Re: [RFC PATCH 2/2] bpf: Initialise mod[] in bpf_trace_printk

[Daniel Borkmann]

Hi James,

On 08/09/2017 10:34 PM, Daniel Borkmann wrote:
> On 08/09/2017 09:39 AM, James Hogan wrote:
> [...]
> > time (but please consider looking at the other patch which is certainly
> > a more real issue).
>
> Sorry for the delay, started looking into that and whether we
> have some other options, I'll get back to you on this.

Could we solve this more generically (as in: originally intended) in
the sense that we don't need to trust the gcc va_list handling; I feel
this is relying on an implementation detail? Perhaps something like
below poc patch?

Thanks again,
Daniel

From 71f16544d455abb6bb82f7253c17c14d2a395e91 Mon Sep 17 00:00:00 2001
Message-Id: <71f16544d455abb6bb82f7253c17c14d2a395e91.1502469361.git.daniel@xxxxxxxxxxxxx>
From: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Date: Fri, 11 Aug 2017 15:56:32 +0200
Subject: [PATCH] bpf: fix bpf_trace_printk on 32 bit

Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
 kernel/trace/bpf_trace.c | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 3738519..d4cb36f 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -204,10 +204,33 @@ static const struct bpf_func_proto *bpf_get_probe_write_proto(void)
 		fmt_cnt++;
 	}

-	return __trace_printk(1/* fake ip will not be printed */, fmt,
-			      mod[0] == 2 ? arg1 : mod[0] == 1 ? (long) arg1 : (u32) arg1,
-			      mod[1] == 2 ? arg2 : mod[1] == 1 ? (long) arg2 : (u32) arg2,
-			      mod[2] == 2 ? arg3 : mod[2] == 1 ? (long) arg3 : (u32) arg3);
+#define __BPF_TP_EMIT()	__BPF_ARG3_TP()
+#define __BPF_TP(...)							\
+	__trace_printk(1 /* fake ip will not be printed */,		\
+		       fmt, ##__VA_ARGS__)
+
+#define __BPF_ARG1_TP(...)						\
+	((mod[0] == 2 || (mod[0] == 1 && __BITS_PER_LONG == 64))	\
+	  ? __BPF_TP(arg1, ##__VA_ARGS__)				\
+	  : ((mod[0] == 1 || (mod[0] == 0 && __BITS_PER_LONG == 32))	\
+	      ? __BPF_TP((long)arg1, ##__VA_ARGS__)			\
+	      : __BPF_TP((u32)arg1, ##__VA_ARGS__)))
+
+#define __BPF_ARG2_TP(...)						\
+	((mod[1] == 2 || (mod[1] == 1 && __BITS_PER_LONG == 64))	\
+	  ? __BPF_ARG1_TP(arg2, ##__VA_ARGS__)				\
+	  : ((mod[1] == 1 || (mod[1] == 0 && __BITS_PER_LONG == 32))	\
+	      ? __BPF_ARG1_TP((long)arg2, ##__VA_ARGS__)		\
+	      : __BPF_ARG1_TP((u32)arg2, ##__VA_ARGS__)))
+
+#define __BPF_ARG3_TP(...)						\
+	((mod[2] == 2 || (mod[2] == 1 && __BITS_PER_LONG == 64))	\
+	  ? __BPF_ARG2_TP(arg3, ##__VA_ARGS__)				\
+	  : ((mod[2] == 1 || (mod[2] == 0 && __BITS_PER_LONG == 32))	\
+	      ? __BPF_ARG2_TP((long)arg3, ##__VA_ARGS__)		\
+	      : __BPF_ARG2_TP((u32)arg3, ##__VA_ARGS__)))
+
+	return __BPF_TP_EMIT();
 }

 static const struct bpf_func_proto bpf_trace_printk_proto = {
--
1.9.3