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

[Manfred Spraul]

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@xxxxxxxxxx>
> >
> > 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?

What about intentionally setting the value to something useful?

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