Re: [LTP] [PATCH v2] overcommit_memory: Fix integer overflow and 32-bit limits

[Petr Vorel <pvorel@suse.cz>]

> On Wed, Oct 15, 2025 at 04:42:47PM +0200, Petr Vorel wrote:
> > > The overcommit test uses sum_total, the sum (memory and swap) to test
> > > the overcommit settings.

> > > This fixes two problems on 32-bit systems. The first is seen with a
> > > integer overflow can occur when calculating sum_total * 2, if the
> > > sum_total is larger than 2GB. The second is limited virtual address

> > You still mention GB ...


> Yep.  It is GB.

> > > space (2-3GB) means the test can fail from address space exhaustion
> > > before overcommit has been tested.

> > > Now the test runs correctly on low-memory 32-bit systems while avoiding
> > > both the overflow bug and virtual address space issues.

> > > Signed-off-by: Ben Copeland <ben.copeland@linaro.org>
> > > Acked-by: Arnd Bergmann <arnd@arndb.de>
> > > Reviewed-by: Petr Vorel <pvorel@suse.cz>
> > > ---
> > >  .../kernel/mem/tunable/overcommit_memory.c    | 33 +++++++++++++++----
> > >  1 file changed, 27 insertions(+), 6 deletions(-)

> > > diff --git a/testcases/kernel/mem/tunable/overcommit_memory.c b/testcases/kernel/mem/tunable/overcommit_memory.c
> > > index 9b2cb479d..7ff5a98d0 100644
> > > --- a/testcases/kernel/mem/tunable/overcommit_memory.c
> > > +++ b/testcases/kernel/mem/tunable/overcommit_memory.c
> > > @@ -131,24 +131,45 @@ static void overcommit_memory_test(void)
> > >  	TST_SYS_CONF_LONG_SET(OVERCOMMIT_MEMORY, 2, 1);

> > >  	update_mem_commit();
> > > -	alloc_and_check(commit_left * 2, EXPECT_FAIL);
> > > -	alloc_and_check(commit_limit + total_batch_size, EXPECT_FAIL);
> > > +	/* Skip tests that would overflow or exceed 32-bit address space */
> > > +	if (tst_kernel_bits() == 64 || (unsigned long)commit_left <= TST_GB / TST_KB) {

> > ... but TST_GB / TST_KB is actually MB (you could use TST_MB).


> The sizes in this test are measured in KB, so it's 1GB but measured in
> terms of KB not bytes.  Using TST_MB would work mathematically but it's
> misleading.

Ah, I'm sorry to overlook an obvious point.

Implementation details: thinking about the code twice, shouldn't be the check
for overflow in alloc_and_check() instead of outside (to keep the condition on
single place)?

Also, if kept outside the 1st and 2nd if could be joined:

	/* Skip tests that would overflow or exceed 32-bit address space */
	if (tst_kernel_bits() == 64 || (unsigned long)commit_left <= TST_GB / TST_KB) {
		alloc_and_check(commit_left * 2, EXPECT_FAIL);
		alloc_and_check(commit_limit + total_batch_size, EXPECT_FAIL);
		update_mem_commit();
		alloc_and_check(commit_left / 2, EXPECT_PASS);
	} else {
		tst_res(TCONF, "Skipping large allocation tests due to address space limits");
	}

instead of your proposal:

	update_mem_commit();
	/* Skip tests that would overflow or exceed 32-bit address space */
	if (tst_kernel_bits() == 64 || (unsigned long)commit_left <= TST_GB / TST_KB) {
		alloc_and_check(commit_left * 2, EXPECT_FAIL);
		alloc_and_check(commit_limit + total_batch_size, EXPECT_FAIL);
	} else {
		tst_res(TCONF, "Skipping large allocation tests due to address space limits");
	}
	update_mem_commit();
	if (tst_kernel_bits() == 64 || (unsigned long)commit_left <= TST_GB / TST_KB) {
		alloc_and_check(commit_left / 2, EXPECT_PASS);
	} else {
		tst_res(TCONF, "Skipping commit_left/2 allocation test due to address space limits");
	}

because update_mem_commit() IMHO just evaluates /proc/meminfo values, but
when alloc_and_check() is skipped nothing has changed.

Kind regards,
Petr

> regards,
> dan carpenter


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