[PATCH] gdb: handle endbr64 instruction in amd64_analyze_prologue

[Simon Marchi <simon.marchi@efficios.com>]

Some GCCs now enable -fcf-protection by default.  This is the case, for
example, with GCC 9.3.0 on Ubuntu 20.04.  Enabling it causes the
`endbr64` instruction to be inserted at the beginning of all functions
and that breaks GDB's prologue analysis.

I noticed this because it gives many failures in gdb.base/break.exp.
But let's take this dummy program and put a breakpoint on main:

    int main(void)
    {
        return 0;
    }

Without -fcf-protection, the breakpoint is correctly put after the prologue:

    $ gcc test.c -g3 -O0 -fcf-protection=none
    $ ./gdb -q -nx --data-directory=data-directory a.out
    Reading symbols from a.out...
    (gdb) disassemble main
    Dump of assembler code for function main:
       0x0000000000001129 <+0>:     push   %rbp
       0x000000000000112a <+1>:     mov    %rsp,%rbp
       0x000000000000112d <+4>:     mov    $0x0,%eax
       0x0000000000001132 <+9>:     pop    %rbp
       0x0000000000001133 <+10>:    retq
    End of assembler dump.
    (gdb) b main
    Breakpoint 1 at 0x112d: file test.c, line 3.

With -fcf-protection, the breakpoint is incorrectly put on the first
byte of the function:

    $ gcc test.c -g3 -O0 -fcf-protection=full
    $ ./gdb -q -nx --data-directory=data-directory a.out
    Reading symbols from a.out...
    (gdb) disassemble main
    Dump of assembler code for function main:
       0x0000000000001129 <+0>:     endbr64
       0x000000000000112d <+4>:     push   %rbp
       0x000000000000112e <+5>:     mov    %rsp,%rbp
       0x0000000000001131 <+8>:     mov    $0x0,%eax
       0x0000000000001136 <+13>:    pop    %rbp
       0x0000000000001137 <+14>:    retq
    End of assembler dump.
    (gdb) b main
    Breakpoint 1 at 0x1129: file test.c, line 2.

Stepping in amd64_skip_prologue, we can see that the prologue analysis,
for GCC-compiled programs, is done in amd64_analyze_prologue by decoding
the instructions and looking for typical patterns.  This patch changes
the analysis to check for a prologue starting with the `endbr64`
instruction, and skip it if it's there.

gdb/ChangeLog:

	* amd64-tdep.c (amd64_analyze_prologue): Check for `endbr64`
	instruction, skip it if it's there.

gdb/testsuite/ChangeLog:

	* gdb.arch/amd64-prologue-skip-cf-protection.exp: New file.
	* gdb.arch/amd64-prologue-skip-cf-protection.c: New file.
---
 gdb/amd64-tdep.c                              | 19 +++++++
 .../amd64-prologue-skip-cf-protection.c       | 21 +++++++
 .../amd64-prologue-skip-cf-protection.exp     | 56 +++++++++++++++++++
 3 files changed, 96 insertions(+)
 create mode 100644 gdb/testsuite/gdb.arch/amd64-prologue-skip-cf-protection.c
 create mode 100644 gdb/testsuite/gdb.arch/amd64-prologue-skip-cf-protection.exp

diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
index 5c56a970d8c..c846447a8e0 100644
--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -2362,6 +2362,9 @@ amd64_x32_analyze_stack_align (CORE_ADDR pc, CORE_ADDR current_pc,
       pushq %rbp        0x55
       movl %esp, %ebp   0x89 0xe5 (or 0x8b 0xec)
 
+   The `endbr64` instruction can be found before these sequences, and will be
+   skipped if found.
+
    Any function that doesn't start with one of these sequences will be
    assumed to have no prologue and thus no valid frame pointer in
    %rbp.  */
@@ -2372,6 +2375,8 @@ amd64_analyze_prologue (struct gdbarch *gdbarch,
 			struct amd64_frame_cache *cache)
 {
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
+  /* The `endbr64` instruction.  */
+  static const gdb_byte endbr64[4] = { 0xf3, 0x0f, 0x1e, 0xfa };
   /* There are two variations of movq %rsp, %rbp.  */
   static const gdb_byte mov_rsp_rbp_1[3] = { 0x48, 0x89, 0xe5 };
   static const gdb_byte mov_rsp_rbp_2[3] = { 0x48, 0x8b, 0xec };
@@ -2392,6 +2397,20 @@ amd64_analyze_prologue (struct gdbarch *gdbarch,
 
   op = read_code_unsigned_integer (pc, 1, byte_order);
 
+  /* Check for the `endbr64` instruction, skip it if found.  */
+  if (op == endbr64[0])
+    {
+      read_code (pc + 1, buf, 3);
+
+      if (memcmp (buf, &endbr64[1], 3) == 0)
+	pc += 4;
+
+      op = read_code_unsigned_integer (pc, 1, byte_order);
+    }
+
+  if (current_pc <= pc)
+    return current_pc;
+
   if (op == 0x55)		/* pushq %rbp */
     {
       /* Take into account that we've executed the `pushq %rbp' that
diff --git a/gdb/testsuite/gdb.arch/amd64-prologue-skip-cf-protection.c b/gdb/testsuite/gdb.arch/amd64-prologue-skip-cf-protection.c
new file mode 100644
index 00000000000..968f1832d8c
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/amd64-prologue-skip-cf-protection.c
@@ -0,0 +1,21 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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/>.  */
+
+int foo (void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.arch/amd64-prologue-skip-cf-protection.exp b/gdb/testsuite/gdb.arch/amd64-prologue-skip-cf-protection.exp
new file mode 100644
index 00000000000..c3348054ae0
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/amd64-prologue-skip-cf-protection.exp
@@ -0,0 +1,56 @@
+# Copyright 2020 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/>.
+
+# Test skipping a prologue that was generated with gcc's -fcf-protection=full
+# (control flow protection) option.
+#
+# This option places an `endbr64` instruction at the start of all functions,
+# which can interfere with prologue analysis.
+
+standard_testfile .c
+set binfile ${binfile}.o
+
+if { ![istarget x86_64-*-* ] || ![is_lp64_target] } {
+    verbose "Skipping ${testfile}."
+    return
+}
+
+set opts {debug additional_flags=-fcf-protection=full}
+
+if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" object $opts] != "" } {
+    untested "failed to compile"
+    return
+}
+
+clean_restart ${binfile}
+
+# Get start address of function foo.
+set foo_addr [get_integer_valueof &foo -1]
+gdb_assert {$foo_addr != -1}
+
+# Put breakpoint on foo, get the address where the breakpoint was installed.
+gdb_test_multiple "break foo" "break on foo, get address" {
+    -re "Breakpoint $decimal at ($hex)" {
+	set bp_addr $expect_out(1,string)
+
+	# Convert to decimal.
+	set bp_addr [expr $bp_addr]
+
+	pass $gdb_test_name
+    }
+}
+
+# Make sure some prologue was skipped.
+gdb_assert {$bp_addr > $foo_addr}
-- 
2.26.2