commit b6a663905c2a1bf9b96c0f406e8e8488045d2dbe
author Sterling Augustine <saugustine@google.com> Thu Oct 31 12:45:20 2019 -0700
committer Sterling Augustine <saugustine@google.com> Thu Nov 07 14:48:35 2019 -0800
tree 188c6a74a05d9b66296bdd1bc335a4c1d2e8b387
parent e7f96ac3c4a29cea46c27cfc9cf236ead102e5a3

Correctly update isSignalFrame when unwinding the stack via dwarf.

A "signal frame" is a function or block of code where execution arrives via a signal or interrupt, rather than via a normal call instruction. In fact, a particular instruction is interrupted by the signal and needs to be restarted. Therefore, when the signal handler is complete, execution needs to return to the interrupted instruction, rather than the instruction immediately following the call instruction, as in a normal call.

Stack unwinders need to know this to correctly unwind signal frames. Dwarf handily provides an "S" in the CIE augmentation string to describe this case, and the libunwind API provides various functions to for unwinders to determine it,.

The llvm libunwind implementation correctly sets it's internal variable "isSignalFrame" when initializing an unwind context. However, upon stepping up the stack, the current implementation correctly reads the augmentation string and sets it in the CIE info (which it then discards), libunwind doesn't update it's internal unwind context data structure.

This change fixes that, and provides compatibility with both the canonical libunwind and the libgcc implementation.

Reviewers: jfb

Subscribers: christof, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D69677

Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: d3c744313c3cca0c076f031ec71e66ca74b12f2a

src/DwarfInstructions.hpp

diff --git a/src/DwarfInstructions.hpp b/src/DwarfInstructions.hpp
index 29a070f..48ef186 100644
--- a/src/DwarfInstructions.hpp
+++ b/src/DwarfInstructions.hpp
@@ -34,7 +34,7 @@
   typedef typename A::sint_t sint_t;
 
   static int stepWithDwarf(A &addressSpace, pint_t pc, pint_t fdeStart,
-                           R &registers);
+                           R &registers, bool &isSignalFrame);
 
 private:
 
@@ -150,7 +150,8 @@
 
 template <typename A, typename R>
 int DwarfInstructions<A, R>::stepWithDwarf(A &addressSpace, pint_t pc,
-                                           pint_t fdeStart, R &registers) {
+                                           pint_t fdeStart, R &registers,
+                                           bool &isSignalFrame) {
   FDE_Info fdeInfo;
   CIE_Info cieInfo;
   if (CFI_Parser<A>::decodeFDE(addressSpace, fdeStart, &fdeInfo,
@@ -196,6 +197,8 @@
       // restoring SP means setting it to CFA.
       newRegisters.setSP(cfa);
 
+      isSignalFrame = cieInfo.isSignalFrame;
+
 #if defined(_LIBUNWIND_TARGET_AARCH64)
       // If the target is aarch64 then the return address may have been signed
       // using the v8.3 pointer authentication extensions. The original

src/UnwindCursor.hpp

diff --git a/src/UnwindCursor.hpp b/src/UnwindCursor.hpp
index b4d44e1..4c18614 100644
--- a/src/UnwindCursor.hpp
+++ b/src/UnwindCursor.hpp
@@ -929,7 +929,7 @@
     return DwarfInstructions<A, R>::stepWithDwarf(_addressSpace,
                                               (pint_t)this->getReg(UNW_REG_IP),
                                               (pint_t)_info.unwind_info,
-                                              _registers);
+                                              _registers, _isSignalFrame);
   }
 #endif
 

src/UnwindLevel1-gcc-ext.c

diff --git a/src/UnwindLevel1-gcc-ext.c b/src/UnwindLevel1-gcc-ext.c
index 63e4083..008df81 100644
--- a/src/UnwindLevel1-gcc-ext.c
+++ b/src/UnwindLevel1-gcc-ext.c
@@ -221,7 +221,14 @@
 _LIBUNWIND_EXPORT uintptr_t _Unwind_GetIPInfo(struct _Unwind_Context *context,
                                               int *ipBefore) {
   _LIBUNWIND_TRACE_API("_Unwind_GetIPInfo(context=%p)", (void *)context);
-  *ipBefore = 0;
+  int isSignalFrame = __unw_is_signal_frame((unw_cursor_t *)context);
+  // Negative means some kind of error (probably UNW_ENOINFO), but we have no
+  // good way to report that, and this maintains backward compatibility with the
+  // implementation that hard-coded zero in every case, even signal frames.
+  if (isSignalFrame <= 0)
+    *ipBefore = 0;
+  else
+    *ipBefore = 1;
   return _Unwind_GetIP(context);
 }