Re: [PATCH] Add MALLOC_CHECK_ and MALLOC_PERTURB_ libc env to the test suite for detecting heap corruption

Elia Pinto [off-list ref] writes:

quoted

 - That "165" thing I mentioned earlier.

Thank you so much for the comments, that's fine. A single
consideration for  MALLOC_PERTURB.

You can use any value between 1..255 for MALLOC_PERTURB_
That chooses the byte that glibc will use to memset all freed buffers.
In general it is defined as

    export MALLOC_PERTURB_=$(($RANDOM % 255 + 1))

(as drepper pointed out http://udrepper.livejournal.com/11429.html)

Drepper never recommends RANDOM there.

Using a random value is slightly better than using a fixed one
in case your fixed value is someday just the right/wrong value to mask
a problem.

Quite the contrary.  When you use a fixed pattern, it is easy which
other pieces of memory has uninitailized contents.  When you use a
random value, you sometimes get an error and sometimes the test
mysteriously pass, which does not help debugging.

openSUSE folks seem to use a fixed value for this exact reason of
repeatability of tests.

http://jaegerandi.blogspot.com/2012/01/finding-subtile-malloc-bugs.html

So OK per the original expression?

No.

I am not convinced 165 is the perfect value, but I am fairly certain
any fixed value is better than using a random to deliberately worsen
repeatability of the tests.

Ok.

Please use the patch that you have already queued in the ep integration branch.

Thank you

2012/9/17, Junio C Hamano [off-list ref]:

Elia Pinto [off-list ref] writes:

quoted

quoted

 - That "165" thing I mentioned earlier.

Thank you so much for the comments, that's fine. A single
consideration for  MALLOC_PERTURB.

You can use any value between 1..255 for MALLOC_PERTURB_
That chooses the byte that glibc will use to memset all freed buffers.
In general it is defined as

    export MALLOC_PERTURB_=$(($RANDOM % 255 + 1))

(as drepper pointed out http://udrepper.livejournal.com/11429.html)

Drepper never recommends RANDOM there.

quoted

Using a random value is slightly better than using a fixed one
in case your fixed value is someday just the right/wrong value to mask
a problem.

Quite the contrary.  When you use a fixed pattern, it is easy which
other pieces of memory has uninitailized contents.  When you use a
random value, you sometimes get an error and sometimes the test
mysteriously pass, which does not help debugging.

openSUSE folks seem to use a fixed value for this exact reason of
repeatability of tests.

http://jaegerandi.blogspot.com/2012/01/finding-subtile-malloc-bugs.html

quoted

So OK per the original expression?

No.

I am not convinced 165 is the perfect value, but I am fairly certain
any fixed value is better than using a random to deliberately worsen
repeatability of the tests.

-- 
Inviato dal mio dispositivo mobile