Re: [PATCH v4 07/11] gdb: Handle shadow stack pointer register unwinding for amd64 linux.

Luis Machado
2025-06-23T15:07:29Z

On 6/23/25 16:00, Schimpe, Christina wrote:
> Hi Luis,
>
> Thanks for the feedback. Please find my comments below.
>
>> -----Original Message-----
>> From: Luis Machado <luis.machado@arm.com>
>> Sent: Thursday, June 19, 2025 11:25 AM
>> To: Schimpe, Christina <christina.schimpe@intel.com>; gdb-
>> patches@sourceware.org
>> Cc: thiago.bauermann@linaro.org; eliz@gnu.org
>> Subject: Re: [PATCH v4 07/11] gdb: Handle shadow stack pointer register
>> unwinding for amd64 linux.
>>
>> On 6/17/25 13:11, Christina Schimpe 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.
>>>
>>> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
>>> ---
>>> gdb/amd64-linux-tdep.c | 84 ++++++++++++++++++
>>> 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, 239 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
>>> d806d3cb1f7..9436f0b190c 100644
>>> --- a/gdb/amd64-linux-tdep.c
>>> +++ b/gdb/amd64-linux-tdep.c
>>> @@ -47,6 +47,8 @@
>>> #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"
>>> @@ -1917,6 +1919,87 @@ amd64_linux_get_tls_dtv_addr (struct gdbarch
>> *gdbarch, ptid_t ptid,
>>> return dtv_addr;
>>> }
>>>
>>> +/* Return the number of bytes required to update the shadow stack pointer
>>> + by one element. For x32 the shadow stack elements are still 64-bit
>>> + aligned. Thus, gdbarch_addr_bit cannot be used to compute the new
>>> + stack pointer. */
>>> +
>>> +static inline int
>>> +amd64_linux_shadow_stack_element_size_aligned (gdbarch *gdbarch) {
>>> + const bfd_arch_info *binfo = gdbarch_bfd_arch_info (gdbarch);
>>> + return (binfo->bits_per_word / binfo->bits_per_byte); }
>>> +
>>> +
>>> +/* Implement shadow stack pointer unwinding. For each new shadow stack
>>> + pointer check if its address is still in the shadow stack memory range.
>>> + If it's outside the range set the returned value to unavailable,
>>> + otherwise return a value containing the new shadow stack pointer.
>>> +*/
>>> +
>>> +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.
>>
>> Same typo as before, period after "v6.6".
>
> Will fix.
>
>>
>>> + 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. */
>>> + CORE_ADDR new_ssp
>>> + = ssp + amd64_linux_shadow_stack_element_size_aligned (gdbarch);
>>> +
>>> + /* If NEW_SSP points to the end of or before (<=) the current
>>> + shadow stack memory range we consider NEW_SSP as valid (but
>>> + empty). */
>>
>> I couldn't quite understand the difference between the empty case and the
>> unavailable case. But maybe I just don't fully understand the feature.
>>
>> Would it be possible to make the comment a bit more clear?
>
> Is this a bit clearer?
>
> "There can be scenarios where we have a shadow stack pointer but the shadow stack
> is empty, as no call instruction has been executed yet. If NEW_SSP points to the end
> of or before (<=) the current shadow stack memory range we consider NEW_SSP as
> valid (but empty). "

Yes, that clear it up. Thanks!

>
> Please also see my answer to Thiago:
> https://sourceware.org/pipermail/gdb-patches/2025-June/218908.html

Thanks. It would be nice if both series were consistent in this regard (<= or <).