netdev - [PATCH net] bpf: fix bpf_trace

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Message-Id: <dd122e1ec0eadaf3f8fc11107f7a57177dcd8588.1502840567.git.daniel@iogearbox.net>
Date:   Wed, 16 Aug 2017 01:45:33 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     davem@...emloft.net
Cc:     james.hogan@...tec.com, ast@...com, netdev@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH net] bpf: fix bpf_trace_printk on 32 bit archs

James reported that on MIPS32 bpf_trace_printk() is currently
broken while MIPS64 works fine:

  bpf_trace_printk() uses conditional operators to attempt to
  pass different types to __trace_printk() depending on the
  format operators. This doesn't work as intended on 32-bit
  architectures where u32 and long are passed differently to
  u64, since the result of C conditional operators follows the
  "usual arithmetic conversions" rules, such that the values
  passed to __trace_printk() will always be u64 [causing issues
  later in the va_list handling for vscnprintf()].

  For example the samples/bpf/tracex5 test printed lines like
  below on MIPS32, where the fd and buf have come from the u64
  fd argument, and the size from the buf argument:

    [...] 1180.941542: 0x00000001: write(fd=1, buf=  (null), size=6258688)

  Instead of this:

    [...] 1625.616026: 0x00000001: write(fd=1, buf=009e4000, size=512)

One way to get it working is to expand various combinations
of argument types into 8 different combinations for 32 bit
and 64 bit kernels. Fix tested by James on MIPS32 and MIPS64
as well that it resolves the issue.

Fixes: 9c959c863f82 ("tracing: Allow BPF programs to call bpf_trace_printk()")
Reported-by: James Hogan <james.hogan@...tec.com>
Tested-by: James Hogan <james.hogan@...tec.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
---
 kernel/trace/bpf_trace.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 3738519..dc498b6 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -204,10 +204,36 @@ 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);
+/* Horrid workaround for getting va_list handling working with different
+ * argument type combinations generically for 32 and 64 bit archs.
+ */
+#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