mips: Fix bracktrace result for signal frames

MIPS fallback code handle a frame where its FDE can not be obtained
(for instance a signal frame) by reading the kernel allocated signal frame
and adding '2' to the value of 'sc_pc' [1].  The added value is used to
recognize an end of an EH region on mips16 [2].

The fix adjust the obtained signal frame value and remove the libgcc added
value by checking if the previous frame is a signal frame one.

Checked with backtrace and tst-sigcontext-get_pc tests on mips-linux-gnu
and mips64-linux-gnu.

[1] libgcc/config/mips/linux-unwind.h from gcc code.
[2] gcc/config/mips/mips.h from gcc code.  */

1 files changed, 30 insertions, 0 deletions

diff --git a/sysdeps/generic/unwind-arch.h b/sysdeps/generic/unwind-arch.h
new file mode 100644
index 0000000000..d712e5e11d
--- /dev/null
+++ b/sysdeps/generic/unwind-arch.h
@@ -0,0 +1,30 @@
+/* Return backtrace of current program state.  Arch-specific bits.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _UNWIND_ARCH_H
+#define _UNWIND_ARCH_H
+
+#include <unwind.h>
+
+static inline void *
+unwind_arch_adjustment (void *prev, void *addr)
+{
+  return addr;
+}
+
+#endif