[PATCH] riscv/ptrace: add new regset to get original a0 register

[Celeste Liu]

The orig_a0 is missing in struct user_regs_struct of riscv, and there is
no way to add it without breaking UAPI. (See Link tag below)

Like NT_ARM_SYSTEM_CALL do, we add a new regset name NT_RISCV_ORIG_A0 to
access original a0 register from userspace via ptrace API.

Link: https://lore.kernel.org/all/59505464-c84a-403d-972f-d4b2055eeaac@gmail.com/
Signed-off-by: Celeste Liu <uwu at coelacanthus.name>
---
 arch/riscv/kernel/ptrace.c | 33 +++++++++++++++++++++++++++++++++
 include/uapi/linux/elf.h   |  1 +
 2 files changed, 34 insertions(+)

diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c
index ea67e9fb7a583683b922fe2c017ea61f3bc848db..faa46de9000376eb445a32d43a40210d7b846844 100644
--- a/arch/riscv/kernel/ptrace.c
+++ b/arch/riscv/kernel/ptrace.c
@@ -31,6 +31,7 @@ enum riscv_regset {
 #ifdef CONFIG_RISCV_ISA_SUPM
 	REGSET_TAGGED_ADDR_CTRL,
 #endif
+	REGSET_ORIG_A0,
 };
 
 static int riscv_gpr_get(struct task_struct *target,
@@ -184,6 +185,30 @@ static int tagged_addr_ctrl_set(struct task_struct *target,
 }
 #endif
 
+static int riscv_orig_a0_get(struct task_struct *target,
+			     const struct user_regset *regset,
+			     struct membuf to)
+{
+	return membuf_store(&to, task_pt_regs(target)->orig_a0);
+}
+
+static int riscv_orig_a0_set(struct task_struct *target,
+			     const struct user_regset *regset,
+			     unsigned int pos, unsigned int count,
+			     const void *kbuf, const void __user *ubuf)
+{
+	int orig_a0 = task_pt_regs(target)->orig_a0;
+	int ret;
+
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &orig_a0, 0, -1);
+	if (ret)
+		return ret;
+
+	task_pt_regs(target)->orig_a0 = orig_a0;
+	return ret;
+}
+
+
 static const struct user_regset riscv_user_regset[] = {
 	[REGSET_X] = {
 		.core_note_type = NT_PRSTATUS,
@@ -224,6 +249,14 @@ static const struct user_regset riscv_user_regset[] = {
 		.set = tagged_addr_ctrl_set,
 	},
 #endif
+	[REGSET_ORIG_A0] = {
+		.core_note_type = NT_RISCV_ORIG_A0,
+		.n = 1,
+		.size = sizeof(elf_greg_t),
+		.align = sizeof(elf_greg_t),
+		.regset_get = riscv_orig_a0_get,
+		.set = riscv_orig_a0_set,
+	},
 };
 
 static const struct user_regset_view riscv_user_native_view = {
diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
index b44069d29cecc0f9de90ee66bfffd2137f4275a8..390060229601631da2fb27030d9fa2142e676c14 100644
--- a/include/uapi/linux/elf.h
+++ b/include/uapi/linux/elf.h
@@ -452,6 +452,7 @@ typedef struct elf64_shdr {
 #define NT_RISCV_CSR	0x900		/* RISC-V Control and Status Registers */
 #define NT_RISCV_VECTOR	0x901		/* RISC-V vector registers */
 #define NT_RISCV_TAGGED_ADDR_CTRL 0x902	/* RISC-V tagged address control (prctl()) */
+#define NT_RISCV_ORIG_A0	  0x903	/* RISC-V original a0 register */
 #define NT_LOONGARCH_CPUCFG	0xa00	/* LoongArch CPU config registers */
 #define NT_LOONGARCH_CSR	0xa01	/* LoongArch control and status registers */
 #define NT_LOONGARCH_LSX	0xa02	/* LoongArch Loongson SIMD Extension registers */

---
base-commit: 0e287d31b62bb53ad81d5e59778384a40f8b6f56
change-id: 20241201-riscv-new-regset-d529b952ad0d

Best regards,
-- 
Celeste Liu <uwu at coelacanthus.name>