Re: [PATCH v4 02/13] aarch64: Implement gdbarch function top_addr_empty_shadow_stack.

[Luis <luis.machado.foss@gmail.com>]

On 08/07/2026 15:36, Christina Schimpe wrote:
> With the implementation of that gdbarch function we can remove
> the arch specific function aarch64_linux_dwarf2_prev_gcspr and
> use the generalized function dwarf2_prev_ssp instead.
> 
> Co-Authored-By: Christina Schimpe <christina.schimpe@intel.com>
> ---
>   gdb/aarch64-linux-tdep.c | 51 ++--------------------------------------
>   gdb/aarch64-tdep.c       | 28 ++++++++++++++++++++--
>   2 files changed, 28 insertions(+), 51 deletions(-)
> 
> diff --git a/gdb/aarch64-linux-tdep.c b/gdb/aarch64-linux-tdep.c
> index f11eccc1bc1..b5959057972 100644
> --- a/gdb/aarch64-linux-tdep.c
> +++ b/gdb/aarch64-linux-tdep.c
> @@ -66,6 +66,7 @@
>   #include "elf/common.h"
>   #include "elf/aarch64.h"
>   #include "arch/aarch64-insn.h"
> +#include "shadow-stack.h"
>   
>   /* For std::pow */
>   #include <cmath>
> @@ -2617,54 +2618,6 @@ aarch64_linux_get_shadow_stack_pointer (gdbarch *gdbarch, regcache *regcache,
>     return gcspr;
>   }
>   
> -/* Implement Guarded Control Stack Pointer Register unwinding.  For each
> -   previous GCS pointer check if its address is still in the GCS memory
> -   range.  If it's outside the range set the returned value to unavailable,
> -   otherwise return a value containing the new GCS pointer.  */
> -
> -static value *
> -aarch64_linux_dwarf2_prev_gcspr (const frame_info_ptr &this_frame,
> -				 void **this_cache, int regnum)
> -{
> -  value *v = frame_unwind_got_register (this_frame, regnum, regnum);
> -  gdb_assert (v != nullptr);
> -
> -  gdbarch *gdbarch = get_frame_arch (this_frame);
> -
> -  if (v->entirely_available () && !v->optimized_out ())
> -    {
> -      int size = register_size (gdbarch, regnum);
> -      bfd_endian byte_order = gdbarch_byte_order (gdbarch);
> -      CORE_ADDR gcspr = extract_unsigned_integer (v->contents_all ().data (),
> -						  size, byte_order);
> -
> -      /* Starting with v6.13, the Linux kernel supports Guarded Control
> -	 Stack.  Using /proc/PID/smaps we can only check if the current
> -	 GCSPR points to GCS memory.  Only if this is the case a valid
> -	 previous GCS pointer can be calculated.  */
> -      std::pair<CORE_ADDR, CORE_ADDR> range;
> -      if (linux_address_in_shadow_stack_mem_range (gcspr, &range))
> -	{
> -	  /* The GCS grows downwards.  To compute the previous GCS pointer,
> -	     we need to increment the GCSPR.  */
> -	  CORE_ADDR new_gcspr = gcspr + 8;
> -
> -	  /* If NEW_GCSPR still points within the current GCS memory range
> -	     we consider it to be valid.  */
> -	  if (new_gcspr < range.second)
> -	    return frame_unwind_got_address (this_frame, regnum, new_gcspr);
> -	}
> -    }
> -
> -  /* Return a value which is marked as unavailable in case we could not
> -     calculate a valid previous GCS pointer.  */
> -  value *retval
> -    = value::allocate_register (get_next_frame_sentinel_okay (this_frame),
> -				regnum, register_type (gdbarch, regnum));
> -  retval->mark_bytes_unavailable (0, retval->type ()->length ());
> -  return retval;
> -}
> -
>   /* AArch64 Linux implementation of the report_signal_info gdbarch
>      hook.  Displays information about possible memory tag violations.  */
>   
> @@ -3243,7 +3196,7 @@ aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
>       {
>         set_gdbarch_get_shadow_stack_pointer (gdbarch,
>   					aarch64_linux_get_shadow_stack_pointer);
> -      tdep->fn_prev_gcspr = aarch64_linux_dwarf2_prev_gcspr;
> +      tdep->fn_prev_gcspr = dwarf2_prev_ssp;
>       }
>   }
>   
> diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
> index 848cee3043c..d789b8569d2 100644
> --- a/gdb/aarch64-tdep.c
> +++ b/gdb/aarch64-tdep.c
> @@ -1908,6 +1908,26 @@ aarch64_pop_gcs_entry (regcache *regs)
>     regcache_cooked_write_unsigned (regs, tdep->gcs_reg_base, gcs_addr + 8);
>   }
>   
> +/* Implement the "top_addr_empty_shadow_stack" gdbarch method.  */
> +
> +static bool
> +aarch64_top_addr_empty_shadow_stack
> +  (const CORE_ADDR addr,
> +   const std::pair<CORE_ADDR, CORE_ADDR> range)
> +{
> +  gdb_assert (addr >= range.first);
> +
> +  /* For AArch64, addr must be strictly less than the upper address in the
> +     range, but other architectures allow it to be equal to the upper
> +     address when the stack is empty so GDB core works with those addresses
> +     and can send them our way.  */
> +  gdb_assert (addr <= range.second);
> +
> +  /* The GCS grows down, and the oldest entry isn't an address.
> +     Just the value '0'.  */

Nit: Might read better if...

"The GCS grows down, and the oldest entry is the value '0', not an address."

> +  return addr >= range.second - 8;
> +}
> +
>   /* Implement the "push_dummy_call" gdbarch method.  */
>   
>   static CORE_ADDR
> @@ -4783,9 +4803,13 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
>     /* Register a hook for converting a memory tag to a string.  */
>     set_gdbarch_memtag_to_string (gdbarch, aarch64_memtag_to_string);
>   
> -  /* AArch64's shadow stack pointer is the GCSPR.  */
>     if (tdep->has_gcs ())
> -    set_gdbarch_ssp_regnum (gdbarch, tdep->gcs_reg_base);
> +    {
> +      /* AArch64's shadow stack pointer is the GCSPR.  */
> +      set_gdbarch_ssp_regnum (gdbarch, tdep->gcs_reg_base);

Should we have a small comment here stating why we're setting up this hook?

> +      set_gdbarch_top_addr_empty_shadow_stack
> +	(gdbarch, aarch64_top_addr_empty_shadow_stack);
> +    }
>   
>     /* ABI */
>     set_gdbarch_short_bit (gdbarch, 16);

Otherwise looks OK to me.