[PATCH] arm64: Don't insert a BTI instruction at inner labels

Jean-Philippe Brucker jean-philippe at linaro.org
Wed Jun 24 07:22:54 EDT 2020

Some ftrace features are broken since commit 714a8d02ca4d ("arm64: asm:
Override SYM_FUNC_START when building the kernel with BTI"). For example
the function_graph tracer:

$ echo function_graph > /sys/kernel/debug/tracing/current_tracer
[   36.107016] WARNING: CPU: 0 PID: 115 at kernel/trace/ftrace.c:2691 ftrace_modify_all_code+0xc8/0x14c

When ftrace_modify_graph_caller() attempts to write a branch at
ftrace_graph_call, it finds the "BTI J" instruction inserted by
SYM_INNER_LABEL() instead of a NOP, and aborts.

It turns out we don't currently need the BTI landing pads inserted by

* ftrace_call and ftrace_graph_call are only used for runtime patching
  of the active tracer. The patched code is not reached from a branch.
* install_el2_stub is reached from a CBZ instruction, which doesn't
  change PSTATE.BTYPE.
* __guest_exit is reached from B instructions in the hyp-entry vectors,
  which aren't subject to BTI checks either.

Remove the BTI annotation from SYM_INNER_LABEL.

Fixes: 714a8d02ca4d ("arm64: asm: Override SYM_FUNC_START when building the kernel with BTI")
Signed-off-by: Jean-Philippe Brucker <jean-philippe at linaro.org>
Tested on QEMU with and without BTI, but only ftrace not KVM.
 arch/arm64/include/asm/linkage.h | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
index 81fefd2a1d023..ba89a9af820ab 100644
--- a/arch/arm64/include/asm/linkage.h
+++ b/arch/arm64/include/asm/linkage.h
@@ -12,7 +12,6 @@
  * instead.
 #define BTI_C hint 34 ;
-#define BTI_J hint 36 ;
  * When using in-kernel BTI we need to ensure that PCS-conformant assembly
@@ -43,11 +42,6 @@
-#define SYM_INNER_LABEL(name, linkage)			\
-	.type name SYM_T_NONE ASM_NL			\
-	SYM_ENTRY(name, linkage, SYM_A_NONE)		\

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