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.

Fixes: e2c0cdfba7f6 ("RISC-V: User-facing API")
Link: https://lore.kernel.org/all/59505464-c84a-403d-972f-d4b2055eeaac@gmail.com/
Cc: stable@vger.kernel.org
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Celeste Liu <uwu@coelacanthus.name>
---
 arch/riscv/kernel/ptrace.c | 32 ++++++++++++++++++++++++++++++++
 include/uapi/linux/elf.h   |  1 +
 2 files changed, 33 insertions(+)

diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c
index ea67e9fb7a583683b922fe2c017ea61f3bc848db..ef9ab74c8575a5c440155973b1c625c06a867c97 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 +185,29 @@ 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)
+{
+	unsigned long 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 0;
+}
+
 static const struct user_regset riscv_user_regset[] = {
 	[REGSET_X] = {
 		.core_note_type = NT_PRSTATUS,
@@ -224,6 +248,14 @@ 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 */

-- 
2.48.0

On Wed, Jan 15, 2025 at 07:13:27PM +0800, Celeste Liu wrote:
> 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.
> 
> Fixes: e2c0cdfba7f6 ("RISC-V: User-facing API")
> Link: https://lore.kernel.org/all/59505464-c84a-403d-972f-d4b2055eeaac@gmail.com/
> Cc: stable@vger.kernel.org
> Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
> Signed-off-by: Celeste Liu <uwu@coelacanthus.name>

Thank you!

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>

> ---
>  arch/riscv/kernel/ptrace.c | 32 ++++++++++++++++++++++++++++++++
>  include/uapi/linux/elf.h   |  1 +
>  2 files changed, 33 insertions(+)
> 
> diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c
> index ea67e9fb7a583683b922fe2c017ea61f3bc848db..ef9ab74c8575a5c440155973b1c625c06a867c97 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 +185,29 @@ 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)
> +{
> +	unsigned long 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 0;
> +}
> +
>  static const struct user_regset riscv_user_regset[] = {
>  	[REGSET_X] = {
>  		.core_note_type = NT_PRSTATUS,
> @@ -224,6 +248,14 @@ 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 */
> 
> -- 
> 2.48.0
>