On 12/18/2017 10:51 AM, Masami Hiramatsu wrote:

On Fri, 15 Dec 2017 14:12:54 -0500
Josef Bacik [off-list ref] wrote:

quoted

From: Josef Bacik <redacted>

Error injection is sloppy and very ad-hoc.  BPF could fill this niche
perfectly with it's kprobe functionality.  We could make sure errors are
only triggered in specific call chains that we care about with very
specific situations.  Accomplish this with the bpf_override_funciton
helper.  This will modify the probe'd callers return value to the
specified value and set the PC to an override function that simply
returns, bypassing the originally probed function.  This gives us a nice
clean way to implement systematic error injection for all of our code
paths.

OK, got it. I think the error_injectable function list should be defined
in kernel/trace/bpf_trace.c because only bpf calls it and needs to care
the "safeness".

[...]

quoted

diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
index 8dc0161cec8f..1ea748d682fd 100644
--- a/arch/x86/kernel/kprobes/ftrace.c
+++ b/arch/x86/kernel/kprobes/ftrace.c

@@ -97,3 +97,17 @@ int arch_prepare_kprobe_ftrace(struct kprobe *p)
 	p->ainsn.boostable = false;
 	return 0;
 }
+
+asmlinkage void override_func(void);
+asm(
+	".type override_func, @function\n"
+	"override_func:\n"
+	"	ret\n"
+	".size override_func, .-override_func\n"
+);
+
+void arch_ftrace_kprobe_override_function(struct pt_regs *regs)
+{
+	regs->ip = (unsigned long)&override_func;
+}
+NOKPROBE_SYMBOL(arch_ftrace_kprobe_override_function);

Calling this as "override_function" is meaningless. This is a function
which just return. So I think combination of just_return_func() and
arch_bpf_override_func_just_return() will be better.

Moreover, this arch/x86/kernel/kprobes/ftrace.c is an archtecture
dependent implementation of kprobes, not bpf.

Josef, please work out any necessary cleanups that would still need
to be addressed based on Masami's feedback and send them as follow-up
patches, thanks.

Hmm, arch/x86/net/bpf_jit_comp.c will be better place?

(No, it's JIT only and I'd really prefer to keep it that way, mixing
 this would result in a huge mess.)