation. sanitize_uprobe_offset implements that sanitization
logic on a per-architecture basis.

The least significant bit is used when branching to switch to thumb ISA.
However, it's an invalid address for us; mask the least significant bit.

We were not masking the thumb bit, thus using the wrong address offset
by one.

Change-Id: Iaff8ccea3a319f9d9ad80501f1beccd74d1ef56d
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
---

diff --git a/src/bin/lttng-sessiond/kernel.cpp b/src/bin/lttng-sessiond/kernel.cpp
index 5b4fc086b..6e2bdb4c6 100644
--- a/src/bin/lttng-sessiond/kernel.cpp
+++ b/src/bin/lttng-sessiond/kernel.cpp
@@ -63,6 +63,32 @@ static int kernel_tracer_event_notifier_group_fd = -1;
 static int kernel_tracer_event_notifier_group_notification_fd = -1;
 static struct cds_lfht *kernel_token_to_event_notifier_rule_ht;
 
+namespace {
+/*
+ * On some architectures, calling convention details are embedded in the symbol
+ * addresses. Uprobe requires a "clean" symbol offset (or at least, an address
+ * where an instruction boundary would be legal) to add
+ * instrumentation. sanitize_uprobe_offset implements that sanitization logic on
+ * a per-architecture basis.
+ */
+#if defined(__arm__) || defined(__aarch64__)
+static inline uint64_t sanitize_uprobe_offset(uint64_t raw_offset)
+{
+	/*
+	 * The least significant bit is used when branching to switch to thumb
+	 * ISA. However, it's an invalid address for us; mask the least
+	 * significant bit.
+	 */
+	return raw_offset &= ~0b1;
+}
+#else /* defined(__arm__) || defined(__aarch64__) */
+static inline uint64_t sanitize_uprobe_offset(uint64_t raw_offset)
+{
+	return raw_offset;
+}
+#endif
+}
+
 /*
  * Add context on a kernel channel.
  *
@@ -452,7 +478,7 @@ static int userspace_probe_add_callsite(const struct lttng_userspace_probe_locat
 			goto end;
 		}
 
-		callsite.u.uprobe.offset = offset;
+		callsite.u.uprobe.offset = sanitize_uprobe_offset(offset);
 		ret = kernctl_add_callsite(fd, &callsite);
 		if (ret) {
 			WARN("Failed to add callsite to ELF userspace probe.");
@@ -479,7 +505,7 @@ static int userspace_probe_add_callsite(const struct lttng_userspace_probe_locat
 			goto end;
 		}
 		for (i = 0; i < offsets_count; i++) {
-			callsite.u.uprobe.offset = offsets[i];
+			callsite.u.uprobe.offset = sanitize_uprobe_offset(offsets[i]);
 			ret = kernctl_add_callsite(fd, &callsite);
 			if (ret) {
 				WARN("Failed to add callsite to SDT userspace probe");