Re: [patch] ia64: Fix breakpoints memory shadow

Jan Kratochvil
2008-11-26T06:34:53Z

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On Tue, 25 Nov 2008 07:12:35 +0100, Joel Brobecker wrote:
> > Still I rather found another defect compared to mem-break.c in the ia64
> > breakpoints code. It is another problem unrelated to the original intention
> > of this ia64 patch.
>
> Jan, please don't take it personally, I really appreciate the way
> you are trying to make your patch better and better. But since
> this was unrelated, I really wished that you committed the first part
> as approved, and then sent a followup patch that dealt with that part
> on its own.

As I got also this mail before the commit
On Sat, 22 Nov 2008 12:30:04 +0100, Eli Zaretskii wrote:
> However, I'm slightly worried by these parts in your code:

I considered it as a valid veto of your approval until Eli's concerns get
resolved. I appreciate your time although I do not see I would make mistakes
in the unwritten formal approval process I try to follow.

> With the approach you took, I had to fish out the previous version of the
> patch, and then compare the two patches to see exactly what changed.

Using this VIM macro for reviewing "metadiffs":
noremap <Esc>D :set hlsearch<cr>/^[+-][+-]\([^+-].*\\|\)$<cr>

The committed patch has these changes against the last post here:
ia64_memory_insert_breakpoint: make_show_memory_breakpoints_cleanup (0 -> 1);
- A real Bug of my former patch (although it was not a regression).
ia64_memory_insert_breakpoint: == IA64_BREAKPOINT => internal_error
- New sanity check, unaware it would be possible to reach it as a user.

> I propose a warning saying something like this:
>
> cannot remove breakpoint at address %s, no break instruction at such address.

Therefore included this warning again:
ia64_memory_remove_breakpoint: != IA64_BREAKPOINT => warning
- Happens for: gdb.base/checkpoint.exp gdb.base/foll-fork.exp
- IMO such sanity check is missing even in default_memory_remove_breakpoint
and these warnings would be then present also on x86. Not checked, though.

Thanks,
Jan

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gdb/
2008-11-25 Jan Kratochvil <jan.kratochvil@redhat.com>

Fix automatic restoration of breakpoints memory for ia64.
* ia64-tdep.c: New #if check on BREAKPOINT_MAX vs. BUNDLE_LEN.
(ia64_memory_insert_breakpoint): New comment part for SHADOW_CONTENTS
content. Remove variable instr. New variable cleanup. Disable
automatic breakpoints restoration. PLACED_SIZE and SHADOW_LEN are now
set larger, to BUNDLE_LEN - 2. Variable `bundle' type update. Return
error if even just final target_write_memory has failed.
(ia64_memory_remove_breakpoint): Rename variables bundle to bundle_mem
and instr to instr_saved. New variables bundle_saved and
instr_breakpoint. Comment new reasons why we need to disable automatic
restoration of breakpoints. Assert PLACED_SIZE and SHADOW_LEN. New
check of the original memory content. Return error if even just final
target_write_memory has failed.
(ia64_breakpoint_from_pc): Implement the emulation of permanent
breakpoints compatible with current bp_loc_is_permanent.
(template_encoding_table): Make it `const'.
* breakpoint.c (bp_loc_is_permanent): Support unsupported software
breakpoints. New variables `cleanup' and `retval'.
* monitor.c (monitor_insert_breakpoint): Remove unused variable `bp'.

gdb/doc/
2008-11-25 Jan Kratochvil <jan.kratochvil@redhat.com>

* gdbint.texinfo (Target Conditionals): Extend the
gdbarch_breakpoint_from_pc description.

gdb/testsuite/
2008-11-25 Jan Kratochvil <jan.kratochvil@redhat.com>

* gdb.base/breakpoint-shadow.exp, gdb.base/breakpoint-shadow.c: New.
* gdb.base/start.exp: New comment about an alternative - `runto_main'.

===================================================================
RCS file: /cvs/src/src/gdb/breakpoint.c,v
retrieving revision 1.362
retrieving revision 1.363
diff -u -r1.362 -r1.363
--- src/gdb/breakpoint.c 2008/11/22 04:41:45 1.362
+++ src/gdb/breakpoint.c 2008/11/26 05:27:48 1.363
@@ -5010,19 +5010,32 @@
CORE_ADDR addr;
const gdb_byte *brk;
gdb_byte *target_mem;
+ struct cleanup *cleanup;
+ int retval = 0;

gdb_assert (loc != NULL);

addr = loc->address;
brk = gdbarch_breakpoint_from_pc (current_gdbarch, &addr, &len);

+ /* Software breakpoints unsupported? */
+ if (brk == NULL)
+ return 0;
+
target_mem = alloca (len);

+ /* Enable the automatic memory restoration from breakpoints while
+ we read the memory. Otherwise we could say about our temporary
+ breakpoints they are permanent. */
+ cleanup = make_show_memory_breakpoints_cleanup (0);
+
if (target_read_memory (loc->address, target_mem, len) == 0
&& memcmp (target_mem, brk, len) == 0)
- return 1;
+ retval = 1;

- return 0;
+ do_cleanups (cleanup);
+
+ return retval;
}

===================================================================
RCS file: /cvs/src/src/gdb/ia64-tdep.c,v
retrieving revision 1.185
retrieving revision 1.186
diff -u -r1.185 -r1.186
--- src/gdb/ia64-tdep.c 2008/11/13 05:05:07 1.185
+++ src/gdb/ia64-tdep.c 2008/11/26 05:27:48 1.186
@@ -110,6 +110,12 @@

#define BUNDLE_LEN 16

+/* See the saved memory layout comment for ia64_memory_insert_breakpoint. */
+
+#if BREAKPOINT_MAX < BUNDLE_LEN - 2
+# error "BREAKPOINT_MAX < BUNDLE_LEN - 2"
+#endif
+
static gdbarch_init_ftype ia64_gdbarch_init;

static gdbarch_register_name_ftype ia64_register_name;
@@ -442,7 +448,7 @@
replace_bit_field (bundle, instr, 5+41*slotnum, 41);
}

-static enum instruction_type template_encoding_table[32][3] =
+static const enum instruction_type template_encoding_table[32][3] =
{
{ M, I, I }, /* 00 */
{ M, I, I }, /* 01 */
@@ -545,7 +551,45 @@
simulators. So I changed the pattern slightly to do "break.i 0x080001"
instead. But that didn't work either (I later found out that this
pattern was used by the simulator that I was using.) So I ended up
- using the pattern seen below. */
+ using the pattern seen below.
+
+ SHADOW_CONTENTS has byte-based addressing (PLACED_ADDRESS and SHADOW_LEN)
+ while we need bit-based addressing as the instructions length is 41 bits and
+ we must not modify/corrupt the adjacent slots in the same bundle.
+ Fortunately we may store larger memory incl. the adjacent bits with the
+ original memory content (not the possibly already stored breakpoints there).
+ We need to be careful in ia64_memory_remove_breakpoint to always restore
+ only the specific bits of this instruction ignoring any adjacent stored
+ bits.
+
+ We use the original addressing with the low nibble in the range <0..2> which
+ gets incorrectly interpreted by generic non-ia64 breakpoint_restore_shadows
+ as the direct byte offset of SHADOW_CONTENTS. We store whole BUNDLE_LEN
+ bytes just without these two possibly skipped bytes to not to exceed to the
+ next bundle.
+
+ If we would like to store the whole bundle to SHADOW_CONTENTS we would have
+ to store already the base address (`address & ~0x0f') into PLACED_ADDRESS.
+ In such case there is no other place where to store
+ SLOTNUM (`adress & 0x0f', value in the range <0..2>). We need to know
+ SLOTNUM in ia64_memory_remove_breakpoint.
+
+ ia64 16-byte bundle layout:
+ | 5 bits | slot 0 with 41 bits | slot 1 with 41 bits | slot 2 with 41 bits |
+
+ The current addressing used by the code below:
+ original PC placed_address placed_size required covered
+ == bp_tgt->shadow_len reqd \subset covered
+ 0xABCDE0 0xABCDE0 0xE <0x0...0x5> <0x0..0xD>
+ 0xABCDE1 0xABCDE1 0xE <0x5...0xA> <0x1..0xE>
+ 0xABCDE2 0xABCDE2 0xE <0xA...0xF> <0x2..0xF>
+
+ `objdump -d' and some other tools show a bit unjustified offsets:
+ original PC byte where starts the instruction objdump offset
+ 0xABCDE0 0xABCDE0 0xABCDE0
+ 0xABCDE1 0xABCDE5 0xABCDE6
+ 0xABCDE2 0xABCDEA 0xABCDEC
+ */

#define IA64_BREAKPOINT 0x00003333300LL

@@ -554,34 +598,55 @@
struct bp_target_info *bp_tgt)
{
CORE_ADDR addr = bp_tgt->placed_address;
- char bundle[BUNDLE_LEN];
+ gdb_byte bundle[BUNDLE_LEN];
int slotnum = (int) (addr & 0x0f) / SLOT_MULTIPLIER;
- long long instr;
+ long long instr_breakpoint;
int val;
int template;
+ struct cleanup *cleanup;

if (slotnum > 2)
error (_("Can't insert breakpoint for slot numbers greater than 2."));

addr &= ~0x0f;

+ /* Disable the automatic memory restoration from breakpoints while
+ we read our instruction bundle. Otherwise, the general restoration
+ mechanism kicks in and we would possibly remove parts of the adjacent
+ placed breakpoints. It is due to our SHADOW_CONTENTS overlapping the real
+ breakpoint instruction bits region. */
+ cleanup = make_show_memory_breakpoints_cleanup (1);
val = target_read_memory (addr, bundle, BUNDLE_LEN);

- /* Check for L type instruction in 2nd slot, if present then
- bump up the slot number to the 3rd slot */
+ /* Check for L type instruction in slot 1, if present then bump up the slot
+ number to the slot 2. */
template = extract_bit_field (bundle, 0, 5);
- if (slotnum == 1 && template_encoding_table[template][1] == L)
- {
- slotnum = 2;
- }
+ if (slotnum == 1 && template_encoding_table[template][slotnum] == L)
+ slotnum = 2;

- instr = slotN_contents (bundle, slotnum);
- memcpy (bp_tgt->shadow_contents, &instr, sizeof (instr));
- bp_tgt->placed_size = bp_tgt->shadow_len = sizeof (instr);
+ /* Slot number 2 may skip at most 2 bytes at the beginning. */
+ bp_tgt->placed_size = bp_tgt->shadow_len = BUNDLE_LEN - 2;
+
+ /* Store the whole bundle, except for the initial skipped bytes by the slot
+ number interpreted as bytes offset in PLACED_ADDRESS. */
+ memcpy (bp_tgt->shadow_contents, bundle + slotnum, bp_tgt->shadow_len);
+
+ /* Breakpoints already present in the code will get deteacted and not get
+ reinserted by bp_loc_is_permanent. Multiple breakpoints at the same
+ location cannot induce the internal error as they are optimized into
+ a single instance by update_global_location_list. */
+ instr_breakpoint = slotN_contents (bundle, slotnum);
+ if (instr_breakpoint == IA64_BREAKPOINT)
+ internal_error (__FILE__, __LINE__,
+ _("Address %s already contains a breakpoint."),
+ paddr_nz (bp_tgt->placed_address));
replace_slotN_contents (bundle, IA64_BREAKPOINT, slotnum);
+
if (val == 0)
- target_write_memory (addr, bundle, BUNDLE_LEN);
+ val = target_write_memory (addr + slotnum, bundle + slotnum,
+ bp_tgt->shadow_len);

+ do_cleanups (cleanup);
return val;
}

@@ -590,9 +655,9 @@
struct bp_target_info *bp_tgt)
{
CORE_ADDR addr = bp_tgt->placed_address;
- char bundle[BUNDLE_LEN];
+ gdb_byte bundle_mem[BUNDLE_LEN], bundle_saved[BUNDLE_LEN];
int slotnum = (addr & 0x0f) / SLOT_MULTIPLIER;
- long long instr;
+ long long instr_breakpoint, instr_saved;
int val;
int template;
struct cleanup *cleanup;
@@ -601,40 +666,96 @@

/* Disable the automatic memory restoration from breakpoints while
we read our instruction bundle. Otherwise, the general restoration
- mechanism kicks in and ends up corrupting our bundle, because it
- is not aware of the concept of instruction bundles. */
+ mechanism kicks in and we would possibly remove parts of the adjacent
+ placed breakpoints. It is due to our SHADOW_CONTENTS overlapping the real
+ breakpoint instruction bits region. */
cleanup = make_show_memory_breakpoints_cleanup (1);
- val = target_read_memory (addr, bundle, BUNDLE_LEN);
+ val = target_read_memory (addr, bundle_mem, BUNDLE_LEN);

- /* Check for L type instruction in 2nd slot, if present then
- bump up the slot number to the 3rd slot */
- template = extract_bit_field (bundle, 0, 5);
- if (slotnum == 1 && template_encoding_table[template][1] == L)
- {
- slotnum = 2;
- }
-
- memcpy (&instr, bp_tgt->shadow_contents, sizeof instr);
- replace_slotN_contents (bundle, instr, slotnum);
+ /* Check for L type instruction in slot 1, if present then bump up the slot
+ number to the slot 2. */
+ template = extract_bit_field (bundle_mem, 0, 5);
+ if (slotnum == 1 && template_encoding_table[template][slotnum] == L)
+ slotnum = 2;
+
+ gdb_assert (bp_tgt->placed_size == BUNDLE_LEN - 2);
+ gdb_assert (bp_tgt->placed_size == bp_tgt->shadow_len);
+
+ instr_breakpoint = slotN_contents (bundle_mem, slotnum);
+ if (instr_breakpoint != IA64_BREAKPOINT)
+ {
+ warning (_("Cannot remove breakpoint at address %s, "
+ "no break instruction at such address."),
+ paddr_nz (bp_tgt->placed_address));
+ return -1;
+ }
+
+ /* Extract the original saved instruction from SLOTNUM normalizing its
+ bit-shift for INSTR_SAVED. */
+ memcpy (bundle_saved, bundle_mem, BUNDLE_LEN);
+ memcpy (bundle_saved + slotnum, bp_tgt->shadow_contents, bp_tgt->shadow_len);
+ instr_saved = slotN_contents (bundle_saved, slotnum);
+
+ /* In BUNDLE_MEM be careful to modify only the bits belonging to SLOTNUM and
+ never any other possibly also stored in SHADOW_CONTENTS. */
+ replace_slotN_contents (bundle_mem, instr_saved, slotnum);
if (val == 0)
- target_write_memory (addr, bundle, BUNDLE_LEN);
+ val = target_write_memory (addr, bundle_mem, BUNDLE_LEN);

do_cleanups (cleanup);
return val;
}

-/* We don't really want to use this, but remote.c needs to call it in order
- to figure out if Z-packets are supported or not. Oh, well. */
-const unsigned char *
+/* As gdbarch_breakpoint_from_pc ranges have byte granularity and ia64
+ instruction slots ranges are bit-granular (41 bits) we have to provide an
+ extended range as described for ia64_memory_insert_breakpoint. We also take
+ care of preserving the `break' instruction 21-bit (or 62-bit) parameter to
+ make a match for permanent breakpoints. */
+
+static const gdb_byte *
ia64_breakpoint_from_pc (struct gdbarch *gdbarch, CORE_ADDR *pcptr, int *lenptr)
{
- static unsigned char breakpoint[] =
- { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
- *lenptr = sizeof (breakpoint);
-#if 0
- *pcptr &= ~0x0f;
-#endif
- return breakpoint;
+ CORE_ADDR addr = *pcptr;
+ static gdb_byte bundle[BUNDLE_LEN];
+ int slotnum = (int) (*pcptr & 0x0f) / SLOT_MULTIPLIER;
+ long long instr_fetched;
+ int val;
+ int template;
+ struct cleanup *cleanup;
+
+ if (slotnum > 2)
+ error (_("Can't insert breakpoint for slot numbers greater than 2."));
+
+ addr &= ~0x0f;
+
+ /* Enable the automatic memory restoration from breakpoints while
+ we read our instruction bundle to match bp_loc_is_permanent. */
+ cleanup = make_show_memory_breakpoints_cleanup (0);
+ val = target_read_memory (addr, bundle, BUNDLE_LEN);
+ do_cleanups (cleanup);
+
+ /* The memory might be unreachable. This can happen, for instance,
+ when the user inserts a breakpoint at an invalid address. */
+ if (val != 0)
+ return NULL;
+
+ /* Check for L type instruction in slot 1, if present then bump up the slot
+ number to the slot 2. */
+ template = extract_bit_field (bundle, 0, 5);
+ if (slotnum == 1 && template_encoding_table[template][slotnum] == L)
+ slotnum = 2;
+
+ /* A break instruction has its all its opcode bits cleared except for
+ the parameter value. For L+X slot pair we are at the X slot (slot 2) so
+ we should not touch the L slot - the upper 41 bits of the parameter. */
+ instr_fetched = slotN_contents (bundle, slotnum);
+ instr_fetched &= 0x1003ffffc0;
+ replace_slotN_contents (bundle, instr_fetched, slotnum);
+
+ *lenptr = BUNDLE_LEN - 2;
+
+ /* SLOTNUM is possibly already locally modified - use caller's *PCPTR. */
+ return bundle + (*pcptr & 0x0f);
}

static CORE_ADDR
===================================================================
RCS file: /cvs/src/src/gdb/doc/gdbint.texinfo,v
retrieving revision 1.292
retrieving revision 1.293
diff -u -r1.292 -r1.293
--- src/gdb/doc/gdbint.texinfo 2008/10/27 11:37:40 1.292
+++ src/gdb/doc/gdbint.texinfo 2008/11/26 05:26:40 1.293
@@ -3430,16 +3430,23 @@
@findex gdbarch_breakpoint_from_pc
@anchor{gdbarch_breakpoint_from_pc} Use the program counter to determine the
contents and size of a breakpoint instruction. It returns a pointer to
-a string of bytes that encode a breakpoint instruction, stores the
+a static string of bytes that encode a breakpoint instruction, stores the
length of the string to @code{*@var{lenptr}}, and adjusts the program
counter (if necessary) to point to the actual memory location where the
-breakpoint should be inserted.
+breakpoint should be inserted. May return @code{NULL} to indicate that
+software breakpoints are not supported.

Although it is common to use a trap instruction for a breakpoint, it's
not required; for instance, the bit pattern could be an invalid
instruction. The breakpoint must be no longer than the shortest
instruction of the architecture.

+Provided breakpoint bytes can be also used by @code{bp_loc_is_permanent} to
+detect permanent breakpoints. @code{gdbarch_breakpoint_from_pc} should return
+an unchanged memory copy if it was called for a location with permanent
+breakpoint as some architectures use breakpoint instructions containing
+arbitrary parameter value.
+
Replaces all the other @var{BREAKPOINT} macros.

@item int gdbarch_memory_insert_breakpoint (@var{gdbarch}, @var{bp_tgt})
/cvs/src/src/gdb/testsuite/gdb.base/breakpoint-shadow.c,v --> standard output
revision 1.1
--- src/gdb/testsuite/gdb.base/breakpoint-shadow.c
+++ src/gdb/testsuite/gdb.base/breakpoint-shadow.c 2008-11-26 05:29:49.128279000 +0000
@@ -0,0 +1,27 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2008 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
+main (void)
+{
+ volatile int i;
+
+ i = 1; /* break-first */
+ i = 2; /* break-second */
+
+ return 0;
+}
/cvs/src/src/gdb/testsuite/gdb.base/breakpoint-shadow.exp,v --> standard output
revision 1.1
--- src/gdb/testsuite/gdb.base/breakpoint-shadow.exp
+++ src/gdb/testsuite/gdb.base/breakpoint-shadow.exp 2008-11-26 05:29:49.855090000 +0000
@@ -0,0 +1,63 @@
+# Copyright 2008 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/>.
+
+set testfile breakpoint-shadow
+set srcfile ${testfile}.c
+set binfile ${objdir}/${subdir}/${testfile}
+if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug}] != "" } {
+ untested "Couldn't compile test program"
+ return -1
+}
+
+gdb_exit
+gdb_start
+gdb_reinitialize_dir $srcdir/$subdir
+gdb_load ${binfile}
+
+# We need to start the inferior to place the breakpoints in the memory at all.
+if ![runto_main] {
+ untested start
+ return -1
+}
+
+# The default "auto" mode removes all the breakpoints when we stop (and not
+# running the nonstop mode). We would not be able to test the shadow.
+gdb_test "set breakpoint always-inserted on"
+gdb_test "show breakpoint always-inserted" "Always inserted breakpoint mode is on."
+
+set match "\nDump of assembler code for function main:\r\n(.*)End of assembler dump.\r\n$gdb_prompt $"
+
+set test "disassembly without breakpoints"
+gdb_test_multiple "disass main" $test {
+ -re $match {
+ set orig $expect_out(1,string)
+ pass $test
+ }
+}
+
+gdb_test "b [gdb_get_line_number "break-first"]" "Breakpoint \[0-9\] at .*" "First breakpoint placed"
+gdb_test "b [gdb_get_line_number "break-second"]" "Breakpoint \[0-9\] at .*" "Second breakpoint placed"
+
+set test "disassembly with breakpoints"
+gdb_test_multiple "disass main" $test {
+ -re $match {
+ set got $expect_out(1,string)
+ if [string equal -nocase $orig $got] {
+ pass $test
+ } else {
+ fail $test
+ }
+ }
+}
===================================================================
RCS file: /cvs/src/src/gdb/testsuite/gdb.base/start.exp,v
retrieving revision 1.5
retrieving revision 1.6
diff -u -r1.5 -r1.6
--- src/gdb/testsuite/gdb.base/start.exp 2008/01/01 22:53:19 1.5
+++ src/gdb/testsuite/gdb.base/start.exp 2008/11/26 05:25:56 1.6
@@ -35,6 +35,9 @@
gdb_reinitialize_dir $srcdir/$subdir
gdb_load ${binfile}

+# This is a testcase specifically for the `start' GDB command. For regular
+# stop-in-main goal in the testcases consider using `runto_main' instead.
+
# For C programs, "start" should stop in main().
if { [gdb_start_cmd] < 0 } {
untested start
@@ -44,4 +47,3 @@
gdb_test "" \
"main \\(\\) at .*start.c.*" \
"start"
-