[PATCH][version 3]add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

Richard Sandiford richard.sandiford@arm.com
Tue Jun 22 08:25:57 GMT 2021


Kees Cook <keescook@chromium.org> writes:
> On Mon, Jun 21, 2021 at 03:39:45PM +0000, Qing Zhao wrote:
>> So, if “pattern value” is “0xFFFFFFFFFFFFFFFF”, then it’s a valid canonical virtual memory address.  However, for most OS, “0xFFFFFFFFFFFFFFFF” should be not in user space.
>> 
>> My question is, is “0xFFFFFFFFFFFFFFFFF” good for pointer? Or “0xAAAAAAAAAAAAAAAA” better?
>
> I think 0xFF repeating is fine for this version. Everything else is a
> "nice to have" for the pattern-init, IMO. :)

Sorry to be awkward, but 0xFF seems worse than 0xAA to me.

For integer types, all values are valid representations, and we're
relying on the pattern being “obviously” wrong in context.  0xAAAA…
is unlikely to be a correct integer but 0xFFFF… would instead be a
“nice” -1.  It would be difficult to tell in a debugger that a -1
came from pattern init rather than a deliberate choice.

I agree that, all other things being equal, it would be nice to use NaNs
for floats.  But relying on wrong numerical values for floats doesn't
seem worse than doing that for integers.

0xAA… for float is (if I've got this right) -3.0316488252093987e-13,
which admittedly doesn't stand out as wrong.  But I'm not sure we
should sacrifice integer debugging for float debugging here.

Thanks,
Richard


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