Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas

On Mon, 2021-02-15 at 10:13 +0100, Michal Hocko wrote: > On Sun 14-02-21 11:21:02, James Bottomley wrote: > > On Sun, 2021-02-14 at 10:58 +0100, David Hildenbrand wrote: > > [...] > > > > And here we come to the question "what are the differences that > > > > justify a new system call?" and the answer to this is very > > > > subjective. And as such we can continue bikeshedding forever. > > > > > > I think this fits into the existing memfd_create() syscall just > > > fine, and I heard no compelling argument why it shouldn‘t. That‘s > > > all I can say. > > > > OK, so let's review history. In the first two incarnations of the > > patch, it was an extension of memfd_create(). The specific > > objection by Kirill Shutemov was that it doesn't share any code in > > common with memfd and so should be a separate system call: > > > > https://lore.kernel.org/linux-api/20200713105812.dnwtdhsuyj3xbh4f@box/ > > Thanks for the pointer. But this argument hasn't been challenged at > all. It hasn't been brought up that the overlap would be considerable > higher by the hugetlb/sealing support. And so far nobody has claimed > those combinations as unviable. Kirill is actually interested in the sealing path for his KVM code so we took a look. There might be a two line overlap in memfd_create for the seal case, but there's no real overlap in memfd_add_seals which is the bulk of the code. So the best way would seem to lift the inode ... -> seals helpers to be non-static so they can be reused and roll our own add_seals.

I can't see a use case at all for hugetlb support, so it seems to be a bit of an angels on pin head discussion. However, if one were to come along handling it in the same way seems reasonable.

> > The other objection raised offlist is that if we do use > > memfd_create, then we have to add all the secret memory flags as an > > additional ioctl, whereas they can be specified on open if we do a > > separate system call. The container people violently objected to > > the ioctl because it can't be properly analysed by seccomp and much > > preferred the syscall version. > > > > Since we're dumping the uncached variant, the ioctl problem > > disappears but so does the possibility of ever adding it back if we > > take on the container peoples' objection. This argues for a > > separate syscall because we can add additional features and extend > > the API with flags without causing anti-ioctl riots. > > I am sorry but I do not understand this argument. You don't understand why container guarding technology doesn't like ioctls?

> What kind of flags are we talking about and why would that be a > problem with memfd_create interface? Could you be more specific > please? You mean what were the ioctl flags in the patch series linked above? They were SECRETMEM_EXCLUSIVE and SECRETMEM_UNCACHED in patch 3/5.

They were eventually dropped after v10, because of problems with architectural semantics, with the idea that it could be added back again if a compelling need arose: https://lore.kernel.org/linux-api/20201123095432.5860-1-rppt@kernel.org/ In theory the extra flags could be multiplexed into the memfd_create flags like hugetlbfs is but with 32 flags and a lot already taken it gets messy for expansion. When we run out of flags the first question people will ask is "why didn't you do separate system calls?".