Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

On Thu, Oct 28, 2021 at 2:21 PM Catalin Marinas <[email protected]> wrote:
>
> They do look fairly similar but we should have the information in the
> fault handler to distinguish: not a page fault (pte permission or p*d
> translation), in_task(), user address, fixup handler. But I agree the
> logic looks fragile.

So thinking about this a bit more, I think I have a possible
suggestion for how to handle this..

The pointer color fault (or whatever some other architecture may do to
generate sub-page faults) is not only not recoverable in the sense
that we can't fix it up, it also ends up being a forced SIGSEGV (ie it
can't be blocked - it has to either be caught or cause the process to
be killed).

And the thing is, I think we could just make the rule be that kernel
code that has this kind of retry loop with fault_in_pages() would
force an EFAULT on a pending SIGSEGV.

IOW, the pending SIGSEGV could effectively be exactly that "thread flag".

And that means that fault_in_xyz() wouldn't need to worry about this
situation at all: the regular copy_from_user() (or whatever flavor it
is - to/from/iter/whatever) would take the fault. And if it's a
regular page fault,. it would act exactly like it does now, so no
changes.

If it's a sub-page fault, we'd just make the rule be that we send a
SIGSEGV even if the instruction in question has a user exception
fixup.

Then we just need to add the logic somewhere that does "if active
pending SIGSEGV, return -EFAULT".

Of course, that logic might be in fault_in_xyz(), but it migth also be
a separate function entirely.

So this does effectively end up being a thread flag, but it's also
slightly more than that - it's that a sub-page fault from kernel mode
has semantics that a regular page fault does not.

The whole "kernel access doesn't cause SIGSEGV, but returns -EFAULT
instead" has always been an odd and somewhat wrong-headed thing. Of
course it should cause a SIGSEGV, but that's not how Unix traditionall
worked. We would just say "color faults always raise a signal, even if
the color fault was triggered in a system call".

(And I didn't check: I say "SIGSEGV" above, but maybe the pointer
color faults are actually SIGBUS? Doesn't change the end result).

                 Linus