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

[Guinevere Larsen <guinevere@redhat.com>]

On 1/30/25 1:11 PM, Schimpe, Christina wrote:
>> -----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?
>
This makes sense, but in that case I think it's better to just create 
this function at this point, so that code doesn't get created and 
deleted unnecessarily.

-- 
Cheers,
Guinevere Larsen
She/Her/Hers