RE: [PATCH 08/12] gdb: Handle shadow stack pointer register unwinding for amd64 linux.

["Schimpe, Christina" <christina.schimpe@intel.com>]

> -----Original Message-----
> From: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> Sent: Thursday, February 6, 2025 4:30 AM
> To: Schimpe, Christina <christina.schimpe@intel.com>
> Cc: gdb-patches@sourceware.org
> Subject: Re: [PATCH 08/12] gdb: Handle shadow stack pointer register unwinding
> for amd64 linux.
> 
> 
> "Schimpe, Christina" <christina.schimpe@intel.com> writes:
> 
> > Unwind the $pl3_ssp register.
> > We now have an updated value for the shadow stack pointer when moving
> > up or down the frame level.  Note that $pl3_ssp can become unavailable
> > when moving to a frame before the shadow stack enablement.  In the
> > example below, shadow stack is enabled in the function 'call1'.  Thus,
> > when moving to a frame level above the function, $pl3_ssp will become
> > unavaiable.
> > Following the restriction of the linux kernel, implement the unwinding
> > for amd64 linux only.
> >
> > Before this patch:
> > ~~~
> > Breakpoint 1, call2 (j=3) at sample.c:44
> > 44	  return 42;
> > (gdb) p $pl3_ssp
> > $1 = (void *) 0x7ffff79ffff8
> > (gdb) up
> > 55	  call2 (3);
> > (gdb) p $pl3_ssp
> > $2 = (void *) 0x7ffff79ffff8
> > (gdb) up
> > 68	  call1 (43);
> > (gdb) p $pl3_ssp
> > $3 = (void *) 0x7ffff79ffff8
> > ~~~
> >
> > After this patch:
> > ~~~
> > Breakpoint 1, call2 (j=3) at sample.c:44
> > 44	  return 42;
> > (gdb) p $pl3_ssp
> > $1 = (void *) 0x7ffff79ffff8
> > (gdb) up
> > 55	  call2 (3);
> > (gdb) p $pl3_ssp
> > $2 = (void *) 0x7ffff7a00000
> > (gdb) up
> > 68	  call1 (43i);
> > (gdb) p $pl3_ssp
> > $3 = <unavailable>
> > ~~~
> >
> > As we now have an updated value for each selected frame, the return
> > command is now enabled for shadow stack enabled programs, too.
> >
> > We therefore add a test for the return command and shadow stack
> > support, and for an updated shadow stack pointer after a frame level change.
> > ---
> >  gdb/amd64-linux-tdep.c                        | 69 +++++++++++++++
> >  gdb/linux-tdep.c                              | 47 ++++++++++
> >  gdb/linux-tdep.h                              |  7 ++
> >  .../gdb.arch/amd64-shadow-stack-cmds.exp      | 88 +++++++++++++++++++
> >  gdb/testsuite/gdb.arch/amd64-shadow-stack.c   | 13 +++
> >  5 files changed, 224 insertions(+)
> >  create mode 100644 gdb/testsuite/gdb.arch/amd64-shadow-stack-cmds.exp
> 
> Just some minor comments.
> 
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> 
> > diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c index
> > 95f643b1217..895feac85e8 100644
> > --- a/gdb/amd64-linux-tdep.c
> > +++ b/gdb/amd64-linux-tdep.c
> > @@ -45,6 +45,8 @@
> >  #include "arch/amd64-linux-tdesc.h"
> >  #include "inferior.h"
> >  #include "x86-tdep.h"
> > +#include "dwarf2/frame.h"
> > +#include "frame-unwind.h"
> >
> >  /* The syscall's XML filename for i386.  */  #define
> > XML_SYSCALL_FILENAME_AMD64 "syscalls/amd64-linux.xml"
> > @@ -1873,6 +1875,72 @@
> amd64_linux_remove_non_address_bits_watchpoint (gdbarch *gdbarch,
> >    return (addr & amd64_linux_lam_untag_mask ());  }
> >
> > +static value *
> > +amd64_linux_dwarf2_prev_ssp (const frame_info_ptr &this_frame,
> > +			     void **this_cache, int regnum) {
> 
> Add documentation comment to this function.
> 
> > +  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 ssp = extract_unsigned_integer (v->contents_all ().data (),
> > +						size, byte_order);
> > +
> > +      /* Starting with v6.6., the Linux kernel supports CET shadow stack.
> > +	 Using /proc/PID/smaps we can only check if the current shadow
> > +	 stack pointer SSP points to shadow stack memory.  Only if this is
> > +	 the case a valid previous shadow stack pointer can be
> > +	 calculated.  */
> > +      std::pair<CORE_ADDR, CORE_ADDR> range;
> > +      if (linux_address_in_shadow_stack_mem_range (ssp, &range))
> > +	{
> > +	  /* The shadow stack grows downwards.  To compute the previous
> > +	     shadow stack pointer, we need to increment SSP.
> > +	     For x32 the shadow stack elements are still 64-bit aligned.
> > +	     Thus, we cannot use gdbarch_addr_bit to compute the new stack
> > +	     pointer.  */
> > +	  const bfd_arch_info *binfo = gdbarch_bfd_arch_info (gdbarch);
> > +	  const int bytes_per_word
> > +	    = (binfo->bits_per_word / binfo->bits_per_byte);
> > +	  CORE_ADDR new_ssp = ssp + bytes_per_word;
> 
> I agree with Guinevere's comment about introducing
> amd64_linux_shadow_stack_element_size_aligned in this patch.
> 
> > +	  /* If NEW_SSP points to the end of or before (<=) the current
> > +	     shadow stack memory range we consider NEW_SSP as valid (but
> > +	     empty).  */
> > +	  if (new_ssp <= range.second)
> > +	    return frame_unwind_got_address (this_frame, regnum, new_ssp);
> > +	}
> > +    }
> > +
> > +  /* Return a value which is marked as unavailable in case we could not
> > +     calculate a valid previous shadow stack 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;
> > +}
> > +
> > +static void
> > +amd64_init_reg (gdbarch *gdbarch, int regnum, dwarf2_frame_state_reg
> *reg,
> > +		const frame_info_ptr &this_frame)
> > +{
> > +  if (regnum == gdbarch_pc_regnum (gdbarch))
> > +    reg->how = DWARF2_FRAME_REG_RA;
> > +  else if (regnum == gdbarch_sp_regnum (gdbarch))
> > +    reg->how = DWARF2_FRAME_REG_CFA;
> > +  else if (regnum == AMD64_PL3_SSP_REGNUM)
> > +    {
> > +      reg->how = DWARF2_FRAME_REG_FN;
> > +      reg->loc.fn = amd64_linux_dwarf2_prev_ssp;
> > +    }
> > +}
> 
> Add documentation comment to this function.
> 
> --
> Thiago

Thank you for the review. I'll apply all your comments in the next version.

Christina
Intel Deutschland GmbH
Registered Address: Am Campeon 10, 85579 Neubiberg, Germany
Tel: +49 89 99 8853-0, www.intel.de
Managing Directors: Sean Fennelly, Jeffrey Schneiderman, Tiffany Doon Silva
Chairperson of the Supervisory Board: Nicole Lau
Registered Office: Munich
Commercial Register: Amtsgericht Muenchen HRB 186928