[libunwind] 08d30c6 - [libunwind] Don't store a predecremented PC when using SEH

[Martin Storsjö via cfe-commits]

Author: Martin Storsjö
Date: 2022-06-06T23:25:24+03:00
New Revision: 08d30c602bcf3bc25a02938f2663a1b9e96a7cbe

URL: https://github.com/llvm/llvm-project/commit/08d30c602bcf3bc25a02938f2663a1b9e96a7cbe
DIFF: https://github.com/llvm/llvm-project/commit/08d30c602bcf3bc25a02938f2663a1b9e96a7cbe.diff

LOG: [libunwind] Don't store a predecremented PC when using SEH

This fixes unwinding in boundary cases on ARM with SEH.

In the case of ARM/Thumb, disp->ControlPc points at the following
instruction, with the thumb bit set. Thus by decrementing 1,
it still points at the next instruction. To achieve the desired
effect of pointing at the previous instruction, one first has to strip
out the thumb bit, then do the decrement by 1 to reach the previous
instruction.

When libcxxabi looks for call site ranges, it already does
`_Unwind_GetIP(context) - 1` (in `scan_eh_tab` in
libcxxabi/src/cxa_personality.cpp), so we shouldn't do the
corresponding `- 1` multiple times.

In the case of libcxxabi on Thumb, `funcStart` (still in `scan_eh_tab`)
may have the thumb bit set. If the program counter address is
decremented both in libunwind (first removing the thumb bit, then
decremented), and then libcxxabi decrements it further, and compares
with a `funcStart` with the thumb bit set, it could point to one byte
before the start of the call site.

Thus: This modification makes libunwind with SEH work with libcxxabi
on Thumb, in settings where libunwind and libcxxabi worked fine with
Dwarf before.

For existing cases with libunwind with SEH (on x86_64 and aarch64),
this modification doesn't break any of my testcases.

Differential Revision: https://reviews.llvm.org/D126869

Added: 
    

Modified: 
    libunwind/src/Unwind-seh.cpp
    libunwind/src/UnwindCursor.hpp

Removed: 
    


################################################################################
diff  --git a/libunwind/src/Unwind-seh.cpp b/libunwind/src/Unwind-seh.cpp
index 64aaf0a383b8..c2c31045a37b 100644
--- a/libunwind/src/Unwind-seh.cpp
+++ b/libunwind/src/Unwind-seh.cpp
@@ -104,7 +104,7 @@ _GCC_specific_handler(PEXCEPTION_RECORD ms_exc, PVOID frame, PCONTEXT ms_ctx,
   if (!ctx) {
     __unw_init_seh(&cursor, disp->ContextRecord);
     __unw_seh_set_disp_ctx(&cursor, disp);
-    __unw_set_reg(&cursor, UNW_REG_IP, disp->ControlPc - 1);
+    __unw_set_reg(&cursor, UNW_REG_IP, disp->ControlPc);
     ctx = (struct _Unwind_Context *)&cursor;
 
     if (!IS_UNWINDING(ms_exc->ExceptionFlags)) {

diff  --git a/libunwind/src/UnwindCursor.hpp b/libunwind/src/UnwindCursor.hpp
index a46bbfbccdb5..e363f9e59c9a 100644
--- a/libunwind/src/UnwindCursor.hpp
+++ b/libunwind/src/UnwindCursor.hpp
@@ -518,6 +518,14 @@ class UnwindCursor : public AbstractUnwindCursor {
   pint_t getLastPC() const { return _dispContext.ControlPc; }
   void setLastPC(pint_t pc) { _dispContext.ControlPc = pc; }
   RUNTIME_FUNCTION *lookUpSEHUnwindInfo(pint_t pc, pint_t *base) {
+#ifdef __arm__
+    // Remove the thumb bit; FunctionEntry ranges don't include the thumb bit.
+    pc &= ~1U;
+#endif
+    // If pc points exactly at the end of the range, we might resolve the
+    // next function instead. Decrement pc by 1 to fit inside the current
+    // function.
+    pc -= 1;
     _dispContext.FunctionEntry = RtlLookupFunctionEntry(pc,
                                                         &_dispContext.ImageBase,
                                                         _dispContext.HistoryTable);