On 25 August 2014 23:03, Laszlo Ersek <ler...@redhat.com> wrote:
> On 08/25/14 21:19, Ard Biesheuvel wrote:
>> This series adds a platform config to support QEMU based virtual machines,
>> either in TCG or KVM mode. These virtual machines declare their platform
>> configuration by passing a device tree which needs to be parsed by Tianocore
>> rather than relying on hardcoded peripherals.
>>
>> Currently, the only assumptions made with respect to the platform config are:
>> - at least 1 MB of DRAM at 0x4000_0000
>> - NOR flash at 0x0
>
> I'll strive to find the time to review this great series.
>
Thanks.
> But, in advance, can we put the flash somewhere else, like at 0x8 or
> 0x10? Every time we treat NULL as a valid pointer to a firmware volume
> *in C*, the fabric of the universe comes a bit more unstitched.
>
Fair enough. With the following added to the .fdf, I am able to drop
the patch to allow FVs at 0x0.
-0x00000000|0x00200000
+0x00000000|0x00001000
+DATA = {
+ 0x0, 0x4, 0x0, 0x14 # b 0x1000
+}
+
+0x00001000|0x001ff000
--
Ard.
> For one, if a standards-conformant compiler manages to deduce that your
> FV pointer is a null pointer, and then you dereference that pointer, the
> compiler will have carte blanche to do whatever it pleases (undefined
> behavior). We should not base this patchset on undefined behavior.
>
> Using a zero physical address might be fine in assembly, but it isn't
> fine in C, for two reasons:
>
> - On this specific implementation, the bit pattern for the zero
> address, and the bit pattern for null pointers, coincide.
>
> - Per the C99 standard, 6.3.2.3 Pointers,
>
> 3 An integer constant expression with the value 0, or such an
> expression cast to type void *, is called a null pointer constant.
> [...]
>
> (As soon as the NOR flash's base address appears in the source code, in
> the form of an integer constant, and it's 0, we have big trouble. Not
> sure if this case ever occurs, but we shouldn't tempt it.)
>
> Again -- this would be perfectly fine in assembly, but given the C
> standard, it is not fine in C.
>
> Thanks
> Laszlo
>
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