Re: [PATCH 1/3] gdb/breakpoint: do not update the condition string if parsing the condition fails

[Simon Marchi <simark@simark.ca>]

On 2020-06-29 9:48 a.m., Tankut Baris Aktemur via Gdb-patches wrote:
> The condition of a breakpoint can be set with the 'cond' command.  If
> the condition has errors that make it problematic to evaluate, it
> appears like GDB rejects the condition, but updates the breakpoint's
> condition string, which causes incorrect/unintuitive behavior.
> 
> For instance:
> 
>   $ gdb ./test
>   Reading symbols from ./test...
>   (gdb) break 5
>   Breakpoint 1 at 0x1155: file test.c, line 5.
>   (gdb) cond 1 gibberish
>   No symbol "gibberish" in current context.
> 
> At this point, it looks like the condition was rejected.
> But "info breakpoints" shows the following:
> 
>   (gdb) info breakpoints
>   Num     Type           Disp Enb Address            What
>   1       breakpoint     keep y   0x0000000000001155 in main at test.c:5
>           stop only if gibberish
> 
> Running the code gives the following behavior, where re-insertion of
> the breakpoint causes failures.
> 
>   (gdb) run
>   Starting program: test
>   warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
>   warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
>   warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
>   warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
>   warning: failed to reevaluate condition for breakpoint 1: No symbol "gibberish" in current context.
>   [Inferior 1 (process 19084) exited normally]
>   (gdb)
> 
> This broken behavior occurs because GDB updates the condition string
> of the breakpoint *before* checking that it parses successfully.
> When parsing fails, the update has already taken place.
> 
> Fix the problem by updating the condition string *after* parsing the
> condition.  We get the following behavior when this patch is applied:
> 
>   $ gdb ./test
>   Reading symbols from ./test...
>   (gdb) break 5
>   Breakpoint 1 at 0x1155: file test.c, line 5.
>   (gdb) cond 1 gibberish
>   No symbol "gibberish" in current context.
>   (gdb) info breakpoints
>   Num     Type           Disp Enb Address            What
>   1       breakpoint     keep y   0x0000000000001155 in main at test.c:5
>   (gdb) run
>   Starting program: test
> 
>   Breakpoint 1, main () at test.c:5
>   5         a = a + 1; /* break-here */
>   (gdb) c
>   Continuing.
>   [Inferior 1 (process 15574) exited normally]
>   (gdb)
> 
> A side note: The problem does not occur if the condition is given
> at the time of breakpoint definition, as in "break 5 if gibberish",
> because the parsing of the condition fails during symtab-and-line
> creation, before the breakpoint is created.
> 
> Finally, the code included the following comment:
> 
>   /* I don't know if it matters whether this is the string the user
>      typed in or the decompiled expression.  */
> 
> This comment did not make sense to me because the condition string is
> the user-typed input.  The patch updates this comment, too.
> 
> gdb/ChangeLog:
> 2020-06-29  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
> 
> 	* breakpoint.c (set_breakpoint_condition): Update the
> 	condition string after parsing the new condition successfully.
> 
> gdb/testsuite/ChangeLog:
> 2020-06-29  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
> 
> 	* gdb.base/condbreak-bad.c: New test.
> 	* gdb.base/condbreak-bad.exp: New file.
> ---
>  gdb/breakpoint.c                         | 17 +++++-----
>  gdb/testsuite/gdb.base/condbreak-bad.c   | 24 ++++++++++++++
>  gdb/testsuite/gdb.base/condbreak-bad.exp | 40 ++++++++++++++++++++++++
>  3 files changed, 73 insertions(+), 8 deletions(-)
>  create mode 100644 gdb/testsuite/gdb.base/condbreak-bad.c
>  create mode 100644 gdb/testsuite/gdb.base/condbreak-bad.exp
> 
> diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
> index 6d81323dd92..1fc2d1b8966 100644
> --- a/gdb/breakpoint.c
> +++ b/gdb/breakpoint.c
> @@ -834,9 +834,6 @@ void
>  set_breakpoint_condition (struct breakpoint *b, const char *exp,
>  			  int from_tty)
>  {
> -  xfree (b->cond_string);
> -  b->cond_string = NULL;
> -
>    if (is_watchpoint (b))
>      {
>        struct watchpoint *w = (struct watchpoint *) b;
> @@ -859,6 +856,9 @@ set_breakpoint_condition (struct breakpoint *b, const char *exp,
>  
>    if (*exp == 0)
>      {
> +      xfree (b->cond_string);
> +      b->cond_string = nullptr;
> +
>        if (from_tty)
>  	printf_filtered (_("Breakpoint %d now unconditional.\n"), b->number);
>      }
> @@ -866,11 +866,6 @@ set_breakpoint_condition (struct breakpoint *b, const char *exp,
>      {
>        const char *arg = exp;
>  
> -      /* I don't know if it matters whether this is the string the user
> -	 typed in or the decompiled expression.  */
> -      b->cond_string = xstrdup (arg);
> -      b->condition_not_parsed = 0;
> -
>        if (is_watchpoint (b))
>  	{
>  	  struct watchpoint *w = (struct watchpoint *) b;
> @@ -896,6 +891,12 @@ set_breakpoint_condition (struct breakpoint *b, const char *exp,
>  		error (_("Junk at end of expression"));
>  	    }
>  	}
> +
> +      /* We know that the new condition parsed successfully.  The
> +	 condition string of the breakpoint can be safely updated.  */
> +      xfree (b->cond_string);
> +      b->cond_string = xstrdup (exp);
> +      b->condition_not_parsed = 0;
>      }
>    mark_breakpoint_modified (b);
>  
> diff --git a/gdb/testsuite/gdb.base/condbreak-bad.c b/gdb/testsuite/gdb.base/condbreak-bad.c
> new file mode 100644
> index 00000000000..58283b75ca7
> --- /dev/null
> +++ b/gdb/testsuite/gdb.base/condbreak-bad.c
> @@ -0,0 +1,24 @@
> +/* 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
> +main ()
> +{
> +  int a = 10;
> +  a = a + 1; /* break-here */
> +  return 0;
> +}
> diff --git a/gdb/testsuite/gdb.base/condbreak-bad.exp b/gdb/testsuite/gdb.base/condbreak-bad.exp
> new file mode 100644
> index 00000000000..a01ba2a9340
> --- /dev/null
> +++ b/gdb/testsuite/gdb.base/condbreak-bad.exp
> @@ -0,0 +1,40 @@
> +# 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 defining bad conditions for breakpoints.
> +
> +standard_testfile
> +
> +if {[prepare_for_testing "failed to prepare" ${binfile} ${srcfile}]} {
> +    return
> +}
> +
> +set bp_location [gdb_get_line_number "break-here"]
> +gdb_breakpoint "$bp_location"
> +set bpnum [get_integer_valueof "\$bpnum" 0 "get bpnum"]
> +
> +# Define a 'bad' condition.  The breakpoint should stay unconditional.
> +gdb_test "cond $bpnum gibberish" \
> +    "No symbol \"gibberish\" in current context." \
> +    "attempt a bad condition"
> +
> +set fill "\[^\r\n\]*"
> +
> +gdb_test "info break" \
> +    [multi_line \
> +	 "Num${fill}What" \
> +	 "${decimal}${fill}breakpoint${fill}keep y${fill}:${bp_location}"] \
> +    "breakpoint is unconditional"
> +

Here, could you also test that when a valid condition exists and we try to change
it for a bad one, the previous condition is still there after the failed attempt
to change it?

Simon