Step over fork/vfork syscall insn in gdbserver

We can also extend disp-step-syscall.exp to test GDBserver step over
breakpoint on syscall instruction.  That is, we set a breakpoint
with a false condition on syscall instruction, so that GDBserver will
step over it.

This test triggers a GDBserver internal error, which can be fixed by
this series.

(gdb) PASS: gdb.base/disp-step-syscall.exp: fork: break cond on target: break on syscall insns
continue^M
Continuing.^M
Remote connection closed^M
(gdb) FAIL: gdb.base/disp-step-syscall.exp: fork: break cond on target: continue to fork again

In GDBserver, there is an internal error,

/home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/linux-low.c:1922: A problem internal to GDBserver has been detected.
unsuspend LWP 25554, suspended=-1

the simplified reproducer is like,

$ ./gdb ./testsuite/outputs/gdb.base/disp-step-syscall/disp-step-fork
(gdb) b main
(gdb) c
(gdb) disassemble fork // in order to find the address of insn 'syscall'
....
   0x00007ffff7ad6023 <+179>:	syscall
(gdb) b *0x00007ffff7ad6023 if main == 0
(gdb) c

gdb/testsuite:

2016-03-03  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/disp-step-syscall.exp (break_cond_on_syscall): New.
	If target supports condition evaluation on target, invoke
	break_cond_on_syscall for fork and vfork.

2 files changed, 63 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 244efcc..eab1255 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,11 @@
 2016-03-03  Yao Qi  <yao.qi@linaro.org>
 
+	* gdb.base/disp-step-syscall.exp (break_cond_on_syscall): New.
+	If target supports condition evaluation on target, invoke
+	break_cond_on_syscall for fork and vfork.
+
+2016-03-03  Yao Qi  <yao.qi@linaro.org>
+
 	* gdb.base/disp-step-syscall.exp: Don't invoke
 	support_displaced_stepping.
 	(disp_step_cross_syscall): Test with displaced stepping off and
diff --git a/gdb/testsuite/gdb.base/disp-step-syscall.exp b/gdb/testsuite/gdb.base/disp-step-syscall.exp
index 3cf436d..0aac649 100644
--- a/gdb/testsuite/gdb.base/disp-step-syscall.exp
+++ b/gdb/testsuite/gdb.base/disp-step-syscall.exp
@@ -168,5 +168,62 @@ proc disp_step_cross_syscall { syscall } {
     }
 }
 
+# Set a breakpoint with a condition that evals false on syscall
+# instruction.  In fact, it tests GDBserver steps over syscall
+# instruction.
+
+proc break_cond_on_syscall { syscall } {
+    with_test_prefix "break cond on target : $syscall" {
+	set testfile "disp-step-$syscall"
+
+	set ret [setup $syscall]
+
+	set syscall_insn_addr [lindex $ret 0]
+	set syscall_insn_next_addr [lindex $ret 1]
+	if { $syscall_insn_addr == -1 } {
+	    return -1
+	}
+
+	gdb_test "continue" "Continuing\\..*Breakpoint \[0-9\]+, (.* in |__libc_|)$syscall \\(\\).*" \
+	    "continue to $syscall"
+	# Delete breakpoint syscall insns to avoid interference with other syscalls.
+	delete_breakpoints
+
+
+	# Create a breakpoint with a condition that evals false.
+	gdb_test "break \*$syscall_insn_addr if main == 0" \
+	    "Breakpoint \[0-9\]* at .*"
+
+	gdb_test "break marker" "Breakpoint.*at.* file .*${testfile}.c, line.*"
+	gdb_test "continue" "Continuing\\..*Breakpoint \[0-9\]+, marker \\(\\) at.*" \
+	    "continue to marker ($syscall)"
+    }
+}
+
 disp_step_cross_syscall "fork"
 disp_step_cross_syscall "vfork"
+
+set testfile "disp-step-fork"
+clean_restart $testfile
+if { ![runto main] } then {
+    fail "run to main"
+    return -1
+}
+
+set cond_bp_target 1
+
+set test "set breakpoint condition-evaluation target"
+gdb_test_multiple $test $test {
+    -re "warning: Target does not support breakpoint condition evaluation.\r\nUsing host evaluation mode instead.\r\n$gdb_prompt $" {
+	# Target doesn't support breakpoint condition
+	# evaluation on its side.
+	set cond_bp_target 0
+    }
+    -re "^$test\r\n$gdb_prompt $" {
+    }
+}
+
+if { $cond_bp_target } {
+    break_cond_on_syscall "fork"
+    break_cond_on_syscall "vfork"
+}