Implement <unwind-link.h> for dynamically loading the libgcc_s unwinder

This will be used to consolidate the libgcc_s access for backtrace
and pthread_cancel.

Unlike the existing backtrace implementations, it provides some
hardening based on pointer mangling.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 74 insertions, 0 deletions

diff --git a/sysdeps/mips/unwind-arch.h b/sysdeps/mips/unwind-arch.h
new file mode 100644
index 0000000000..78de42e001
--- /dev/null
+++ b/sysdeps/mips/unwind-arch.h
@@ -0,0 +1,74 @@
+/* Dynamic loading of the libgcc unwinder.  MIPS customization.
+   Copyright (C) 2020-2021 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 _ARCH_UNWIND_LINK_H
+#define _ARCH_UNWIND_LINK_H
+
+#include <stdint.h>
+#include <sys/syscall.h>
+
+#define UNWIND_LINK_GETIP 1
+#define UNWIND_LINK_FRAME_STATE_FOR 1
+#define UNWIND_LINK_FRAME_ADJUSTMENT 1
+#define UNWIND_LINK_EXTRA_FIELDS
+#define UNWIND_LINK_EXTRA_INIT
+
+/* 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 idea here is to adjust the obtained signal frame ADDR value and remove
+   the libgcc added value by checking if the previous frame is a signal frame
+   one.
+
+   [1] libgcc/config/mips/linux-unwind.h from gcc code.
+   [2] gcc/config/mips/mips.h from gcc code.  */
+
+static inline void *
+unwind_arch_adjustment (void *prev, void *addr)
+{
+  uint32_t *pc = (uint32_t *) prev;
+
+  if (pc == NULL)
+    return addr;
+
+  /* For MIPS16 or microMIPS frame libgcc makes no adjustment.  */
+  if ((uintptr_t) pc & 0x3)
+    return addr;
+
+  /* The vDSO containes either
+
+     24021061 li v0, 0x1061 (rt_sigreturn)
+     0000000c syscall
+        or
+     24021017 li v0, 0x1017 (sigreturn)
+     0000000c syscall  */
+  if (pc[1] != 0x0000000c)
+    return addr;
+#if _MIPS_SIM == _ABIO32
+  if (pc[0] == (0x24020000 | __NR_sigreturn))
+    return (void *) ((uintptr_t) addr - 2);
+#endif
+  if (pc[0] == (0x24020000 | __NR_rt_sigreturn))
+    return (void *) ((uintptr_t) addr - 2);
+
+  return addr;
+}
+
+#endif /* _ARCH_UNWIND_LINK_H */