[PATCH] RISC-V: Handle different sigcontext struct layout.

[Kito Cheng]

RISC-V glibc intend to removed its own `sigcontext.h`[1] and use the linux
kernel's one, however the struct layout is slightly different between those two
version, fortunately they have identical layout, so we just need a
magical way to detect which one we are used.

libgcc/ChangeLog:

        * config/riscv/linux-unwind.h (SIGCONTEXT_PC): New.
        (riscv_fallback_frame_state): Use SIGCONTEXT_PC rather than
        sc->gregs[i].

[1] https://sourceware.org/pipermail/libc-alpha/2022-January/135417.html
---
 libgcc/config/riscv/linux-unwind.h | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/libgcc/config/riscv/linux-unwind.h 
b/libgcc/config/riscv/linux-unwind.h
index c86df2f85bc..4919facb36b 100644
--- a/libgcc/config/riscv/linux-unwind.h
+++ b/libgcc/config/riscv/linux-unwind.h
@@ -30,6 +30,16 @@
 
 #define MD_FALLBACK_FRAME_STATE_FOR riscv_fallback_frame_state
 
+/* RISC-V Glibc has removed its own sigcontext.h and use the linux kernel's
+   one, however the struct layout is little different between those two
+   version, fortunately they have identical layout, so we just need a
+   magical way to detect which one we are used.  */
+#ifdef _ASM_RISCV_SIGCONTEXT_H
+#define SIGCONTEXT_PC(SC) (SC)->sc_regs.pc
+#else
+#define SIGCONTEXT_PC(SC) (SC)->gregs[0]
+#endif
+
 static _Unwind_Reason_Code
 riscv_fallback_frame_state (struct _Unwind_Context *context,
                            _Unwind_FrameState * fs)
@@ -50,6 +60,8 @@ riscv_fallback_frame_state (struct _Unwind_Context *context,
   uint16_t *pc = context->ra;
   struct sigcontext *sc;
   int i;
+  /* Get regsister offest from register size.  */
+  _Unwind_Ptr reg_offset = __riscv_xlen / 8;
 
   /* A signal frame will have a return address pointing to
      __default_sa_restorer. This code is hardwired as:
@@ -73,17 +85,23 @@ riscv_fallback_frame_state (struct _Unwind_Context *context,
 
   for (i = 0; i < 32; i++)
     {
+      /* Restore all registers value from kernel structures.
+        The corresponding bits in the Linux kernel are in
+        arch/riscv/include/asm/ptrace.h.  */
       fs->regs.reg[i].how = REG_SAVED_OFFSET;
-      fs->regs.reg[i].loc.offset = (_Unwind_Ptr) &sc->gregs[i] - new_cfa;
+      fs->regs.reg[i].loc.offset
+       = (_Unwind_Ptr) &SIGCONTEXT_PC (sc) + (i * reg_offset) - new_cfa;
     }
 
   fs->signal_frame = 1;
   fs->retaddr_column = __LIBGCC_DWARF_ALT_FRAME_RETURN_COLUMN__;
   fs->regs.reg[fs->retaddr_column].how = REG_SAVED_VAL_OFFSET;
   fs->regs.reg[fs->retaddr_column].loc.offset =
-    (_Unwind_Ptr) sc->gregs[0] - new_cfa;
+    (_Unwind_Ptr) SIGCONTEXT_PC (sc) - new_cfa;
 
   return _URC_NO_REASON;
 }
 
+#undef SIGCONTEXT_PC
+
 #endif
-- 
2.34.0