From:   Qing Zhang <zhangqing@loongson.cn>
To:     Huacai Chen <chenhuacai@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        WANG Xuerui <kernel@xen0n.name>
Cc:     Jiaxun Yang <jiaxun.yang@flygoat.com>, loongarch@lists.linux.dev,
        linux-kernel@vger.kernel.org
Subject: [PATCH] LoongArch: ptrace: Add function argument access API
Date:   Sat, 11 Feb 2023 16:44:14 +0800
Message-Id: <20230211084414.25998-1-zhangqing@loongson.cn>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Add regs_get_argument() which returns N th argument of the function
call, t show This enables ftrace kprobe events to access kernel function
arguments via $argN syntax.

eg:
echo 'p bio_add_page arg1=$arg1' > kprobe_events
bash: echo: write error: Invalid argument

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
---
 arch/loongarch/Kconfig              |  1 +
 arch/loongarch/include/asm/ptrace.h | 33 +++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index e3eba2eb4b44..103046966893 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -97,6 +97,7 @@ config LOONGARCH
 	select HAVE_EXIT_THREAD
 	select HAVE_FAST_GUP
 	select HAVE_FTRACE_MCOUNT_RECORD
+	select HAVE_FUNCTION_ARG_ACCESS_API
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
 	select HAVE_GENERIC_VDSO
diff --git a/arch/loongarch/include/asm/ptrace.h b/arch/loongarch/include/asm/ptrace.h
index 66a0e6c480a3..f6ffcc00753c 100644
--- a/arch/loongarch/include/asm/ptrace.h
+++ b/arch/loongarch/include/asm/ptrace.h
@@ -120,6 +120,39 @@ static inline long regs_return_value(struct pt_regs *regs)
 	return regs->regs[4];
 }
 
+/**
+ * regs_get_kernel_argument() - get Nth function argument in kernel
+ * @regs:       pt_regs of that context
+ * @n:          function argument number (start from 0)
+ *
+ * regs_get_argument() returns @n th argument of the function call.
+ * Note that this chooses most probably assignment, in some case
+ * it can be incorrect.
+ * This is expected to be called from kprobes or ftrace with regs
+ * where the top of stack is the return address.
+ */
+static inline unsigned long regs_get_kernel_argument(struct pt_regs *regs,
+						     unsigned int n)
+{
+	static const unsigned int argument_offs[] = {
+		offsetof(struct pt_regs, regs[4]),
+		offsetof(struct pt_regs, regs[5]),
+		offsetof(struct pt_regs, regs[6]),
+		offsetof(struct pt_regs, regs[7]),
+		offsetof(struct pt_regs, regs[8]),
+		offsetof(struct pt_regs, regs[9]),
+		offsetof(struct pt_regs, regs[10]),
+		offsetof(struct pt_regs, regs[11]),
+#define NR_REG_ARGUMENTS 8
+	};
+
+	if (n >= NR_REG_ARGUMENTS) {
+		n -= NR_REG_ARGUMENTS;
+		return regs_get_kernel_stack_nth(regs, n);
+	} else
+		return regs_get_register(regs, argument_offs[n]);
+}
+
 #define instruction_pointer(regs) ((regs)->csr_era)
 #define profile_pc(regs) instruction_pointer(regs)
 
-- 
2.36.0