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

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

> -----Original Message-----
> From: Guinevere Larsen <guinevere@redhat.com>
> Sent: Thursday, January 30, 2025 3:29 PM
> To: Schimpe, Christina <christina.schimpe@intel.com>; gdb-
> patches@sourceware.org
> Subject: Re: [PATCH 08/12] gdb: Handle shadow stack pointer register unwinding
> for amd64 linux.
> 
> On 12/20/24 5:04 PM, Schimpe, Christina wrote:
> > 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
> >
> > 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) {
> > +  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);
> In patch 10 of this series, you introduce
> amd64_linux_shadow_stack_element_size_aligned to simplify this calculation. is
> there any reason you didn't introduce it here?

Thanks a lot for looking at this.

The reason is that at this state of the series I only had one occurrence of this
particular code line and its comment. To avoid duplication I decided to make a 
small function for it in patch 10, but before it seemed to introduce more
overhead. Would that make sense to you? 

Christina
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