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Message-ID: <20251103185237.2284456-1-benniu@meta.com>
Date: Mon, 3 Nov 2025 10:52:35 -0800
From: Ben Niu <niuben003@...il.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Mark Rutland <mark.rutland@....com>,
Will Deacon <will@...nel.org>,
tytso@....edu,
Jason@...c4.com,
Ben Niu <benniu@...a.com>,
linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v2] tracing: Enable kprobe for selected Arm64 assembly
When ftrace is enabled, a function can only be kprobe'd if
(1) the function has proper nops inserted at the beginning
(2) the first inserted nop's address is added in section
__patchable_function_entries.
See function within_notrace_func in kernel/trace/trace_kprobe.c
for more details.
This patch adds a new asm function macro SYM_FUNC_START_TRACE
that makes an asm funtion satisfy the above two conditions so that
it can be kprobe'd. In addition, the macro is applied to
__arch_copy_to_user and __arch_copy_from_user, which were found
to be hot in certain workloads.
Note: although this patch unblocks kprobe tracing, fentry is still
broken because no BTF info gets generated from assembly files. A
separate patch is needed to fix that.
This patch was built with different combos of the following
features and kprobe tracing for asm functions __arch_copy_to_user
and __arch_copy_from_user worked in all cases:
CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
CONFIG_DYNAMIC_FTRACE_WITH_ARGS
CONFIG_ARM64_BTI_KERNEL
Signed-off-by: Ben Niu <benniu@...a.com>
---
v2: only opt-in __arch_copy_from_user and __arch_copy_to_user for
tracing since tracing asm functions in general may be unsafe
v1: aQOpd14RXnyQLOWR@...a.com
---
arch/arm64/include/asm/linkage.h | 91 ++++++++++++++++++++++++--------
arch/arm64/lib/copy_from_user.S | 2 +-
arch/arm64/lib/copy_to_user.S | 2 +-
3 files changed, 70 insertions(+), 25 deletions(-)
diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
index d3acd9c87509..b9a7513dc7d9 100644
--- a/arch/arm64/include/asm/linkage.h
+++ b/arch/arm64/include/asm/linkage.h
@@ -5,8 +5,47 @@
#include <asm/assembler.h>
#endif
-#define __ALIGN .balign CONFIG_FUNCTION_ALIGNMENT
-#define __ALIGN_STR ".balign " #CONFIG_FUNCTION_ALIGNMENT
+#define __ALIGN .balign CONFIG_FUNCTION_ALIGNMENT
+#define __ALIGN_STR ".balign " #CONFIG_FUNCTION_ALIGNMENT
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
+
+#define PRE_FUNCTION_NOPS \
+ ALIGN; \
+ nops CONFIG_FUNCTION_ALIGNMENT / 4 - 2; \
+ .pushsection __patchable_function_entries, "awo", @progbits, .text; \
+ .p2align 3; \
+ .8byte 1f; \
+ .popsection; \
+ 1 :; \
+ nops 2;
+
+#define PRE_PROLOGUE_NOPS nops 2;
+
+#elif defined(CONFIG_DYNAMIC_FTRACE_WITH_ARGS)
+
+#define PRE_FUNCTION_NOPS
+
+#define PRE_PROLOGUE_NOPS \
+ .pushsection __patchable_function_entries, "awo", @progbits, .text; \
+ .p2align 3; \
+ .8byte 1f; \
+ .popsection; \
+ 1 :; \
+ nops 2;
+
+#else
+
+#define PRE_FUNCTION_NOPS
+#define PRE_PROLOGUE_NOPS
+
+#endif
+
+#ifdef CONFIG_ARM64_BTI_KERNEL
+#define BTI_C bti c;
+#else
+#define BTI_C
+#endif
/*
* When using in-kernel BTI we need to ensure that PCS-conformant
@@ -15,32 +54,38 @@
* everything, the override is done unconditionally so we're more
* likely to notice any drift from the overridden definitions.
*/
-#define SYM_FUNC_START(name) \
- SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
- bti c ;
+#define SYM_FUNC_START_TRACE(name) \
+ PRE_FUNCTION_NOPS \
+ SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
+ BTI_C \
+ PRE_PROLOGUE_NOPS
+
+#define SYM_FUNC_START(name) \
+ SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
+ BTI_C
-#define SYM_FUNC_START_NOALIGN(name) \
- SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE) \
- bti c ;
+#define SYM_FUNC_START_NOALIGN(name) \
+ SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE) \
+ BTI_C \
-#define SYM_FUNC_START_LOCAL(name) \
- SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN) \
- bti c ;
+#define SYM_FUNC_START_LOCAL(name) \
+ SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN) \
+ BTI_C \
-#define SYM_FUNC_START_LOCAL_NOALIGN(name) \
- SYM_START(name, SYM_L_LOCAL, SYM_A_NONE) \
- bti c ;
+#define SYM_FUNC_START_LOCAL_NOALIGN(name) \
+ SYM_START(name, SYM_L_LOCAL, SYM_A_NONE) \
+ BTI_C \
-#define SYM_FUNC_START_WEAK(name) \
- SYM_START(name, SYM_L_WEAK, SYM_A_ALIGN) \
- bti c ;
+#define SYM_FUNC_START_WEAK(name) \
+ SYM_START(name, SYM_L_WEAK, SYM_A_ALIGN) \
+ BTI_C \
-#define SYM_FUNC_START_WEAK_NOALIGN(name) \
- SYM_START(name, SYM_L_WEAK, SYM_A_NONE) \
- bti c ;
+#define SYM_FUNC_START_WEAK_NOALIGN(name) \
+ SYM_START(name, SYM_L_WEAK, SYM_A_NONE) \
+ BTI_C \
-#define SYM_TYPED_FUNC_START(name) \
- SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
- bti c ;
+#define SYM_TYPED_FUNC_START(name) \
+ SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
+ BTI_C \
#endif
diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
index 400057d607ec..07b6d6c9573f 100644
--- a/arch/arm64/lib/copy_from_user.S
+++ b/arch/arm64/lib/copy_from_user.S
@@ -61,7 +61,7 @@
end .req x5
srcin .req x15
-SYM_FUNC_START(__arch_copy_from_user)
+SYM_FUNC_START_TRACE(__arch_copy_from_user)
add end, x0, x2
mov srcin, x1
#include "copy_template.S"
diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
index 819f2e3fc7a9..e7e663aea8ca 100644
--- a/arch/arm64/lib/copy_to_user.S
+++ b/arch/arm64/lib/copy_to_user.S
@@ -60,7 +60,7 @@
end .req x5
srcin .req x15
-SYM_FUNC_START(__arch_copy_to_user)
+SYM_FUNC_START_TRACE(__arch_copy_to_user)
add end, x0, x2
mov srcin, x1
#include "copy_template.S"
--
2.47.3
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