[LTP] [PATCH v1] overcommit_memory: Fix integer overflow and 32-bit limits

[Ben Copeland]

Hello Petr,

On Mon, 13 Oct 2025 at 16:00, Petr Vorel <pvorel@suse.cz> wrote:
>
> Hi all,
>
> > On Fri, Oct 10, 2025, at 11:04, Ben Copeland 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
> > > 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.
>
> Thanks for looking into the issue.
>
> > > Signed-off-by: Ben Copeland <ben.copeland@linaro.org>
>
> > The logic makes sense to me.
>
> > Acked-by: Arnd Bergmann <arnd@arndb.de>
>
> > >     update_mem();
> > >     alloc_and_check(free_total / 2, EXPECT_PASS);
> > > -   alloc_and_check(sum_total * 2, EXPECT_FAIL);
> > > +   /* Skip if sum_total * 2 would exceed address space.
> > > +    * On 32-bit, skip when sum_total > ULONG_MAX/4 (~1GB).
> > > +    * Most 32-bit systems with <=1GB RAM can map 2x that in 3GB vaddr space.
> > > +    * Systems with 2GB+ RAM likely cannot fit allocations in vaddr space. */
> > > +   if (tst_kernel_bits() == 64 || (unsigned long)sum_total <= ONE_GB) {
>
> Why not use TST_GB instead of creating new constant ONE_GB?

This makes sense, I will do that.

>
> > >  #define ONE_GB                 (1024 * 1024 * TST_KB)
>
> Maybe even use static variable holding the condition and set it in setup()?
>
> Otherwise LGTM.
>
> Reviewed-by: Petr Vorel <pvorel@suse.cz>
>
> > > +           alloc_and_check(sum_total * 2, EXPECT_FAIL);
> > > +   } else {
> > > +           tst_res(TCONF, "Skipping large allocation test due to address space limits");
> > > +   }
>
> > It would be nice if it was possible to express this in terms of
> > the kernel's TASK_SIZE constant and not have to check for
> > 64-bit kernels.
>
> > Unfortunately I have not been able find TASK_SIZE from userspace
> > directly other than maybe probing MAP_FIXED mmap() calls which
> > would be overly complicated. Your approach is probably as
> > good as it gets.
>
> Thanks for your time!
>
> BTW in our case (openSUSE Tumbleweed 32 bit) it fails on commit_left,
> which is slightly smaller:
> commit_left: 2221904
> sum_total: 2884328

Ah, good spot. This is failing further up the code; this fix was for
overcommit_memory=1. It's hitting the same integer overflow issue. I
will apply the same fix to overcommit_memory=2 and spin up a v2 patch.

Thanks for the feedback!

Regards,

Ben

>
> tst_kernel.c:90: TINFO: uname.machine=i686 kernel is 32bit
> tst_test.c:2021: TINFO: LTP version: 20251010.79b7f212
> tst_test.c:2024: TINFO: Tested kernel: 6.17.1-1-default #1 SMP PREEMPT_DYNAMIC Tue Oct  7 04:26:30 UTC 2025 (1bf5c2e) i686
> tst_kconfig.c:88: TINFO: Parsing kernel config '/proc/config.gz'
> tst_kconfig.c:676: TINFO: CONFIG_FAULT_INJECTION kernel option detected which might slow the execution
> tst_test.c:1842: TINFO: Overall timeout per run is 0h 02m 00s
> overcommit_memory.c:103: TINFO: MemTotal is 2050380 kB
> overcommit_memory.c:105: TINFO: SwapTotal is 2098152 kB
> overcommit_memory.c:109: TINFO: CommitLimit is 3121292 kB
> overcommit_memory.c:122: TINFO: Setting /proc/sys/vm/overcommit_ratio to 50
> overcommit_memory.c:125: TINFO: TotalBatchSize is 8008 kB
> overcommit_memory.c:131: TINFO: Setting /proc/sys/vm/overcommit_memory to 2
> overcommit_memory.c:161: TINFO: malloc 5738112 kB successfully
> overcommit_memory.c:188: TFAIL: alloc passed, expected to fail
> overcommit_memory.c:165: TINFO: malloc 3129300 kB failed
> overcommit_memory.c:186: TPASS: alloc failed as expected
> overcommit_memory.c:161: TINFO: malloc 1434528 kB successfully
> overcommit_memory.c:180: TPASS: alloc passed as expected
> overcommit_memory.c:140: TINFO: Setting /proc/sys/vm/overcommit_memory to 0
> overcommit_memory.c:161: TINFO: malloc 1804260 kB successfully
> overcommit_memory.c:180: TPASS: alloc passed as expected
> overcommit_memory.c:165: TINFO: malloc 8297064 kB failed
> overcommit_memory.c:186: TPASS: alloc failed as expected
> overcommit_memory.c:147: TINFO: Setting /proc/sys/vm/overcommit_memory to 1
> overcommit_memory.c:161: TINFO: malloc 2074266 kB successfully
> overcommit_memory.c:180: TPASS: alloc passed as expected
> overcommit_memory.c:165: TINFO: malloc 4148532 kB failed
> overcommit_memory.c:182: TFAIL: alloc failed, expected to pass
> overcommit_memory.c:165: TINFO: malloc 8297064 kB failed
> overcommit_memory.c:182: TFAIL: alloc failed, expected to pass
>
> Kind regards,
> Petr
>
> >       Arnd