Re: [PATCH] Change handling of '!' operator in stap probes

[Sergio Durigan Junior via Gdb-patches <gdb-patches@sourceware.org>]

On Tuesday, January 12 2021, Sergio Durigan Junior via Gdb-patches wrote:

> On Sunday, January 03 2020, Sergio Durigan Junior via Gdb-patches wrote:
>
>> On Thursday, December 31 2020, Tom Tromey wrote:
>>
>>> While working on the expression rewrite, I noticed that the '!'
>>> operator is handled as if it were a binary operator.  However, this
>>> seems incorrect.  Instead, I think it should be handled alongside the
>>> other unary operators in stap_parse_single_operand.
>>>
>>> This patch makes this change.
>
> Ping.

Ping^2.

>>
>> Thanks for the patch, Tom.
>>
>> This code was written based on the information contained here:
>>
>>   https://sourceware.org/binutils/docs/as/Infix-Ops.html
>>
>> You can see that it says:
>>
>>   !
>>     Bitwise Or Not
>>
>> The problem is that, when we examine gas' source code, we see that it
>> doesn't treat '!' as "bitwise or not" (fortunately!), but as the
>> "logical not":
>>
>>   https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/symbols.c;h=f6e4b718ef719eef8a7e6e29dbadf3605058403d;hb=HEAD#l1128
>>
>> OK, this means that our stap-probe.c is not completely wrong: the enum
>> should indeed be named UNOP_LOGICAL_NOT.
>>
>> Now, when we check to see if the precedence assigned to UNOP_LOGICAL_NOT
>> is the right one, we see that it is:
>>
>>   https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/expr.c;h=1b420d928cde50456af0137529b8215294ba521d;hb=HEAD#l1481
>>
>>
>>   /* Rank	Examples
>>      0	operand, (expression)
>>      1	||
>>      2	&&
>>      3	== <> < <= >= >
>>      4	+ -
>>      5	used for * / % in MRI mode
>>      6	& ^ ! |
>>   ...
>>
>> Huh, interesting.  Anyway, this whole investigation (plus the fact that,
>> as you mentioned, we don't have tests to exercise this part of the code)
>> made me uncover a few latent bugs on the stap parser code, so I'm
>> sending a patch that incorporates part of your changes and also fixes
>> the issues I found.  As a bonus, I'm proposing a new testcase
>> (amd64-only) to make sure that we expand the coverage of this code.
>>
>> WDYT?
>>
>> -- 
>> Sergio
>> GPG key ID: 237A 54B1 0287 28BF 00EF  31F4 D0EB 7628 65FC 5E36
>> Please send encrypted e-mail if possible
>> https://sergiodj.net/
>>
>> From da02db7cacef62ce085d5715b1339b24d8f2260c Mon Sep 17 00:00:00 2001
>> From: Sergio Durigan Junior <sergiodj@sergiodj.net>
>> Date: Sun, 3 Jan 2021 02:42:52 -0500
>> Subject: [PATCH] Fix a few stap parser issues and add a new test for probe
>>  expressions
>>
>> The creation of this patch was motivated by Tom's "Change handling of
>> '!' operator in stap probes" patch.
>>
>> While reviewing his patch, I stumbled upon a few issues with the stap
>> expression parser.  They are:
>>
>> - As it turns out, even with Tom's patch applied the parser doesn't
>>   properly handle the '!' operator.  The underlying issue was the fact
>>   that stap_parse_argument_conditionally also needed to be patched in
>>   order to recognize '!' as an operator that is part of a single
>>   operand, and parse it accordingly.
>>
>> - While writing the testcase I'm proposing on this patch, I found that
>>   parenthesized sub-expressions were not being parsed correctly when
>>   there was another term after them.  For example:
>>
>>     1 - (2 + 3) + 4
>>
>>   In this case, the parser was considering "1" to be the left-side of
>>   the expression, and "(2 + 3) + 4" to be the right-side.  The patch
>>   fixes the parser by making it identify whether a parenthesized
>>   sub-expression has just been parsed, and act accordingly.
>>
>> I've tested this on my Debian testing amd64, and everything seems OK.
>>
>> gdb/ChangeLog:
>> 2021-01-03  Sergio Durigan Junior  <sergiodj@sergiodj.net>
>> 	    Tom Tromey <tom@tromey.com>
>>
>> 	* stap-probe.c (stap_parse_single_operand): Handle '!'
>> 	operator.
>> 	(stap_parse_argument_conditionally): Likewise.
>> 	Skip spaces after processing open-parenthesis sub-expression.
>> 	(stap_parse_argument_1): Skip spaces after call to
>> 	stap_parse_argument_conditionally.
>> 	Handle case when right-side expression is a parenthesized
>> 	sub-expression.
>> 	Skip spaces after call to stap_parse_argument_1.
>>
>> gdb/testsuite/ChangeLog:
>> 2021-01-03  Sergio Durigan Junior  <sergiodj@sergiodj.net>
>>
>> 	* gdb.arch/amd64-stap-expressions.S: New file.
>> 	* gdb.arch/amd64-stap-expressions.exp: New file.
>> ---
>>  gdb/stap-probe.c                              | 31 +++++++--
>>  .../gdb.arch/amd64-stap-expressions.S         | 43 ++++++++++++
>>  .../gdb.arch/amd64-stap-expressions.exp       | 68 +++++++++++++++++++
>>  3 files changed, 137 insertions(+), 5 deletions(-)
>>  create mode 100644 gdb/testsuite/gdb.arch/amd64-stap-expressions.S
>>  create mode 100644 gdb/testsuite/gdb.arch/amd64-stap-expressions.exp
>>
>> diff --git a/gdb/stap-probe.c b/gdb/stap-probe.c
>> index c2ddd047f6..224dd5714f 100644
>> --- a/gdb/stap-probe.c
>> +++ b/gdb/stap-probe.c
>> @@ -870,7 +870,7 @@ stap_parse_single_operand (struct stap_parse_info *p)
>>        return;
>>      }
>>  
>> -  if (*p->arg == '-' || *p->arg == '~' || *p->arg == '+')
>> +  if (*p->arg == '-' || *p->arg == '~' || *p->arg == '+' || *p->arg == '!')
>>      {
>>        char c = *p->arg;
>>        /* We use this variable to do a lookahead.  */
>> @@ -924,6 +924,8 @@ stap_parse_single_operand (struct stap_parse_info *p)
>>  	    write_exp_elt_opcode (&p->pstate, UNOP_NEG);
>>  	  else if (c == '~')
>>  	    write_exp_elt_opcode (&p->pstate, UNOP_COMPLEMENT);
>> +	  else if (c == '!')
>> +	    write_exp_elt_opcode (&p->pstate, UNOP_LOGICAL_NOT);
>>  	}
>>      }
>>    else if (isdigit (*p->arg))
>> @@ -1012,7 +1014,7 @@ stap_parse_argument_conditionally (struct stap_parse_info *p)
>>  {
>>    gdb_assert (gdbarch_stap_is_single_operand_p (p->gdbarch));
>>  
>> -  if (*p->arg == '-' || *p->arg == '~' || *p->arg == '+' /* Unary.  */
>> +  if (*p->arg == '-' || *p->arg == '~' || *p->arg == '+' || *p->arg == '!'
>>        || isdigit (*p->arg)
>>        || gdbarch_stap_is_single_operand (p->gdbarch, p->arg))
>>      stap_parse_single_operand (p);
>> @@ -1027,11 +1029,12 @@ stap_parse_argument_conditionally (struct stap_parse_info *p)
>>  
>>        stap_parse_argument_1 (p, 0, STAP_OPERAND_PREC_NONE);
>>  
>> -      --p->inside_paren_p;
>> +      p->arg = skip_spaces (p->arg);
>>        if (*p->arg != ')')
>> -	error (_("Missign close-paren on expression `%s'."),
>> +	error (_("Missign close-parenthesis on expression `%s'."),
>>  	       p->saved_arg);
>>  
>> +      --p->inside_paren_p;
>>        ++p->arg;
>>        if (p->inside_paren_p)
>>  	p->arg = skip_spaces (p->arg);
>> @@ -1067,6 +1070,9 @@ stap_parse_argument_1 (struct stap_parse_info *p, bool has_lhs,
>>        stap_parse_argument_conditionally (p);
>>      }
>>  
>> +  if (p->inside_paren_p)
>> +    p->arg = skip_spaces (p->arg);
>> +
>>    /* Start to parse the right-side, and to "join" left and right sides
>>       depending on the operation specified.
>>  
>> @@ -1104,8 +1110,21 @@ stap_parse_argument_1 (struct stap_parse_info *p, bool has_lhs,
>>        if (p->inside_paren_p)
>>  	p->arg = skip_spaces (p->arg);
>>  
>> -      /* Parse the right-side of the expression.  */
>> +      /* Parse the right-side of the expression.
>> +
>> +	 We save whether the right-side is a parenthesized
>> +	 subexpression because, if it is, we will have to finish
>> +	 processing this part of the expression before continuing.  */
>> +      bool paren_subexp = *p->arg == '(';
>> +
>>        stap_parse_argument_conditionally (p);
>> +      if (p->inside_paren_p)
>> +	p->arg = skip_spaces (p->arg);
>> +      if (paren_subexp)
>> +	{
>> +	  write_exp_elt_opcode (&p->pstate, opcode);
>> +	  continue;
>> +	}
>>  
>>        /* While we still have operators, try to parse another
>>  	 right-side, but using the current right-side as a left-side.  */
>> @@ -1130,6 +1149,8 @@ stap_parse_argument_1 (struct stap_parse_info *p, bool has_lhs,
>>  	  /* Parse the right-side of the expression, but since we already
>>  	     have a left-side at this point, set `has_lhs' to 1.  */
>>  	  stap_parse_argument_1 (p, 1, lookahead_prec);
>> +	  if (p->inside_paren_p)
>> +	    p->arg = skip_spaces (p->arg);
>>  	}
>>  
>>        write_exp_elt_opcode (&p->pstate, opcode);
>> diff --git a/gdb/testsuite/gdb.arch/amd64-stap-expressions.S b/gdb/testsuite/gdb.arch/amd64-stap-expressions.S
>> new file mode 100644
>> index 0000000000..76a47aa9b5
>> --- /dev/null
>> +++ b/gdb/testsuite/gdb.arch/amd64-stap-expressions.S
>> @@ -0,0 +1,43 @@
>> +/* Copyright (C) 2021 Free Software Foundation, Inc.
>> +
>> +   This file is part of GDB.
>> +
>> +   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/>.  */
>> +
>> +#include <sys/sdt.h>
>> +
>> +	.file	"amd64-stap-expressions.S"
>> +	.text
>> +	.globl	main
>> +main:
>> +	/* We use a nop here because we don't want the first probe to
>> +	   be placed at the same location as the main label.  */
>> +	nop
>> +
>> +	/* Single operands.  */
>> +	STAP_PROBE1(probe, log_neg, 8@!($0+$1))
>> +	STAP_PROBE1(probe, minus, -8@-($3+$4))
>> +	STAP_PROBE1(probe, bit_neg, -8@~$22)
>> +
>> +	/* Arithmetic expressions.  */
>> +	STAP_PROBE1(probe, plus1, 8@$3+($10-$8)-$1)
>> +	STAP_PROBE1(probe, plus2, 8@$100-(  ($8+$10) -$50)+$3)
>> +	STAP_PROBE1(probe, plus3, 8@$100-(($8+$10)-$50)+((($8 - $9) + $40) - $4)+$4)
>> +
>> +	/* Bitwise expressions.  */
>> +	STAP_PROBE1(probe, and, 8@$128&$128)
>> +	STAP_PROBE1(probe, or, 8@$8|$4)
>> +
>> +	xor	%rax,%rax
>> +	ret
>> diff --git a/gdb/testsuite/gdb.arch/amd64-stap-expressions.exp b/gdb/testsuite/gdb.arch/amd64-stap-expressions.exp
>> new file mode 100644
>> index 0000000000..5e3cb60a9e
>> --- /dev/null
>> +++ b/gdb/testsuite/gdb.arch/amd64-stap-expressions.exp
>> @@ -0,0 +1,68 @@
>> +# Copyright 2021 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/>.
>> +
>> +standard_testfile ".S"
>> +
>> +if { ![istarget "x86_64-*-*"] || ![is_lp64_target] } {
>> +    verbose "Skipping $testfile.exp"
>> +    return
>> +}
>> +
>> +if { [prepare_for_testing "failed to prepare" $testfile $srcfile] } {
>> +    return -1
>> +}
>> +
>> +# Helper procedure to go to probe NAME
>> +
>> +proc goto_probe { name } {
>> +    global decimal hex
>> +
>> +    gdb_test "break -pstap $name" "Breakpoint $decimal at $hex"
>> +    gdb_test "continue" "Breakpoint $decimal, main \\(\\) at .*\r\n.*STAP_PROBE1.*${name},.*\\)"
>> +}
>> +
>> +# Helper procedure to test the probe's argument
>> +
>> +proc test_probe_value { value } {
>> +    gdb_test "print \$_probe_argc" "= 1"
>> +    gdb_test "print \$_probe_arg0" "= $value"
>> +}
>> +
>> +if { ![runto_main] } {
>> +    return -1
>> +}
>> +
>> +# Name and expected value for each probe.
>> +set probe_names_and_values {
>> +    { "log_neg" "0" }
>> +    { "minus" "-7" }
>> +    { "bit_neg" "-23" }
>> +
>> +    { "plus1" "4" }
>> +    { "plus2" "135" }
>> +    { "plus3" "171" }
>> +
>> +    { "and" "128" }
>> +    { "or" "12" }
>> +}
>> +
>> +foreach probe_info $probe_names_and_values {
>> +    set name [lindex $probe_info 0]
>> +    set value [lindex $probe_info 1]
>> +    with_test_prefix $name {
>> +	goto_probe $name
>> +	test_probe_value $value
>> +    }
>> +}
>> -- 
>>
>> 2.29.2
>>
>>
>
> -- 
> Sergio
> GPG key ID: 237A 54B1 0287 28BF 00EF  31F4 D0EB 7628 65FC 5E36
> Please send encrypted e-mail if possible
> https://sergiodj.net/

-- 
Sergio
GPG key ID: 237A 54B1 0287 28BF 00EF  31F4 D0EB 7628 65FC 5E36
Please send encrypted e-mail if possible
https://sergiodj.net/