[LTP] LTP: shmget02 fails on compat mode - 64-bit kernel and 32-bit userspace

[Li Wang]

Hi Manfred,

On Sat, May 20, 2023 at 1:55 AM Manfred Spraul <manfred@colorfullife.com>
wrote:

> Hi all,
>
> On 5/19/23 12:57, Arnd Bergmann wrote:
> > On Fri, May 19, 2023, at 11:17, Naresh Kamboju wrote:
> >> LTP running on compat mode where the tests run on
> >> 64-bit kernel and 32-bit userspace are noticed on a list of failures.
> >>
> >> What would be the best way to handle this rare combination of failures ?
> >>
> >> * arm64: juno-r2-compat, qemu_arm64-compat and qemu_x86_64-compat
> >>      - shmget02
> >>
> >> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> >>
> >> tst_hugepage.c:83: TINFO: 0 hugepage(s) reserved
> >> tst_test.c:1558: TINFO: Timeout per run is 0h 02m 30s
> >> tst_kconfig.c:87: TINFO: Parsing kernel config '/proc/config.gz'
> >> shmget02.c:95: TPASS: shmget(1644199826, 2048, 1024) : ENOENT (2)
> >> shmget02.c:95: TPASS: shmget(1627422610, 2048, 1536) : EEXIST (17)
> >> <4>[   84.678150] __vm_enough_memory: pid: 513, comm: shmget02, not
> >> enough memory for the allocation
> >> shmget02.c:95: TPASS: shmget(1644199826, 0, 1536) : EINVAL (22)
> >> shmget02.c:95: TFAIL: shmget(1644199826, 4278190080, 1536) expected
> >> EINVAL: ENOMEM (12)
> > Adding Liam Howlett, Davidlohr Bueso and Manfred Spraul to Cc, they
> > have worked on the shm code in the past few years.
> >
> > This is the line
> >
> >       {&shmkey1, SHMMAX + 1, IPC_CREAT | IPC_EXCL, 0, 0, EINVAL},
> >
> > from
> >
> >
> https://github.com/linux-test-project/ltp/blob/04e8f2f4fd949/testcases/kernel/syscalls/ipc/shmget/shmget02.c#LL59C1-L59C61
> >
> > right?
> >
> > I think this is a result of SHMMAX being defined as
> > #define SHMMAX (ULONG_MAX - (1UL << 24)), so the kernel would
> > likely use a large 64-bit value here, while the 32-bit user
> > space uses a much smaller limit.
> >
> > The expected return code likely comes from
> >
> > static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
> > {
> > ...
> >          if (size < SHMMIN || size > ns->shm_ctlmax)
> >                  return -EINVAL;
> >
> > but if ns->shm_ctlmax is probably set to the 64-bit value here.
> > It would then trigger the accounting limit in __shmem_file_setup():
> >
> >          if (shmem_acct_size(flags, size))
> >                  return ERR_PTR(-ENOMEM);
> >
> > My feeling is that the kernel in this case works as expected,
> > and I wouldn't see this as a bug. On the other hand, this
> > can probably be addressed in the kernel by adding a check for
> > compat tasks like
> >
> > --- a/ipc/shm.c
> > +++ b/ipc/shm.c
> > @@ -714,7 +714,8 @@ static int newseg(struct ipc_namespace *ns, struct
> ipc_params *params)
> >          char name[13];
> >          vm_flags_t acctflag = 0;
> >
> > -       if (size < SHMMIN || size > ns->shm_ctlmax)
> > +       if (size < SHMMIN || size > ns->shm_ctlmax ||
> > +          in_compat_syscall() && size > COMPAT_SHMMAX))
> >                  return -EINVAL;
> >
> >          if (numpages << PAGE_SHIFT < size)
> >
> I would consider this as ugly: ns->shm_ctlmax can be configured by
> writing to /proc/sys/kernel/shmmax.
>
> You can break the test case on 64-bit as well, just by writing SHMMAX+1
> to /proc/sys/kernel/shmmax
>
> Thus I think the test case is flawed:
>
> It is testing the overflow behavior for a configurable value by testing
> with default+1. But sometimes the actual value is not the default.
>
> Are the tests running as root?
>

Yes.



>
> What about intentionally setting the value to something useful?
>


This suggest sounds reasonable, but I have a question:
is there any upper limit for setting the /proc/sys/kernel/shmmax?

The test seems to try to test the bounder and as a
corner case for covering that scenario.



>
> tmp=$(cat /proc/sys/kernel/shmmax)
>
> echo "1234" > /proc/sys/kernel/shmmax
>
> semget() based on {&shmkey1, 1234 + 1, IPC_CREAT | IPC_EXCL, 0, 0, EINVAL},
> echo $tmp >/proc/sys/kernel/shmmax
>
> Or, alternatively: read /proc/sys/kernel/shmmax, and skip the test if
> the value is larger than ULONG_MAX-1.
>
> --
>         Manfred
>
>
>

-- 
Regards,
Li Wang