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

[Liam R. Howlett]

* Arnd Bergmann <arnd@xxxxxxxx> [230519 06:57]:
> 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.
> >

Is this new?

> > 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.

There was an issue with returning ENOMEM on 32bit which may be causing
this failure - if it's new.

> 
> 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)
> 
> 
>       Arnd