[PATCH 1/2 v3] ftrace: don't allow IPMODIFY without proper compiler support

[Jiri Kosina]

Using IPMODIFY needs to be allowed only with compilers which are 
guaranteed to generate function prologues compatible with function 
redirection through changing instruction pointer in saved regs.

For example changing regs->ip on x86_64 in cases when gcc is using mcount 
(and not fentry) is not allowed, because at the time mcount call is 
issued, the original function's prologue has already been executed, which 
leads to all kinds of inconsistent havoc.

There is currently no way to express dependency on gcc features in 
Kconfig, (CC_USING_FENTRY is defined only during build, so it's not 
possible for Kconfig symbol to depend on it) so this needs to be checked 
in runtime.

Mark x86_64 with fentry supported for now. Other archs can be added
gradually.

Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
---

v1 -> v2: turn macro function into a plain macro
v2 -> v3: return ENOTSUPP and reject registering ftrace_ops in 
	  ftrace_hash_move() already

 arch/x86/include/asm/ftrace.h | 2 ++
 include/linux/ftrace.h        | 5 +++++
 kernel/trace/ftrace.c         | 5 +++++
 3 files changed, 12 insertions(+)

diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
index f45acad..f84eaaf 100644
--- a/arch/x86/include/asm/ftrace.h
+++ b/arch/x86/include/asm/ftrace.h
@@ -4,8 +4,10 @@
 #ifdef CONFIG_FUNCTION_TRACER
 #ifdef CC_USING_FENTRY
 # define MCOUNT_ADDR		((long)(__fentry__))
+# define ftrace_ipmodify_supported 1
 #else
 # define MCOUNT_ADDR		((long)(mcount))
+# define ftrace_ipmodify_supported 0
 #endif
 #define MCOUNT_INSN_SIZE	5 /* sizeof mcount call */
 
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 1da6029..837153a 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -244,6 +244,11 @@ static inline int ftrace_function_local_disabled(struct ftrace_ops *ops)
 extern void ftrace_stub(unsigned long a0, unsigned long a1,
 			struct ftrace_ops *op, struct pt_regs *regs);
 
+#ifndef ftrace_ipmodify_supported
+/* let's not make any implicit assumptions about profiling call placement */
+# define ftrace_ipmodify_supported 0
+
+#endif
 #else /* !CONFIG_FUNCTION_TRACER */
 /*
  * (un)register_ftrace_function must be a macro since the ops parameter
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 929a733..3c1261c 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1378,6 +1378,11 @@ ftrace_hash_move(struct ftrace_ops *ops, int enable,
 	if (ops->flags & FTRACE_OPS_FL_IPMODIFY && !enable)
 		return -EINVAL;
 
+	if ((ops->flags & FTRACE_OPS_FL_IPMODIFY) && !ftrace_ipmodify_supported) {
+		WARN(1, "Your compiler doesn't support features necessary for IPMODIFY");
+		return -ENOTSUPP;
+	}
+
 	/*
 	 * If the new source is empty, just free dst and assign it
 	 * the empty_hash.

-- 
Jiri Kosina
SUSE Labs
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