Re: RFA: Need to extend x86 psABI to support thread cancellation and alternate signal stack

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Mar 27, 2018 at 8:55 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Mar 27, 2018 at 8:43 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 03/27/2018 01:26 PM, H.J. Lu wrote:
>>
>>> 2. Since shadow stack is never saved and restored by compiler, unwinder
>>> in libgcc counts how many stack frame it has to unwind and uses INCSSP
>>> to pop shadow stack.  This can't unwind the original shadow stack when
>>> the alternate shadow stack is used.  _URC_NO_REASON_CANCEL
>>> works only if longjmp will be used to finish stack unwinding, which is
>>> the case for thread cancellation in glibc.
>>>
>>> Here are patches for GCC:
>>>
>>>
>>> https://github.com/hjl-tools/gcc/commit/e9ff815941406e38fa629947af4d809b9129e860
>>>
>>> and glibc:
>>>
>>>
>>> https://github.com/hjl-tools/glibc/commit/1aec81528ab26aa8a8a7965317b6e1a8ba4526aa
>>>
>>> They fixed the issue.
>>
>>
>> The patches are nice and short, but: Do they really fix the issue?  They
>> make cancellation work again, but they do not fix the general unwinding
>> issue with alternate signal handler stacks AFAICS.
>
> That is true.   We do support unwinding with alternate signal handler stack
> using longjmp.   If there is another use case of unwinding with alternate signal
> handler stack, we can investigate.  If this isn't a valid use case, we
> don't want
> to create a very complex scheme to support it.

Here is a GCC only patch without changing glibc.   If we are in alternate
signal stack, don't try to unwind shadow stack.

diff --git a/libgcc/config/i386/linux-unwind.h
b/libgcc/config/i386/linux-unwind.h
index f1f52334d8d..0e082873a5f 100644
--- a/libgcc/config/i386/linux-unwind.h
+++ b/libgcc/config/i386/linux-unwind.h
@@ -24,6 +24,7 @@ see the files COPYING3 and COPYING.RUNTIME
respectively.  If not, see

 /* Unwind shadow stack for -fcf-protection -mshstk.  */
 #if defined __SHSTK__ && defined __CET__
+# define _Unwind_Frames_Extra_Skip_Alternate_Signal_Stack
 # include "config/i386/shadow-stack-unwind.h"
 #endif

diff --git a/libgcc/unwind.inc b/libgcc/unwind.inc
index 68c08964d30..9fbde024135 100644
--- a/libgcc/unwind.inc
+++ b/libgcc/unwind.inc
@@ -152,6 +152,19 @@ _Unwind_ForcedUnwind_Phase2 (struct _Unwind_Exception *exc,
   void *stop_argument = (void *) (_Unwind_Ptr) exc->private_2;
   _Unwind_Reason_Code code, stop_code;
   unsigned long frames = 1;
+  int in_signal_stack = 0;
+#ifdef _Unwind_Frames_Extra_Skip_Alternate_Signal_Stack
+  stack_t signal_stack;
+
+  if (sigaltstack (NULL, &signal_stack) == 0
+      && signal_stack.ss_flags == SS_ONSTACK)
+    {
+      void *stack = __builtin_frame_address (0);
+      if (stack >= signal_stack.ss_sp
+   && stack < (signal_stack.ss_sp + signal_stack.ss_size))
+ in_signal_stack = 1;
+    }
+#endif

   while (1)
     {
@@ -193,6 +206,9 @@ _Unwind_ForcedUnwind_Phase2 (struct _Unwind_Exception *exc,
       frames++;
     }

+  if (in_signal_stack)
+    frames = 0;
+
   *frames_p = frames;
   return code;
 }


-- 
H.J.