Re: [PATCH] selftests/mm: compaction_test: Fix off by one in check_compaction()

[Dan Carpenter]

On Fri, Aug 09, 2024 at 11:20:48AM -0600, Shuah Khan wrote:
> On 8/9/24 06:32, Dan Carpenter wrote:
> > The "initial_nr_hugepages" variable is unsigned long so it takes up to
> > 20 characters to print, plus 1 more character for the NUL terminator.
> > Unfortunately, this buffer is not quite large enough for the terminator
> > to fit.  Also use snprintf() for a belt and suspenders approach.
> > 
> > Fixes: fb9293b6b015 ("selftests/mm: compaction_test: fix bogus test success and reduce probability of OOM-killer invocation")
> > Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > ---
> >   tools/testing/selftests/mm/compaction_test.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/mm/compaction_test.c b/tools/testing/selftests/mm/compaction_test.c
> > index e140558e6f53..2c3a0eb6b22d 100644
> > --- a/tools/testing/selftests/mm/compaction_test.c
> > +++ b/tools/testing/selftests/mm/compaction_test.c
> > @@ -89,9 +89,10 @@ int check_compaction(unsigned long mem_free, unsigned long hugepage_size,
> >   	int fd, ret = -1;
> >   	int compaction_index = 0;
> >   	char nr_hugepages[20] = {0};
> > -	char init_nr_hugepages[20] = {0};
> > +	char init_nr_hugepages[24] = {0};
> 
> Can we exceed this limit too? Can you make this a define?
> 

It's based on counting the digits in U64_MAX.

18446744073709551615X
123456789012345678901

We don't have any defines for that kind of thing.  It's not a bad idea.
#define STRLEN_U64_MAX 20
char init_nr_hugepages[STRLEN_U64_MAX + 1];

But it should be done as part of a kernel wide clean up and not part of this
buffer overflow fix.  The line above it could be changed as well, for example.
Let me create a KTODO and hope the internet will take care of it.

KTODO: create defines for STRLEN_[SU]8/16/32/64_MIN/MAX.

Btw, I rounded up to 24 just because I like buffer sizes that are divisible
by 4 but the compiler is probably going to do that automatically either way.

regards,
dan carpenter