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SubjectRE: [PATCH 03/11] fs: add new read_uptr and write_uptr file operations
From: Linus Torvalds
> Sent: 27 June 2020 17:33
> On Sat, Jun 27, 2020 at 3:49 AM David Laight <> wrote:
> >
> > > Just keep the existing "set_fs()". It's not harmful if it's only used
> > > occasionally. We should rename it once it's rare enough, though.
> >
> > Am I right in thinking that it just sets a flag in 'current' ?
> Basically, yes. That's what it has always done.

I could check, but I suspect it sets what TASK_SIZE uses to ~0u
so that access_ok() can't fail.

> Well "always" is not true - it used to set the %fs segment register
> originally (thus the name), but _conceptually_ it sets a flag for
> "should user accesses be kernel accesses instead".
> On x86 - and most other architectures where user space and kernel
> space are in the same address space and accessed with the same
> instructions, that has then been implemented as just a "what is the
> limit for an access".
> On other architectures - architectures that need different access
> methods (or different flags to the load/store instruction) - it's an
> actual flag that changes which access method you use.
> > Although I don't remember access_ok() doing a suitable check
> > (would need to be (address - base) < limit).
> So again, on the architectures with a unified address space,
> access_ok() is exactly that "address + access_size <= limit", although
> often done with some inline asm just to get the overflow case done
> efficiently.

I realised afterwards that the 'kernel address is actually user'
check isn't really done on architectures like x86 until stac/clac.

I had another thought.
While setting up a full-blown scatter-gather 'iter' structure for
functions like [gs]etsockopt, ioctl and fcntl is OTT and probably
measurably expensive a lightweight 'buffer' structure that just
contained address, length and user/kernel flag could be used.

Although the uses would need an extra level of indirection this
would be offset by reducing the number of parameters passed
through all the layers.

> I thought there was just one very specific case of "oh, in certain
> cases of setsockopt we don't know what size this address is and optlen
> is ignored", so we have to just pass the pointer down to the protocol,
> which is the point that knows how much of an address it wants..

I can't help feeling that userspace passes a suitable length but
the kernel doesn't verify it.

It is worse than that, one of the SCTP getsockopt() calls has to return
a length that is shorter than the buffer it wrote.

So any buffer descriptor length would have to be advisory.


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