[1/2] Fix PR breakpoints/16889: gdb segfaults when printing ASM SDT arguments

  This commit fixes PR breakpoints/16889, which is about a bug that
triggers when GDB tries to parse probes whose arguments do not contain
the initial (and optional) "N@" part.  For reference sake, the de
facto format is described here:

  <https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation>

Anyway, this PR actually uncovered two bugs (related) that were
happening while parsing the arguments.  The first one was that the
parser *was* catching *some* arguments that were missing the "N@"
part, but it wasn't correctly setting the argument's type.  This was
causing a NULL pointer being dereferenced, ouch...

The second bug uncovered was that the parser was not catching all of
the cases for a probe which did not provide the "N@" part.  The fix
for that was to simplify the check that the code was making to
identify non-prefixed probes.  The code is simpler and easier to read
now.

I am also providing a testcase for this bug, only for x86_64
architectures.

gdb/
2014-05-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR breakpoints/16889
	* stap-probe.c (stap_parse_probe_arguments): Simplify
	check for non-prefixed probes (i.e., probes whose
	arguments do not start with "N@").  Always set the
	argument type to a sane value.

gdb/testsuite/
2014-05-01  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR breakpoints/16889
	* gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.S: New file.
	* gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.exp: Likewise.
---
 gdb/stap-probe.c                                   | 11 ++---
 .../gdb.arch/amd64-stap-optional-prefix.S          | 29 +++++++++++++
 .../gdb.arch/amd64-stap-optional-prefix.exp        | 50 ++++++++++++++++++++++
 3 files changed, 85 insertions(+), 5 deletions(-)
 create mode 100644 gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.S
 create mode 100644 gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.exp
  

On 05/01/2014 10:52 PM, Sergio Durigan Junior wrote:

> gdb/testsuite/
> 2014-05-01  Sergio Durigan Junior  <sergiodj@redhat.com>
> 
> 	PR breakpoints/16889
> 	* gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.S: New file.
> 	* gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.exp: Likewise.

No "gdb/testsuite/" in the entries.

> ---

l-prefix.S b/gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.S
> new file mode 100644
> index 0000000..9c86f2d
> --- /dev/null
> +++ b/gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.S
> @@ -0,0 +1,29 @@
> +/* Copyright (C) 2014 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-optional-prefix.S"

I think a line break after the include would read better.

> +	.text
> +	.globl	main
> +main:



> +	mov	%rsp,%rbp
> +	pushq	$42
> +	STAP_PROBE1(probe, foo, (%rsp))
> +	STAP_PROBE1(probe, bar, -8(%rbp))

What controls whether the optional prefix is included?  Could
we perhaps add two extra probes that probe the same values,
but use the optional prefix?  Something to the effect of:

	STAP_PROBE1(probe, foo, (%rsp))
	STAP_PROBE1(probe, bar, -8(%rbp))
	STAP_PROBE1(probe, foo_prefix, 8@(%rsp))
	STAP_PROBE1(probe, bar_prefix, 8@-8(%rbp))

(but I'm clueless on how that is actually written.)


> +	mov	%rbp,%rsp
> +	xor	%rax,%rax
> +	ret
> diff --git a/gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.exp b/gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.exp
> new file mode 100644
> index 0000000..9747dc8
> --- /dev/null
> +++ b/gdb/testsuite/gdb.arch/amd64-stap-optional-prefix.exp
> @@ -0,0 +1,50 @@
> +# 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 testcase is for PR breakpoints/16889
> +
> +if { ![istarget "x86_64-*-*"] } {
> +    verbose "Skipping amd64-stap-special-operands.exp"
> +    return
> +}
> +
> +standard_testfile ".S"

If you swap these, you can use $testfile:

standard_testfile ".S"

if { ![istarget "x86_64-*-*"] } {
    verbose "Skipping $testfile.exp"
    return
}

> +
> +if { [prepare_for_testing $testfile.exp $testfile $srcfile] } {
> +    untested amd64-stap-optional-prefix.exp
> +    return -1
> +}

Here too.  But, prepare_for_testing already calls untested for
you, using the text passed as first argument.  Write:

if { [prepare_for_testing "failed to prepare" $testfile $srcfile] } {
    return -1
}

> +
> +# Run to the first probe (foo).
> +
> +if { ![runto "-pstap foo"] } {
> +    fail "run to probe foo"
> +    return
> +}
> +
> +# Probe foo's first argument must me == $rsp

s/me/be ?  I think you meant:

 # Probe foo's first argument must be == ($rsp)

Might even make it easier to read to spell that out
in plain English.

Otherwise this looks good to me.