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

[Richard Biener <rguenther@suse.de>]

On June 21, 2021 5:11:30 PM GMT+02:00, Qing Zhao <qing.zhao@oracle.com> wrote:
>HI, Richard,
>
>> On Jun 21, 2021, at 2:53 AM, Richard Biener <rguenther@suse.de>
>wrote:
>> 
>>> 
>>> 
>>> This is for the compatibility with CLANG. -:).
>(https://reviews.llvm.org/D54604)
>> 
>> I don't care about functional 1:1 "compatibility" with CLANG.
>
>Okay.  -:)
>
>> 
>>> 1. Pattern initialization
>>> 
>>>  This is the recommended initialization approach. Pattern
>initialization's
>> 
>> But elsewhere you said pattern initialization is only for debugging,
>> not production …
>
>Yes. Pattern initialization is only for debugging purpose during
>development phase.
>
>> 
>>> Use a pattern that fits them all.  I mean memory allocation
>hardening
>>> fills allocated storage with a repeated (byte) pattern and people
>are
>>> happy with that.  It also makes it easy to spot uninitialized
>storage
>>> from a debugger.  So please, do not over-design this, it really
>doesn't
>>> make any sense and the common case you are inevitably chasing here
>>> would already be fine with a random repeated pattern.
>>> 
>>> So, My question is:
>>> 
>>> If we want to pattern initialize with the single repeated pattern
>for all types, with one is better to use:  “0xAAAAAAAA”
>>> or “0xFFFFFFFF” , or other pattern that our current glibc used?
>What’s that pattern?
>> 
>> It's set by the user.
>
>Yes, looks like that glibc uses a byte-repeated pattern that is set by
>the user through environment variable.
>
>> 
>>> Will  “0xAAAAAAAA” in a floating type auto variable crash the
>program?
>>> Will “0xFFFFFFFF” in a pointer type auto variable crash the program?
>(Might crash?)
>>> 
>>> 
>>> (thus also my suggestion to split out
>>> padding handling - now we can also split out pattern init handling,
>>> maybe somebody else feels like reviewing and approving this, who
>knows).
>>> 
>>> I am okay with further splitting pattern initialization part to a
>separate patch. Then we will
>>> have 4 independent patches in total:
>>> 
>>> 1. -fauto-var-init=zero and all the handling in other passes to the
>new added call to .DEFERRED_INIT.
>>> 2. Add -fauto-var-init=pattern
>>> 3. Add -fauto-var-init-padding
>>> 4. Add -ftrivial-auto-var-init for CLANG compatibility.
>>> 
>>> Are the above the correct understanding?
>> 
>> I think we can drop -fauto-var-init=pattern and just go with block
>> initializing which will cover padding as well which means we can
>> stay with the odd -ftrivial-auto-var-init name used by CLANG and
>> add no additional options.
>
>Yes, this is a good idea. 
>
>block initializing will cover all paddings automatically. 
>
>Shall we do block initializing for both “zero initialization” and
>“pattern initialization”?
>
>Currently, for zero initialization, I used the following:
>
>>>> +    case AUTO_INIT_ZERO:
>>>> +      init = build_zero_cst (TREE_TYPE (var));
>>>> +      expand_assignment (var, init, false);
>>>> +      break;
>
>Looks like that the current “expand_assignment” does not initialize
>paddings with zeroes. 
>Shall I also use “memset” for “zero initialization”?

I'd say so, yes. 

>> 
>>> As said, block-initializing with a repeated pattern is OK and I can
>see
>>> that being useful.  Trying to produce "nicer" values for floats,
>bools
>>> and pointers on 32bit platforms is IMHO not going to fix anything
>and
>>> introduce as many problems as it will "fix".
>>> 
>>> Yes, I agree, if we can find a good repeated pattern for all types’s
>
>>> pattern initialization, that will be much easier and simpler to 
>>> implement, I am happy to do that.  (Honestly, the part of
>implementation 
>>> that took me most of the time is pattern-initialization.. and I am
>still 
>>> not very comfortable with this part Of the code myself.  -:)
>> 
>> There's no "safe" pattern besides all-zero for all "undefined" uses
>> (note that uses do not necessarily use declared types).  Which is why
>> recommending pattern init is somewhat misguided.  There's maybe 
>> some useful pattern that more readily produces crashes, those that
>> produce a FP sNaN for all of the float types.
>
>So, pattern value as 0xFF might be better than 0xAA since 0xFFFFFFFF
>will be a NaN value for floating type?

I think for debugging NaNs are quite nice, yes. 

>> 
>>> And if you block-initialize stuff you then automagically cover
>padding.
>>> I call this a win-win, no?
>>> 
>>> Yes, this will also initialize paddings with patterns (Not zeroes as
>CLANG did).
>>> Shall we compatible with CLANG on this?
>> 
>> No, why?
>
>Okay.
>
>>> in my example code (untested) you then still need
>>> 
>>>  expand_assignment (var, ctor, false);
>>> 
>>> it would be the easiest way to try pattern init with a pattern
>that's
>>> bigger than a byte (otherwise of course the memset path is optimal).
>>> 
>>> If the pattern that is used to initialize all types is
>byte-repeatable, for example, 0xA or 0xF, then
>>> We can use memset to initialize all types, however, the potential
>problem is, if later we decide
>>> To change to another pattern that might not be byte-repeatable, then
>the memset implementation
>>> is not proper at that time.
>>> 
>>> Is it possible that we might change the pattern later?
>> 
>> The pattern should be documented as an implementation detail unless
>> we want to expose it to the user via, say, -fpattern-init=0xdeadbeef.
>
>Not sure whether it’s necessary to expose this to user.
>
>One question that is important to the implementation is:
>
>Shall we use “byte-repeated” or “word-repeated” pattern?
>Is “word-repeated” pattern better than “byte-repeated” pattern?
>
>For implementation, “byte-repeated” pattern will make the whole
>implementation much simpler since both “zero initialization” 
>and “pattern initialization” can be implemented with “memset” with
>different “value”.  
>
>So, if “word-repeated” pattern will not have too much more benefit, I
>will prefer “byte-repeated” pattern.
>
>Let me know your comments here.

I have no strong opinion and prefer byte repetition for simplicity. But I would document this as implementation detail that can change. 

Richard. 

>> 
>>> 
>>> 
>>> As said, for example glibc allocator hardening with MALLOC_PERTURB_
>>> uses simple byte-init.
>>> 
>>> What’s the pattern glibc used?
>> 
>> The value of the MALLOC_PERTURB_ environment truncated to a byte.
>
>Okay.
>
>thanks.
>
>Qing
>> 
>> Richard.
>>