Hi,

Thanks for catching a bug!

On 7/23/25 5:00 AM, Catalin Marinas wrote:
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()?

Seemed to make more sense to do the adds in the function using the offset values.

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).

Yes I 'fixed' this in simulate ret below, which is probably when I dropped the xn because I was just testing it with compiled code that was always using lr.



  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?


I broke it because my testing code/app never generated it.




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