gdb: Disable address space randomization for a test

  The gdb.opt/inline-break.exp test script relies on capturing a $pc
value in one run of GDB, then restarting GDB and placing a breakpoint
at the previously captured $pc and checking we get the expected
behaviour.

With address space randomization on then the captured $pc value is
almost never valid in the second run of GDB, and so the test fails.

Disabling address space randomization where appropriate fixes this
issue.

gdb/testsuite/ChangeLog:

	* gdb.opt/inline-break.exp: Disable address space randomization
	for the two runs where we care about addresses being consistent.
---
 gdb/testsuite/ChangeLog                |  5 +++++
 gdb/testsuite/gdb.opt/inline-break.exp | 12 ++++++++++++
 2 files changed, 17 insertions(+)
  

> On Jul 30, 2018, at 2:50 PM, Andrew Burgess <andrew.burgess@embecosm.com> wrote:
> 
> The gdb.opt/inline-break.exp test script relies on capturing a $pc
> value in one run of GDB, then restarting GDB and placing a breakpoint
> at the previously captured $pc and checking we get the expected
> behaviour.
> 
> With address space randomization on then the captured $pc value is
> almost never valid in the second run of GDB, and so the test fails.
> 
> Disabling address space randomization where appropriate fixes this
> issue.
> 
> ...
> 
> +    # At the end of this test we store a $pc value, then restart GDB,
> +    # and use this $pc value to place a breakpoint.  If address space
> +    # randomization is on, then the chance of this $pc value being
> +    # correct in a second GDB instance is pretty small.
> +    gdb_test "set disable-randomization off"

Does that turn off address randomization?  The keyword implies the opposite (double negative, so "disable... off" means "turn on").

Also, does this work on every target OS?

	paul
  

* Paul Koning <paulkoning@comcast.net> [2018-07-30 14:56:32 -0400]:

> 
> 
> > On Jul 30, 2018, at 2:50 PM, Andrew Burgess <andrew.burgess@embecosm.com> wrote:
> > 
> > The gdb.opt/inline-break.exp test script relies on capturing a $pc
> > value in one run of GDB, then restarting GDB and placing a breakpoint
> > at the previously captured $pc and checking we get the expected
> > behaviour.
> > 
> > With address space randomization on then the captured $pc value is
> > almost never valid in the second run of GDB, and so the test fails.
> > 
> > Disabling address space randomization where appropriate fixes this
> > issue.
> > 
> > ...
> > 
> > +    # At the end of this test we store a $pc value, then restart GDB,
> > +    # and use this $pc value to place a breakpoint.  If address space
> > +    # randomization is on, then the chance of this $pc value being
> > +    # correct in a second GDB instance is pretty small.
> > +    gdb_test "set disable-randomization off"
> 
> Does that turn off address randomization?  The keyword implies the
> opposite (double negative, so "disable... off" means "turn on").

That's a very good question... now I just have to figure out why my
incorrect patch fixed the problem I was seeing.... watch this space...

> Also, does this work on every target OS?

That's a good question, I don't have an answer for you.

Thanks,
Andrew
  

On 2018-07-30 15:01, Andrew Burgess wrote:
>> Does that turn off address randomization?  The keyword implies the
>> opposite (double negative, so "disable... off" means "turn on").
> 
> That's a very good question... now I just have to figure out why my
> incorrect patch fixed the problem I was seeing.... watch this space...

GDB usually disables address randomization by default, so I was also 
wondering why this would be needed.  Do you have a scenario where GDB 
doesn't disable it by default?

Simon
  

* Simon Marchi <simon.marchi@polymtl.ca> [2018-07-30 15:49:01 -0400]:

> On 2018-07-30 15:01, Andrew Burgess wrote:
> > > Does that turn off address randomization?  The keyword implies the
> > > opposite (double negative, so "disable... off" means "turn on").
> > 
> > That's a very good question... now I just have to figure out why my
> > incorrect patch fixed the problem I was seeing.... watch this space...
> 
> GDB usually disables address randomization by default, so I was also
> wondering why this would be needed.  Do you have a scenario where GDB
> doesn't disable it by default?

Thank you to you both for pointing me in the right direction.

It turned out the mistake was user error.  I've recently switched to a
jenkins/docker combo to try and streamline testing my patches.

When running GDB under docker I'm having to supply the '--privileged'
flag in order to allow GDB to function correctly.

The problem was that I use a separate docker instance to build GDB and
then a set of instances to test GDB in different modes (native,
gdbserver, etc).  When testing GDB I supplied the '--privileged', but
not when building GDB.  As a result configure decided that my system
didn't support the personality API, and so all the code to disable
address space randomization was not compiled into GDB.

As for why I failed to spot this, again user error, but far less
interesting, I forgot I pushed a change to the scripts I used for
comparing test results.

Anyway, sorry for the noise, and thanks for the help figuring out my
mistakes.

Thanks,
Andrew
  

On 2018-07-31 08:47, Andrew Burgess wrote:
> * Simon Marchi <simon.marchi@polymtl.ca> [2018-07-30 15:49:01 -0400]:
> 
>> On 2018-07-30 15:01, Andrew Burgess wrote:
>> > > Does that turn off address randomization?  The keyword implies the
>> > > opposite (double negative, so "disable... off" means "turn on").
>> >
>> > That's a very good question... now I just have to figure out why my
>> > incorrect patch fixed the problem I was seeing.... watch this space...
>> 
>> GDB usually disables address randomization by default, so I was also
>> wondering why this would be needed.  Do you have a scenario where GDB
>> doesn't disable it by default?
> 
> Thank you to you both for pointing me in the right direction.
> 
> It turned out the mistake was user error.  I've recently switched to a
> jenkins/docker combo to try and streamline testing my patches.
> 
> When running GDB under docker I'm having to supply the '--privileged'
> flag in order to allow GDB to function correctly.
> 
> The problem was that I use a separate docker instance to build GDB and
> then a set of instances to test GDB in different modes (native,
> gdbserver, etc).  When testing GDB I supplied the '--privileged', but
> not when building GDB.  As a result configure decided that my system
> didn't support the personality API, and so all the code to disable
> address space randomization was not compiled into GDB.
> 
> As for why I failed to spot this, again user error, but far less
> interesting, I forgot I pushed a change to the scripts I used for
> comparing test results.
> 
> Anyway, sorry for the noise, and thanks for the help figuring out my
> mistakes.
> 
> Thanks,
> Andrew

Well, that's good information to know, thanks!

Simon
  

Paul> Also, does this work on every target OS?

It doesn't.  The target has to implement the
supports_disable_randomization method, and then actually implement the
disabling.  At least macOS doesn't do this (see
https://sourceware.org/bugzilla/show_bug.cgi?id=23365), and probably
others as well.

Tom