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

Thank you for the feedback.

> -----Original Message-----
> From: Luis <[email protected]>
> Sent: Donnerstag, 13. August 2026 00:13
> To: Joos, Christina <[email protected]>; [email protected]
> Cc: [email protected]; [email protected]
> Subject: Re: [PATCH v4 02/13] aarch64: Implement gdbarch function
> top_addr_empty_shadow_stack.
> 
> 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 <[email protected]>
> > ---
> >   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."

Will fix.

> > +  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?

I can for sure add a comment that this is required for ARM to allow unwinding of
the shadow stack pointer in dwarf2_prev_ssp.

> > +      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.

I will wait for more feedback for this series (Thiago indicated he will review it soon)
and then post a new series with your and his feedback included. 

Regards,
Christina
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