Handle NetBSD specific _Unwind_Ptr

[krytarowski]

Summary

NetBSD declares _Unwind_Ptr as void* which is not compatible
without explicit cast with the compiler-rt version uintptr_t.

Sponsored by <The NetBSD Foundation>

[krytarowski]

@kcc this is first step to upstream NetBSD support for sanitizers - to cleanup build issues in compiler-rt.

[kcc]

In D33878#772375, @krytarowski wrote:

@kcc this is first step to upstream NetBSD support for sanitizers - to cleanup build issues in compiler-rt.

Sorry, I don't know this code.

[krytarowski]

Ping?

[krytarowski]

lib/builtins/gcc_personality_v0.c

27

This line must be defined(__NetBSD__) && !defined(__clang__) as the failure is GCC specific. Clang uses custom unwind.h header.

[compnerd]

lib/builtins/gcc_personality_v0.c

27

clang doesn't really use a custom unwind.h per se. It is one of the compiler vended headers, and unless you specifically setup the build, the compiler vended header can be used over the external unwinder. That said, this macro should only be needed at one site, so we should just inline the difference.

215–216

Interesting that this has never been an issue, but the explicit cast here is fine.

248

_Unwind_SetGR is declared as: void _Unwind_SetGR(struct _Unwind_Context *, int, _Unwind_Word);

So this cast should not be needed.

250

Yes, _Unwind_SetIP is declared as void _Unwind_SetIP(struct _Unwind_Context *, _Unwind_Ptr);, and if NetBSD declares it as void *, then the explicit cast is reasonable.

Didn't NetBSD use GCC at one point? It seems that GCC too defines _Unwind_Ptr as unsigned int __attribute__((__mode__(__pointer__))), making it effectively the same as uintptr_t.

I'm tempted to say that this is a bug in the unwinder in NetBSD. However, adding a condition here for NetBSD and a cast is not too terrible.

[joerg]

Kamil, which unwind.h are you using? The outdated copy in libexecinfo.h or the modern one used by libunwind? I see little reason to cater to the bugs in the former...

[krytarowski]

https://github.com/NetBSD/src/blob/trunk/lib/libexecinfo/unwind.h here is the referenced header used by GCC.

For Clang/LLVM if I understand correctly the used header is (it overshadows the libexecinfo one):
https://github.com/NetBSD/src/blob/trunk/external/bsd/llvm/dist/clang/lib/Headers/unwind.h

[krytarowski]

In D33878#847654, @joerg wrote:

Kamil, which unwind.h are you using? The outdated copy in libexecinfo.h or the modern one used by libunwind? I see little reason to cater to the bugs in the former...

So, can we get rid of the libexecinfo one?

[joerg]

Well, the libexecinfo one exists as fallback because gcc doesn't provide one.

[krytarowski]

In D33878#847732, @joerg wrote:

Well, the libexecinfo one exists as fallback because gcc doesn't provide one.

What's the preferred solution to this issue? There is also an option to upfront decide that compiler-rt isn't buildable with GCC.

[krytarowski]

Suspended till getting functional elementary support of LLDB/NetBSD aboard.

[krytarowski]

lib/builtins/gcc_personality_v0.c

248

Pity on NetBSD this is:

void _Unwind_SetGR(struct _Unwind_Context *, int, _Unwind_Ptr);

So the 3rd argument differs.

[krytarowski]

lib/builtins/gcc_personality_v0.c

248

Maybe NetBSD should change the type to _Unwind_Word here? @joerg ?

250

How about _Unwind_SetIP(context, (_Unwind_Ptr)(funcStart + landingPad));?

[joerg]

lib/builtins/gcc_personality_v0.c

248

It is plain uintptr_t. _Unwind_Word isn't part of the IA64 specification, GNU seems to have created it because they didn't want to use uint64_t and somehow didn't think of uintptr_t it seems. I don't really see a point in introducing it.

[krytarowski]

Committed non-controversial chunks of this. The rest is rescheduled for future in separate review.

lib/builtins/gcc_personality_v0.c

  * which are quired for the C personality to conform to the specification.  In
  * order to provide forward compatibility for such compilers, we re-declare the
  * necessary interfaces in the helper to permit a standalone compilation of the
  * builtins (which contains the C unwinding personality for historical reasons).
  */
 #include "unwind-ehabi-helpers.h"
 #endif
 
+#if defined(__NetBSD__)
+#define TYPE_UNWIND_PTR void*
+#else
+#define TYPE_UNWIND_PTR uintptr_t
+#endif
+
 /*
  * Pointer encodings documented at:
  *   http://refspecs.freestandards.org/LSB_1.3.0/gLSB/gLSB/ehframehdr.html
  */
 
 #define DW_EH_PE_omit      0xff  /* no data follows */
 
 #define DW_EH_PE_absptr    0x00

 #endif
         return continueUnwind(exceptionObject, context);
 
     /* There is nothing to do if there is no LSDA for this frame. */
     const uint8_t* lsda = (uint8_t*)_Unwind_GetLanguageSpecificData(context);
     if ( lsda == (uint8_t*) 0 )
         return continueUnwind(exceptionObject, context);
 
-    uintptr_t pc = _Unwind_GetIP(context)-1;
-    uintptr_t funcStart = _Unwind_GetRegionStart(context);
+    uintptr_t pc = (uintptr_t)_Unwind_GetIP(context) - 1;
+    uintptr_t funcStart = (uintptr_t)_Unwind_GetRegionStart(context);
     uintptr_t pcOffset = pc - funcStart;
 
     /* Parse LSDA header. */
     uint8_t lpStartEncoding = *lsda++;
     if (lpStartEncoding != DW_EH_PE_omit) {
         readEncodedPointer(&lsda, lpStartEncoding); 
     }
     uint8_t ttypeEncoding = *lsda++;
while (p < callSiteTableEnd) {
             continue; /* no landing pad for this entry */
         if ( (start <= pcOffset) && (pcOffset < (start+length)) ) {
             /* Found landing pad for the PC.
              * Set Instruction Pointer to so we re-enter function 
              * at landing pad. The landing pad is created by the compiler
              * to take two parameters in registers.
              */
             _Unwind_SetGR(context, __builtin_eh_return_data_regno(0),
-                          (uintptr_t)exceptionObject);
+                          (TYPE_UNWIND_PTR)exceptionObject);
             _Unwind_SetGR(context, __builtin_eh_return_data_regno(1), 0);
-            _Unwind_SetIP(context, (funcStart + landingPad));
+            _Unwind_SetIP(context, (TYPE_UNWIND_PTR)(funcStart + landingPad));
             return _URC_INSTALL_CONTEXT;
         }
     }
 
     /* No landing pad found, continue unwinding. */
     return continueUnwind(exceptionObject, context);
 }
-