Re: [RFC PATCH v4 2/4] x86/sgx: Implement support for MADV_WILLNEED

Haitao Huang
2023-02-14T21:42:16Z

On Tue, 14 Feb 2023 14:54:53 -0600, Huang, Kai <kai.huang@intel.com> wrote:

> On Tue, 2023-02-14 at 13:18 -0600, Haitao Huang wrote:
>> Hi Kai
>>
>> On Tue, 14 Feb 2023 03:47:24 -0600, Huang, Kai <kai.huang@intel.com>
>> wrote:
>>
>> > On Fri, 2023-01-27 at 20:55 -0800, Haitao Huang wrote:
>> > > @@ -97,10 +99,81 @@ static int sgx_mmap(struct file *file, struct
>> > > vm_area_struct *vma)
>> > > vma->vm_ops = &sgx_vm_ops;
>> > > vma->vm_flags |= VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP | VM_IO;
>> > > vma->vm_private_data = encl;
>> > > + vma->vm_pgoff = PFN_DOWN(vma->vm_start - encl->base);
>> > > return 0;
>> > > }
>> >
>> > Perhaps I am missing something, but above change looks weird.
>> > Conceptually, it doesn't/shouldn't belong to this series, which
>> > essentially
>> > preallocates and does EAUG EPC pages for a (or part of) given enclave.
>> > The EAUG
>> > logic should already be working for the normal fault path, which means
>> > the code
>> > change above either: 1) has been done at other place; 2) isn't needed.
>> >
>> > I have kinda forgotten the userspace sequence to create an enclave.
>> If
>> > I recall
>> > correctly, you do below to create an enclave:
>> >
>> > 1) encl_fd = open("/dev/sgx_enclave");
>> > 2) encl_addr = mmap(encl_fd, encl_size, 0 /* pgoff */);
>> > 3) IOCTL(ECREATE, encl_addr, encl_size);
>> >
>> > Would the above code change break the "mmap()" in above step 2?
>> >
>>
>> No, vm_pgoff was not used previously for enclave VMAs. I had to add this
>> because the offset passed to sgx_fadvise is relative to file base and
>> calculated in mm/madvise.c like this:
>>
>> offset = (loff_t)(start - vma->vm_start)
>> + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
>
> But shouldn't 'offset is relative to the file base' be conceptually
> correct from
> the fadvice()'s point of view?
>
> I think you should do:
>
> encl_offset = offset + encl->base;
>
> inside sgx_fadvice()?
>
>>
If we don't set vma->vm_pgoff (default to zero), then offset will be
calculated as (start - vma->vm_start). Then the above calculation is wrong
if we have multiple VMAs for the same enclave, which is usually the case.

>> I had a comment in first version but removed it based on Jarkko's
>> suggestion here:
>> https://lore.kernel.org/all/Y2B0jBsG6HE4KVk7@kernel.org/
>>
>> The original comments probably seemed redundant to the definitions of
>> the
>> vm_pgoff field and the fadvise interface. But let me know if we need
>> add a
>> more helpful version of comments or any suggestion on the comments.
>
> I still think this code change is wrong.
>
> For instance, IIUC, it at least breaks the case where enclave hasn't been
> created/initialized, where encl->base == 0 (although normal code path
> doesn't
> use vm_pgoff, conceptually it's still wrong IIUC).
>
> Maybe I am missing something?

The fadvise interface is only usable for an initialized enclave,
sgx_fadvise will return error otherwise. Conceptually I view enclave base
as "file base", it's just that we don't ever need handle the zero case
caused by uninitialized enclave (kind of like a file never mapped). If an
initialized enclave happens to have zero base, it would also work.

Thanks
Haitao