valgrind-users

[Valgrind-users] How to mark separate bits as undefined?

[juris <ju...@mt...>]

Hello, I was wondering if it is possible to partially mark
a memory location as undefined when running under valgrind.

I am using a custom allocator, that allocates a some bits
in a bitmap for each memory allocation. I would like to catch
access to these bits before they are initialized. Is there some
way to do it?

Thank you for the help.

Re: [Valgrind-users] How to mark separate bits as undefined?

[Azat K. <a3a...@gm...>]

On Wed, Apr 08, 2015 at 08:43:27AM +0300, juris wrote:
> Hello, I was wondering if it is possible to partially mark
> a memory location as undefined when running under valgrind.
> 
> I am using a custom allocator, that allocates a some bits
> in a bitmap for each memory allocation. I would like to catch
> access to these bits before they are initialized. Is there some
> way to do it?
> 
> Thank you for the help.

You could look at VALGRIND_SET_VBITS here:
http://valgrind.org/docs/manual/mc-manual.html

Example here:
http://repo.or.cz/w/valgrind.git/blob/HEAD:/memcheck/tests/metadata.c

Re: [Valgrind-users] How to mark separate bits as undefined?

[juris <ju...@mt...>]

On Wed, 8 Apr 2015 09:00:43 +0300
Azat Khuzhin <a3a...@gm...> wrote:

> You could look at VALGRIND_SET_VBITS here:
> http://valgrind.org/docs/manual/mc-manual.html
> 
> Example here:
> http://repo.or.cz/w/valgrind.git/blob/HEAD:/memcheck/tests/metadata.c

Thank you very much! The example is helpful. (But the documentation isn't :-(
This looks sufficient to implement the marking of separate bits.
Am I safe to assume that V bit value '0' will always mean 'defined' and
'1' will mean 'undefined'? Or at least that this meaning will not be
different for different address ranges?
I cannot find this mentioned in manual...

Re: [Valgrind-users] How to mark separate bits as undefined?

[Florian K. <fl...@ei...>]

On 08.04.2015 11:37, juris wrote:

> Thank you very much! The example is helpful. (But the documentation isn't :-(

You know how it is with documentation... Patches welcome.  :)

> This looks sufficient to implement the marking of separate bits.
> Am I safe to assume that V bit value '0' will always mean 'defined' and
> '1' will mean 'undefined'?

Yes.
memcheck/mc_include.h:#define V_BIT_DEFINED         0
memcheck/mc_include.h:#define V_BIT_UNDEFINED       1

   Florian

Re: [Valgrind-users] How to mark separate bits as undefined?

[John R. <jr...@bi...>]

> Hello, I was wondering if it is possible to partially mark
> a memory location as undefined when running under valgrind.

Quoting the documentation  http://valgrind.org/info/tools.html :
>  Memcheck tracks addressability at the byte-level, and initialisation of values at the bit-level. As a result, it can detect the use of single uninitialised bits, and does not report spurious errors on bitfield operations

Therefore: Run memcheck under a debugger, and watch while memcheck checks this program:
-----
#include <stdlib.h>
int *p;
int main()
{
	p = (int *)malloc(100);
	p[5] &= 0xec6da539;  /* clear some bits, leave the rest uninit */
	return p[0xff & p[5]];  /* indexing by uninit is an immediate error */
}
-----