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    SubjectRe: [PATCH] gup: return -EFAULT on access_ok failure
    On Thu, Apr 05, 2018 at 11:43:27AM -0700, Linus Torvalds wrote:
    > On Thu, Apr 5, 2018 at 11:28 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
    > >
    > > to repeat what you are saying IIUC __get_user_pages_fast returns 0 if it can't
    > > pin any pages and that is by design. Returning 0 on error isn't usual I think
    > > so I guess this behaviour should we well documented.
    >
    > Arguably it happens elsewhere too, and not just in the kernel.
    > "read()" at past the end of a file is not an error, you'll just get 0
    > for EOF.
    >
    > So it's not really "returning 0 on error".
    >
    > It really is simply returning the number of pages it got. End of
    > story. That number of pages can be smaller than the requested number
    > of pages, and _that_ is due to some error, but note how it can return
    > "5" on error too - you asked for 10 pages, but the error happened in
    > the middle!
    >
    > So the right way to check for error is to bverify that you get the
    > number of pages that you asked for. If you don't, something bad
    > happened.
    >
    > Of course, many users don't actually care about "I didn't get
    > everything". They only care about "did I get _something_". Then that 0
    > ends up being the error case, but note how it depends on the caller.
    >
    > > What about get_user_pages_fast though?
    >
    > We do seem to special-case the first page there. I'm not sure it's a
    > good idea. But like the __get_user_pages_fast(), we seem to have users
    > that know about the particular semantics and depend on it.
    >
    > It's all ugly, I agree.
    >
    > End result: we can't just change semantics of either of them.
    >
    > At least not without going through every single user and checking that
    > they are ok with it.
    >
    > Which I guess I could be ok with. Maybe changing the semantics of
    > __get_user_pages_fast() is acceptable, if you just change it
    > *everywhere* (which includes not just he users, but also the couple of
    > architecture-specific versions of that same function that we have.
    >
    > Linus

    OK I hope I understood what you are saying here.

    At least drivers/gpu/drm/i915/i915_gem_userptr.c seems to
    get it wrong:

    pinned = __get_user_pages_fast(obj->userptr.ptr,

    if (pinned < 0) {
    pages = ERR_PTR(pinned);
    pinned = 0;
    } else if (pinned < num_pages) {
    pages = __i915_gem_userptr_get_pages_schedule(obj);
    active = pages == ERR_PTR(-EAGAIN);
    } else {
    pages = __i915_gem_userptr_alloc_pages(obj, pvec, num_pages);
    active = !IS_ERR(pages);
    }

    The <0 path is never taken.

    Cc maintainers - should that driver be changed to use
    get_user_pages_fast?


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