x86 Linux watchpoints: Couldn't write debug register: Invalid argument.

This patch fixes this on x86 Linux:

 (gdb) watch *buf@2
 Hardware watchpoint 8: *buf@2
 (gdb) si
 0x00000000004005a7      34        for (i = 0; i < 100000; i++); /* stepi line */
 (gdb) del
 Delete all breakpoints? (y or n) y
 (gdb) watch *(buf+1)@1
 Hardware watchpoint 9: *(buf+1)@1
 (gdb) si
 0x00000000004005a7 in main () at ../../../src/gdb/testsuite/gdb.base/watchpoint-reuse-slot.c:34
 34        for (i = 0; i < 100000; i++); /* stepi line */
 Couldn't write debug register: Invalid argument.
 (gdb)

In the example above the debug registers are being switched from this
state:

        CONTROL (DR7): 0000000000050101          STATUS (DR6): 0000000000000000
        DR0: addr=0x0000000000601040, ref.count=1  DR1: addr=0x0000000000000000, ref.count=0
        DR2: addr=0x0000000000000000, ref.count=0  DR3: addr=0x0000000000000000, ref.count=0

to this:

        CONTROL (DR7): 0000000000010101          STATUS (DR6): 0000000000000000
        DR0: addr=0x0000000000601041, ref.count=1  DR1: addr=0x0000000000000000, ref.count=0
        DR2: addr=0x0000000000000000, ref.count=0  DR3: addr=0x0000000000000000, ref.count=0

That is, before, DR7 was setup for watching a 2 byte region starting
at what's in DR0 (0x601040).

And after, DR7 is setup for watching a 1 byte region starting at
what's in DR0 (0x601041).

We always write DR0..DR3 before DR7, because if we enable a slot's
bits in DR7, you need to have already written the corresponding
DR0..DR3 registers -- the kernel rejects the DR7 write with EINVAL
otherwise.

The error shown above is the opposite scenario.  When we try to write
0x601041 to DR0, DR7's bits still indicate intent of watching a 2-byte
region.  That DR0/DR7 combination is invalid, because 0x601041 is
unaligned.  To watch two bytes, we'd have to use two slots.  So the
kernel errors out with EINVAL.

Fix this by always first clearing DR7, then writing DR0..DR3, and then
setting DR7's bits.

A little optimization -- if we're disabling the last watchpoint, then
we can clear DR7 just once.  The changes to nat/i386-dregs.c make that
easier to detect, and as bonus, they make it a little easier to make
sense of DR7 in the debug logs, as we no longer need to remember we're
seeing stale bits.

Tested on x86_64 Fedora 20, native and GDBserver.

This adds an exhaustive test that switches between many different
combinations of watchpoint types and addresses and widths.

gdb/
2014-06-23  Pedro Alves  <palves@redhat.com>

	* amd64-linux-nat.c (amd64_linux_prepare_to_resume): Clear
	DR_CONTROL before setting DR0..DR3.
	* i386-linux-nat.c (i386_linux_prepare_to_resume): Likewise.
	* nat/i386-dregs.c (i386_remove_aligned_watchpoint): Clear all
	bits of DR_CONTROL related to the debug register slot being
	disabled.  If all slots are vacant, clear local slowdown as well,
	and assert DR_CONTROL is 0.

gdb/gdbserver/
2014-06-23  Pedro Alves  <palves@redhat.com>

	* linux-x86-low.c (x86_linux_prepare_to_resume): Clear DR_CONTROL
	before setting DR0..DR3.

gdb/testsuite/
2014-06-23  Pedro Alves  <palves@redhat.com>

	* gdb.base/watchpoint-reuse-slot.c: New file.
	* gdb.base/watchpoint-reuse-slot.exp: New file.

1 files changed, 187 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/watchpoint-reuse-slot.exp b/gdb/testsuite/gdb.base/watchpoint-reuse-slot.exp
new file mode 100644
index 0000000000..aa3039886d
--- /dev/null
+++ b/gdb/testsuite/gdb.base/watchpoint-reuse-slot.exp
@@ -0,0 +1,187 @@
+# Copyright 2014 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 alternating between watchpoint types, watching a sliding window
+# of addresses (thus alternating between aligned and unaligned
+# addresses).  Only a single watchpoint exists at any given time.  On
+# targets that only update the debug registers on resume, this
+# stresses the debug register setup code, both in GDB and in the
+# target/kernel as one watchpoint replaces the other in a single
+# operation.  (Note that we don't have any of these watchpoints
+# trigger.)
+
+if [target_info exists gdb,no_hardware_watchpoints] {
+    unsupported "no target support"
+    return
+}
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+if ![runto_main] then {
+    fail "Can't run to main"
+    return 0
+}
+
+# The line we'll be stepping.
+set srcline [gdb_get_line_number "stepi line"]
+
+# The address the program is stopped at currently.
+set cur_addr ""
+
+# Get the current PC.
+
+proc get_pc {} {
+    global hex gdb_prompt
+
+    set addr ""
+    set test "get PC"
+    gdb_test_multiple "p /x \$pc" "$test" {
+	-re " = ($hex).*$gdb_prompt $" {
+	    set addr $expect_out(1,string)
+	    pass "$test"
+	}
+    }
+
+    return $addr
+}
+
+
+# Issue a stepi, and make sure the program advanced past the current
+# instruction (stored in the CUR_ADDR global).
+
+proc stepi {} {
+    global hex gdb_prompt cur_addr
+
+    set srcline "  for (i = 0; i < 100000; i++); /* stepi line */"
+    set test "stepi advanced"
+    gdb_test_multiple "stepi" $test {
+	-re "($hex).*[string_to_regexp $srcline]\r\n$gdb_prompt $" {
+	    set addr $expect_out(1,string)
+	    if {$addr != $cur_addr} {
+		pass $test
+	    } else {
+		fail $test
+	    }
+	    set cur_addr addr
+	}
+    }
+}
+
+gdb_breakpoint $srcline
+gdb_continue_to_breakpoint "stepi line"
+
+# The test tries various sequences of different types of watchpoints.
+# Probe for support first.
+
+# So we get an immediate warning/error if the target doesn't support a
+# given watchpoint type.
+gdb_test_no_output "set breakpoint always-inserted on"
+
+# The list of supported commands.  Below we'll probe for support and
+# add elements to this list.
+set cmds {}
+
+foreach cmd {"watch" "awatch" "rwatch"} {
+    set test $cmd
+    gdb_test_multiple "$cmd buf.byte\[0\]" $test {
+	-re "You may have requested too many.*$gdb_prompt $" {
+	    unsupported $test
+	}
+	-re "$gdb_prompt $" {
+	    pass $test
+	    lappend cmds $cmd
+	}
+    }
+
+   delete_breakpoints
+}
+
+set test "hbreak"
+gdb_test_multiple "hbreak main" $test {
+    -re "You may have requested too many.*$gdb_prompt $" {
+	pass $test
+    }
+    -re "$gdb_prompt $" {
+	pass $test
+	lappend cmds "hbreak"
+    }
+}
+
+delete_breakpoints
+
+set cur_addr [get_pc]
+
+# Watch WIDTH bytes at BASE + OFFSET.  CMD specifices the specific
+# type of watchpoint to use.  If CMD is "hbreak", WIDTH is ignored.
+
+proc watch_command {cmd base offset width} {
+    global srcfile srcline hex
+
+    if {$cmd == "hbreak"} {
+	set expr "*(buf.byte + $base + $offset)"
+	gdb_test "hbreak $expr" "Hardware assisted breakpoint \[0-9\]+ at $hex"
+    } elseif {$cmd == "watch"} {
+	set expr "*(buf.byte + $base + $offset)@$width"
+	gdb_test "$cmd $expr" \
+	    "Hardware watchpoint \[0-9\]+: [string_to_regexp $expr]"
+    } elseif {$cmd == "awatch"} {
+	set expr "*(buf.byte + $base + $offset)@$width"
+	gdb_test "$cmd $expr" \
+	    "Hardware access \\(read/write\\) watchpoint \[0-9\]+: [string_to_regexp $expr]"
+    } elseif {$cmd == "rwatch"} {
+	set expr "*(buf.byte + $base + $offset)@$width"
+	gdb_test "$cmd $expr" \
+	    "Hardware read watchpoint \[0-9\]+: [string_to_regexp $expr]"
+    }
+}
+
+# Run test proper.  See intro for description.
+
+foreach always_inserted {"off" "on" } {
+    gdb_test_no_output "set breakpoint always-inserted $always_inserted"
+    foreach cmd1 $cmds {
+	foreach cmd2 $cmds {
+	    for {set width 1} {$width < 4} {incr width} {
+
+		if {$cmd1 == "hbreak" && $cmd2 == "hbreak" && $width > 1} {
+		    # hbreak ignores WIDTH, no use testing more than
+		    # once.
+		    continue
+		}
+
+		for {set x 0} {$x < 4} {incr x} {
+		    set prefix "always-inserted $always_inserted: "
+		    append prefix "$cmd1 x $cmd2: "
+		    with_test_prefix "$prefix: width $width, iter $x" {
+			with_test_prefix "base + 0" {
+			    watch_command $cmd1 $x 0 $width
+			    stepi
+			    gdb_test_no_output "delete \$bpnum"
+			}
+			with_test_prefix "base + 1" {
+			    watch_command $cmd2 $x 1 $width
+			    stepi
+			    gdb_test_no_output "delete \$bpnum"
+			}
+		    }
+		}
+	    }
+	}
+    }
+}