Re: [PATCH v2] Fix PR12526: -location watchpoints for bitfield arguments

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Wed, Aug 20, 2014 at 11:32 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> { v2: Here is my crude attempt at adding a testcase for this changeset.
>   I also fixed the bug that I alluded to earlier. }
>
> PR 12526 reports that -location watchpoints against bitfield arguments
> trigger false positives when bits around the bitfield, but not the
> bitfield itself, are modified.
>
> This happens because -location watchpoints naturally operate at the byte
> level, not at the bit level.  When the address of a bitfield lvalue is
> taken, information about the bitfield (i.e. its offset and size) is lost
> in the process.
>
> This information must first be retained throughout the lifetime of the
> -location watchpoint.  This patch achieves this by adding two new fields
> to the watchpoint struct: val_bitpos and val_bitsize.  These fields are
> set when a watchpoint is first defined in watch_command_1().  They are
> both equal to zero if the watchpoint is not a -location watchpoint or if
> the argument is not a bitfield.
>
> Then these bitfield parameters are used inside update_watchpoint() and
> watchpoint_check() to extract the actual value of the bitfield from the
> watchpoint address, with the help of a local helper function
> extract_bitfield_from_watchpoint_value().
>
> Finally when creating a HW breakpoint pointing to a bitfield, we
> optimize the address and length of the breakpoint.  By skipping over the
> bytes that don't cover the bitfield, this step reduces the frequency at
> which a read watchpoint for the bitfield is triggered.  It also reduces
> the number of times a false-positive call to check_watchpoint() is
> triggered for a write watchpoint.
>
> gdb/
>         PR breakpoints/12526
>         * breakpoint.h (struct watchpoint): New fields val_bitpos and
>         val_bitsize.
>         * breakpoint.c (watch_command_1): Use these fields to retain
>         bitfield information.
>         (extract_bitfield_from_watchpoint_value): New function.
>         (watchpoint_check): Use it.
>         (update_watchpoint): Use it.  Optimize the address and length
>         of a HW watchpoint pointing to a bitfield.
>
> gdb/testsuite/
>         PR breakpoints/12526
>         * gdb.base/pr12526.exp: New file.
>         * gdb.base/pr12526.c: New file.
> ---
>  gdb/breakpoint.c                   | 67 +++++++++++++++++++++++++++++++++++++-
>  gdb/breakpoint.h                   |  5 +++
>  gdb/testsuite/gdb.base/pr12526.c   | 54 ++++++++++++++++++++++++++++++
>  gdb/testsuite/gdb.base/pr12526.exp | 47 ++++++++++++++++++++++++++
>  4 files changed, 172 insertions(+), 1 deletion(-)
>  create mode 100644 gdb/testsuite/gdb.base/pr12526.c
>  create mode 100644 gdb/testsuite/gdb.base/pr12526.exp
>
> diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
> index 683ed2b..7b7c74b 100644
> --- a/gdb/breakpoint.c
> +++ b/gdb/breakpoint.c
> @@ -1703,6 +1703,31 @@ watchpoint_del_at_next_stop (struct watchpoint *w)
>    b->disposition = disp_del_at_next_stop;
>  }
>
> +/* Extract a bitfield value from value VAL using the bit parameters contained in
> +   watchpoint W.  */
> +
> +static struct value *
> +extract_bitfield_from_watchpoint_value (struct watchpoint *w, struct value *val)
> +{
> +  LONGEST bit_val;
> +  int ok;
> +
> +  if (val == NULL)
> +    return NULL;
> +
> +  ok = unpack_value_bits_as_long (value_type (val),
> +                                 value_contents_for_printing (val),
> +                                 value_offset (val),
> +                                 w->val_bitpos,
> +                                 w->val_bitsize,
> +                                 val,
> +                                 &bit_val);
> +  if (ok)
> +    return value_from_longest (value_type (val), bit_val);
> +
> +  return NULL;
> +}
> +
>  /* Assuming that B is a watchpoint:
>     - Reparse watchpoint expression, if REPARSE is non-zero
>     - Evaluate expression and store the result in B->val
> @@ -1877,6 +1902,12 @@ update_watchpoint (struct watchpoint *b, int reparse)
>          watchpoints.  */
>        if (!b->val_valid && !is_masked_watchpoint (&b->base))
>         {
> +         if (b->val_bitsize)
> +           {
> +             v = extract_bitfield_from_watchpoint_value (b, v);
> +             if (v)
> +               release_value (v);
> +           }
>           b->val = v;
>           b->val_valid = 1;
>         }
> @@ -1906,8 +1937,24 @@ update_watchpoint (struct watchpoint *b, int reparse)
>                   CORE_ADDR addr;
>                   int type;
>                   struct bp_location *loc, **tmp;
> +                 int bitpos = 0, bitsize = 0;
> +
> +                 if (value_bitsize (v))
> +                   {
> +                     bitpos = value_bitpos (v);
> +                     bitsize = value_bitsize (v);
> +                   }
> +                 else if (v == result && b->val_bitsize)
> +                   {
> +                     bitpos = b->val_bitpos;
> +                     bitsize = b->val_bitsize;
> +                   }
>
>                   addr = value_address (v);
> +                 if (bitsize)
> +                   /* Skip the bytes that don't contain the bitfield.  */
> +                   addr += bitpos / 8;
> +
>                   type = hw_write;
>                   if (b->base.type == bp_read_watchpoint)
>                     type = hw_read;
> @@ -1922,7 +1969,13 @@ update_watchpoint (struct watchpoint *b, int reparse)
>
>                   loc->pspace = frame_pspace;
>                   loc->address = addr;
> -                 loc->length = TYPE_LENGTH (value_type (v));
> +
> +                 if (bitsize)
> +                   /* Just cover the bytes that make up the bitfield.  */
> +                   loc->length = ((bitpos % 8) + bitsize + 7) / 8;
> +                 else
> +                   loc->length = TYPE_LENGTH (value_type (v));
> +
>                   loc->watchpoint_type = type;
>                 }
>             }
> @@ -5039,6 +5092,9 @@ watchpoint_check (void *p)
>        mark = value_mark ();
>        fetch_subexp_value (b->exp, &pc, &new_val, NULL, NULL, 0);
>
> +      if (b->val_bitsize)
> +       new_val = extract_bitfield_from_watchpoint_value (b, new_val);
> +
>        /* We use value_equal_contents instead of value_equal because
>          the latter coerces an array to a pointer, thus comparing just
>          the address of the array instead of its contents.  This is
> @@ -11172,6 +11228,7 @@ watch_command_1 (const char *arg, int accessflag, int from_tty,
>    struct expression *exp;
>    const struct block *exp_valid_block = NULL, *cond_exp_valid_block = NULL;
>    struct value *val, *mark, *result;
> +  int saved_bitpos = 0, saved_bitsize = 0;
>    struct frame_info *frame;
>    const char *exp_start = NULL;
>    const char *exp_end = NULL;
> @@ -11305,6 +11362,12 @@ watch_command_1 (const char *arg, int accessflag, int from_tty,
>    mark = value_mark ();
>    fetch_subexp_value (exp, &pc, &val, &result, NULL, just_location);
>
> +  if (val && just_location)
> +    {
> +      saved_bitpos = value_bitpos (val);
> +      saved_bitsize = value_bitsize (val);
> +    }
> +
>    if (just_location)
>      {
>        int ret;
> @@ -11440,6 +11503,8 @@ watch_command_1 (const char *arg, int accessflag, int from_tty,
>    else
>      {
>        w->val = val;
> +      w->val_bitpos = saved_bitpos;
> +      w->val_bitsize = saved_bitsize;
>        w->val_valid = 1;
>      }
>
> diff --git a/gdb/breakpoint.h b/gdb/breakpoint.h
> index f6d06ce..2b80af1 100644
> --- a/gdb/breakpoint.h
> +++ b/gdb/breakpoint.h
> @@ -779,6 +779,11 @@ struct watchpoint
>       then an error occurred reading the value.  */
>    int val_valid;
>
> +  /* When watching the location of a bitfield, contains the offset and size of
> +     the bitfield.  Otherwise contains 0.  */
> +  int val_bitpos;
> +  int val_bitsize;
> +
>    /* Holds the frame address which identifies the frame this
>       watchpoint should be evaluated in, or `null' if the watchpoint
>       should be evaluated on the outermost frame.  */
> diff --git a/gdb/testsuite/gdb.base/pr12526.c b/gdb/testsuite/gdb.base/pr12526.c
> new file mode 100644
> index 0000000..b51926d
> --- /dev/null
> +++ b/gdb/testsuite/gdb.base/pr12526.c
> @@ -0,0 +1,54 @@
> +/* This testcase is part of GDB, the GNU debugger.
> +
> +   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/>.  */
> +
> +struct foo
> +{
> +  unsigned long a:1;
> +  unsigned char b:2;
> +  unsigned long c:3;
> +  char d:4;
> +  int e:5;
> +  char f:6;
> +  int g:7;
> +  long h:8;
> +} q = { 0 };
> +
> +int
> +main (void)
> +{
> +  q.a = 1;
> +  q.b = 2;
> +  q.c = 3;
> +  q.d = 4;
> +  q.e = 5;
> +  q.f = 6;
> +  q.g = -7;
> +  q.h = -8;
> +
> +  q.a--;
> +  q.b--;
> +  q.e--;
> +  q.d--;
> +  q.c--;
> +
> +  q.f--;
> +  q.g--;
> +  q.h--;
> +
> +
> +  return 0;
> +}
> diff --git a/gdb/testsuite/gdb.base/pr12526.exp b/gdb/testsuite/gdb.base/pr12526.exp
> new file mode 100644
> index 0000000..ed99bdc
> --- /dev/null
> +++ b/gdb/testsuite/gdb.base/pr12526.exp
> @@ -0,0 +1,47 @@
> +# 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/>.
> +
> +# This file is part of the gdb testsuite
> +
> +standard_testfile
> +
> +if {[prepare_for_testing $testfile.exp $testfile $srcfile debug]} {
> +    return -1
> +}
> +
> +if {![runto_main]} {
> +    return -1
> +}
> +
> +# Test correctness of watchpoints on bitfields.
> +
> +gdb_test "watch -l q.c"
> +gdb_test "cont" "q\.c.*Old value = 0.*New value = 3.*"
> +gdb_test "watch -l q.d"
> +gdb_test "cont" "q\.d.*Old value = 0.*New value = 4.*"
> +gdb_test "watch q.e"
> +gdb_test "cont" "q\.e.*Old value = 0.*New value = 5.*"
> +
> +gdb_test "cont" "q\.e.*Old value = 5.*New value = 4.*"
> +gdb_test "cont" "q\.d.*Old value = 4.*New value = 3.*"
> +gdb_test "cont" "q\.c.*Old value = 3.*New value = 2.*"
> +
> +delete_breakpoints
> +gdb_test "watch q.f"
> +gdb_test "watch q.g"
> +gdb_test "watch -l q.h"
> +gdb_test "cont" "q\.f.*Old value = 6.*New value = 5.*"
> +gdb_test "cont" "q\.g.*Old value = -7.*New value = -8.*"
> +gdb_test "cont" "q\.h.*Old value = -8.*New value = -9.*"
> --
> 2.1.0
>

Ping.