Re: [PATCH v4 5/8] arm64: probes: Add GCS support to bl/blr/ret

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Jul 18, 2025 at 11:37:37PM -0500, Jeremy Linton wrote:
> diff --git a/arch/arm64/kernel/probes/simulate-insn.c b/arch/arm64/kernel/probes/simulate-insn.c
> index 09a0b36122d0..c75dce7bbe13 100644
> --- a/arch/arm64/kernel/probes/simulate-insn.c
> +++ b/arch/arm64/kernel/probes/simulate-insn.c
> @@ -13,6 +13,7 @@
>  #include <asm/traps.h>
>  
>  #include "simulate-insn.h"
> +#include "asm/gcs.h"
>  
>  #define bbl_displacement(insn)		\
>  	sign_extend32(((insn) & 0x3ffffff) << 2, 27)
> @@ -49,6 +50,20 @@ static inline u32 get_w_reg(struct pt_regs *regs, int reg)
>  	return lower_32_bits(pt_regs_read_reg(regs, reg));
>  }
>  
> +static inline void update_lr(struct pt_regs *regs, long addr)
> +{
> +	int err = 0;
> +
> +	if (user_mode(regs) && task_gcs_el0_enabled(current)) {
> +		push_user_gcs(addr + 4,	 &err);
> +		if (err) {
> +			force_sig(SIGSEGV);
> +			return;
> +		}
> +	}
> +	procedure_link_pointer_set(regs, addr + 4);
> +}
> +
>  static bool __kprobes check_cbz(u32 opcode, struct pt_regs *regs)
>  {
>  	int xn = opcode & 0x1f;
> @@ -107,9 +122,8 @@ simulate_b_bl(u32 opcode, long addr, struct pt_regs *regs)
>  {
>  	int disp = bbl_displacement(opcode);
>  
> -	/* Link register is x30 */
>  	if (opcode & (1 << 31))
> -		set_x_reg(regs, 30, addr + 4);
> +		update_lr(regs, addr);

Why not pass (addr + 4) here and skip the addition in update_lr()?

>  
>  	instruction_pointer_set(regs, addr + disp);
>  }
> @@ -133,17 +147,26 @@ simulate_br_blr(u32 opcode, long addr, struct pt_regs *regs)
>  	/* update pc first in case we're doing a "blr lr" */
>  	instruction_pointer_set(regs, get_x_reg(regs, xn));
>  
> -	/* Link register is x30 */
>  	if (((opcode >> 21) & 0x3) == 1)
> -		set_x_reg(regs, 30, addr + 4);
> +		update_lr(regs, addr);
>  }

I can see why this function was originally updating PC (in case of a blr
lr) but updating the LR was not supposed to fail. With GCS, I think we
should follow similar logic to simulate_b_bl() and skip updating PC/LR
if the write to the GCS failed (assuming that's what the hardware does,
I haven't checked the spec).

>  void __kprobes
>  simulate_ret(u32 opcode, long addr, struct pt_regs *regs)
>  {
> -	int xn = (opcode >> 5) & 0x1f;
> +	u64 ret_addr;
> +	int err = 0;
> +	unsigned long lr = procedure_link_pointer(regs);
>  
> -	instruction_pointer_set(regs, get_x_reg(regs, xn));
> +	if (user_mode(regs) && task_gcs_el0_enabled(current)) {
> +		ret_addr = pop_user_gcs(&err);
> +		if (err || ret_addr != lr) {
> +			force_sig(SIGSEGV);
> +			return;
> +		}
> +	}
> +
> +	instruction_pointer_set(regs, lr);
>  }

What happened to the RET Xn case?

-- 
Catalin