On Tue, 2014-05-13 at 10:49 +0200, Bjørn Mork wrote:
> Oliver Neukum <oneu...@suse.de> writes:
>
> > On Sat, 2014-05-10 at 17:41 +0200, Bjørn Mork wrote:
> >> + /* clamp new_rx to sane values */
> >> + min = min_t(u32, USB_CDC_NCM_NTB_MIN_IN_SIZE,
> >> le32_to_cpu(ctx->ncm_parm.dwNtbInMaxSize));
> >> + max = min_t(u32, CDC_NCM_NTB_MAX_SIZE_RX,
> >> le32_to_cpu(ctx->ncm_parm.dwNtbInMaxSize));
> >
> > Are you sure this makes sense? min_t both times?
>
> Yes, I am sure. At least it made sense when I wrote it. I am more in
> doubt now.
I actually suspected a copy n' paste error; thence the formulation.
> I guess you don't question the max calculation, but just so everyone
> else gets the idea: dwNtbInMaxSize is the buffer size suggested by the
> device. Some devices just specify an insanely large value (132kB has
> been observed). So we need to cap that to CDC_NCM_NTB_MAX_SIZE_RX, which
> is the absolutely largest buffer size we are prepared to support.
Good
> USB_CDC_NCM_NTB_MIN_IN_SIZE is the minimum acceptable buffer size
> according to the spec. dwNtbInMaxSize is not allowed to be smaller than
> this. So if we assume that no device violates the spec, then the above
> should simple be
>
> min = USB_CDC_NCM_NTB_MIN_IN_SIZE;
> max = min_t(u32, CDC_NCM_NTB_MAX_SIZE_RX,
> le32_to_cpu(ctx->ncm_parm.dwNtbInMaxSize));
>
> which is the result for all spec conforming devices.
>
> The reason I put that min_t() there instead was an attempt to deal with
> the (not unlikely) event that some buggy device set dwNtbInMaxSize lower
> than this required minimum value. We then have the choices:
>
> a) fail to support the buggy device
> b) attempt to set a larger buffer size than the device supports
> c) accept the lower size
My preference would be b) > a) > c)
It seems to me that would should respect the spec and if the spec sets
a lower limit then we don't go lower.
> So I chose c) in an attempt to be as gentle as possible. But I am open
> to go for a) instead if you think that is better. After all
> USB_CDC_NCM_NTB_MIN_IN_SIZE is as low as 2048, so it doesn't fit much
> more than the headers and a single full size ethernet frame. And I see
> now that we fail to do further sanity checking after this. What if
> dwNtbInMaxSize is 0? Or smaller than the necessary headers?
Exactly. Some fool may simply overlook setting it at all.
> Should I rewrite the above to do a) instead? I.e.
>
> min = USB_CDC_NCM_NTB_MIN_IN_SIZE;
> max = min_t(u32, CDC_NCM_NTB_MAX_SIZE_RX,
> le32_to_cpu(ctx->ncm_parm.dwNtbInMaxSize));
> if (min > max)
> fail;
>
> I don't think b) is a good idea. It might work, but it might also fail
> in surprising ways making it hard to debug.
Users may prefer working devices to clean failures, but
I primarily care about conforming to spec. We just shouldn't
do such violations in a general case.
Regards
Oliver
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