gdb/riscv: Don't error when decoding a 6 or 8 byte instruction

If the RISC-V prologue scanner finds a 6 or 8 byte instruction we
currently throw an internal error, which is not great for the user.

A mechanism already exists in the prologue scanner to leave
instructions marked as unknown so that we can stop the prologue scan
without raising an error, this is used for all 2 and 4 byte
instructions that are not part of the small set the prologue scanner
actually understands.

This commit changes GDB so that all 6 and 8 byte instructions are
marked as unknown, rather than causing an error.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_insn::decode): Gracefully ignore
	instructions of lengths 6 or 8 bytes.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-unwind-long-insn-6.s: New file.
	* gdb.arch/riscv-unwind-long-insn-8.s: New file.
	* gdb.arch/riscv-unwind-long-insn.c: New file.
	* gdb.arch/riscv-unwind-long-insn.exp: New file.

1 files changed, 61 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.exp b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.exp
new file mode 100644
index 0000000..ddb9142
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/riscv-unwind-long-insn.exp
@@ -0,0 +1,61 @@
+# Copyright 2019 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program 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 General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This tests GDB's ability to handle 6 and 8 byte instructions in the
+# RISC-V prologue scanner.  These instruction should be ignored, but
+# should not result in an error that interrupts the debug session.
+#
+# Each of the files riscv-unwind-long-insn-*.s include a function
+# (func) that contains a fake long instruction (6 or 8 bytes) in the
+# prologue.  We trick GDB into parsing the fake instruction by tail
+# calling from a different function, 'bar' to the middle of 'func'.
+
+if {![istarget "riscv*-*-*"]} {
+    verbose "Skipping ${gdb_test_file_name}."
+    return
+}
+
+foreach_with_prefix {insn_size} {6 8} {
+    standard_testfile riscv-unwind-long-insn.c \
+	riscv-unwind-long-insn-${insn_size}.s
+
+    set testfile "${testfile}-${insn_size}"
+    if {[prepare_for_testing "failed to prepare" $testfile \
+	     "$srcfile $srcfile2"  debug]} {
+	return -1
+    }
+
+    if ![runto_main] then {
+	fail "can't run to main"
+	return 0
+    }
+
+    gdb_breakpoint "bar"
+    gdb_continue_to_breakpoint "bar"
+
+    # This next single instruction step takes us through a tail-call
+    # from 'bar' into 'func'.
+    gdb_test "si" "func \(\).*"
+
+    # Now check that we have a sane backtrace.
+    gdb_test "bt" \
+	[multi_line \
+	     "#0\[ \t\]*func \\\(\\\) at .*$srcfile2:\[0-9\]+" \
+	     "#1\[ \t\]*$hex in main \\\(\\\) at .*$srcfile:\[0-9\]+"] \
+	"Backtrace to the main frame"
+
+    # Finally finish, and we should end up back in main.
+    gdb_test "finish" "main \\\(\\\) at .*$srcfile:.*"
+}