[PATCH v2 1/2] kprobes: introduce ARCH_HANDLES_KPROBES_ON_FTRACE

[Heiko Carstens]

Allow architectures to implement handling of kprobes on function
tracer call sites on their own, without depending on common code.

This patch removes the kprobes check if a kprobe is being placed
on a function tracer call site and therefore gives full responsibility
of handling this correctly to the architecture.

This patch also introduces a user space visible change: if a kprobe
is placed into the middle of an ftrace instruction the return value
is changed from -EINVAL to -EILSEQ also for architectures which do
not support KPROBES_ON_FTRACE.
However in reality this change shouldn't matter at all.

Signed-off-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
---
 arch/Kconfig     |  8 ++++++++
 kernel/kprobes.c | 36 +++++++++++++++++++++---------------
 2 files changed, 29 insertions(+), 15 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 05d7a8a458d5..e1a8e0edf03f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -85,6 +85,14 @@ config KPROBES_ON_FTRACE
 	 passing of pt_regs to function tracing, then kprobes can
 	 optimize on top of function tracing.
 
+config ARCH_HANDLES_KPROBES_ON_FTRACE
+	def_bool n
+	help
+	 If an architecture can handle kprobes on function tracer call
+	 sites on own, then this option should be selected. This option
+	 removes the check which otherwise prevents to set kprobes on
+	 function tracer call sites.
+
 config UPROBES
 	def_bool n
 	select PERCPU_RWSEM
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 3995f546d0f3..4b57fe9fbeb7 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1410,28 +1410,34 @@ static inline int check_kprobe_rereg(struct kprobe *p)
 	return ret;
 }
 
-static int check_kprobe_address_safe(struct kprobe *p,
-				     struct module **probed_mod)
+static int check_ftrace_location(struct kprobe *p)
 {
-	int ret = 0;
 	unsigned long ftrace_addr;
 
-	/*
-	 * If the address is located on a ftrace nop, set the
-	 * breakpoint to the following instruction.
-	 */
 	ftrace_addr = ftrace_location((unsigned long)p->addr);
-	if (ftrace_addr) {
-#ifdef CONFIG_KPROBES_ON_FTRACE
-		/* Given address is not on the instruction boundary */
-		if ((unsigned long)p->addr != ftrace_addr)
-			return -EILSEQ;
+	if (!ftrace_addr)
+		return 0;
+	/* Given address is not on the instruction boundary */
+	if ((unsigned long)p->addr != ftrace_addr)
+		return -EILSEQ;
+	/* If an architecture handles kprobes on ftrace, we're done */
+	if (IS_ENABLED(CONFIG_ARCH_HANDLES_KPROBES_ON_FTRACE))
+		return 0;
+	if (IS_ENABLED(CONFIG_KPROBES_ON_FTRACE)) {
 		p->flags |= KPROBE_FLAG_FTRACE;
-#else	/* !CONFIG_KPROBES_ON_FTRACE */
-		return -EINVAL;
-#endif
+		return 0;
 	}
+	return -EINVAL;
+}
+
+static int check_kprobe_address_safe(struct kprobe *p,
+				     struct module **probed_mod)
+{
+	int ret;
 
+	ret = check_ftrace_location(p);
+	if (ret)
+		return ret;
 	jump_label_lock();
 	preempt_disable();
 
-- 
1.8.5.5

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