[PATCH bpf-next v3 2/7] ftrace: Fix deadloop caused by direct call in ftrace selftest

From: Xu Kuohai <xukuohai@huawei.com>
To: <bpf@vger.kernel.org>, <linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, <netdev@vger.kernel.org>,
	<linux-kselftest@vger.kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	 Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Alexei Starovoitov <ast@kernel.org>,
	Zi Shen Lim <zlim.lnx@gmail.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	"David S . Miller" <davem@davemloft.net>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	David Ahern <dsahern@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, <x86@kernel.org>,
	<hpa@zytor.com>, Shuah Khan <shuah@kernel.org>,
	Jakub Kicinski <kuba@kernel.org>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Pasha Tatashin <pasha.tatashin@soleen.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Daniel Kiss <daniel.kiss@arm.com>,
	Steven Price <steven.price@arm.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Peter Collingbourne <pcc@google.com>,
	Mark Brown <broonie@kernel.org>, Delyan Kratunov <delyank@fb.com>,
	Kumar Kartikeya Dwivedi <memxor@gmail.com>
Subject: [PATCH bpf-next v3 2/7] ftrace: Fix deadloop caused by direct call in ftrace selftest
Date: Sun, 24 Apr 2022 11:40:23 -0400	[thread overview]
Message-ID: <20220424154028.1698685-3-xukuohai@huawei.com> (raw)
In-Reply-To: <20220424154028.1698685-1-xukuohai@huawei.com>

After direct call is enabled for arm64, ftrace selftest enters a
dead loop:

<trace_selftest_dynamic_test_func>:
00  bti     c
01  mov     x9, x30                            <trace_direct_tramp>:
02  bl      <trace_direct_tramp>    ---------->     ret
                                                     |
                                         lr/x30 is 03, return to 03
                                                     |
03  mov     w0, #0x0   <-----------------------------|
     |                                               |
     |                   dead loop!                  |
     |                                               |
04  ret   ---- lr/x30 is still 03, go back to 03 ----|

The reason is that when the direct caller trace_direct_tramp() returns
to the patched function trace_selftest_dynamic_test_func(), lr is still
the address after the instrumented instruction in the patched function,
so when the patched function exits, it returns to itself!

To fix this issue, we need to restore lr before trace_direct_tramp()
exits, so rewrite a dedicated trace_direct_tramp() for arm64.

Reported-by: Li Huafei <lihuafei1@huawei.com>
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
---
 kernel/trace/trace_selftest.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index abcadbe933bb..d2eff2b1d743 100644
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -785,8 +785,24 @@ static struct fgraph_ops fgraph_ops __initdata  = {
 };
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+#ifdef CONFIG_ARM64
+extern void trace_direct_tramp(void);
+
+asm (
+"	.pushsection	.text, \"ax\", @progbits\n"
+"	.type		trace_direct_tramp, %function\n"
+"	.global		trace_direct_tramp\n"
+"trace_direct_tramp:"
+"	mov	x10, x30\n"
+"	mov	x30, x9\n"
+"	ret	x10\n"
+"	.size		trace_direct_tramp, .-trace_direct_tramp\n"
+"	.popsection\n"
+);
+#else
 noinline __noclone static void trace_direct_tramp(void) { }
 #endif
+#endif
 
 /*
  * Pretty much the same than for the function tracer from which the selftest
-- 
2.30.2


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