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

[Kees Cook <keescook@chromium.org>]

On Wed, Jun 16, 2021 at 07:39:02PM +0000, Qing Zhao wrote:
> So, the major question now is:
> 
> Is one single repeatable pattern enough for pattern initialization for all different types of auto variables?
> 
> If YES, then the implementation for pattern initialization will be much easier and simpler
>       as you pointed out. And will save me a lot of pain to implement this part.
> If NO, then we have to keep the current complicate implementation since it provides us
>       the flexibility to assign different patterns to different types.
> 
> Honestly, I don’t have a good justification on this question myself.
> 
> The previous references I have so far are the current behavior of CLANG and Microsoft compiler.
> 
> For your reference,
> . CLANG uses different patterns for INTEGER  (0xAAAAAAAA) and FLOAT (0xFFFFFFFF) and 32-bit pointer (0x000000AA)
> https://reviews.llvm.org/D54604
> . Microsoft uses different patterns for INTEGERS ( 0xE2), FLOAT (1.0)
> https://msrc-blog.microsoft.com/2020/05/13/solving-uninitialized-stack-memory-on-windows/
> 
> My understanding from CLANG’s comment is, the patterns are easier to crash the program for the certain type, therefore easier to
> catch any potential bugs.

Right, this is the justification for the different patterns. I am
fine with a static value for the first version of this functionality,
as long as it's a non-canonical virtual memory address when evaluated
as a pointer (so that the pattern can't be made to aim at a legitimate
fixed allocatable address in memory).

> Don’t know why Microsoft chose the pattern like this.
> 
> So, For GCC, what should we do on the pattern initializations, shall we choose one single repeatable pattern for all the types as you suggested,
> Or chose different patterns for different types as Clang and Microsoft compiler’s behavior?
> 
> Kees, do you have any comment on this?
> 
> How did Linux Kernel use -ftrivial-auto-var-init=pattern feature of CLANG?

It's just used as-is from the compiler, and recommended for "debug
builds".

-- 
Kees Cook