Re: [Tarantool-patches] [PATCH luajit 1/2] build: introduce LUAJIT_USE_UBSAN option

[Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Hi, Sergey,

On 16.05.2024 13:14, Sergey Kaplun wrote:
> Hi, folks!
> Some more thoughts below.
>
> On 15.05.24, Sergey Kaplun wrote:
>
> <snipped>
>
>> +  string(JOIN "," UBSAN_IGNORE_OPTIONS
>> +    # Misaligned pseudo-pointers are used to determine internal
>> +    # variable names inside the `for` cycle.
>> +    alignment
>> +    # Not interested in float cast overflow errors.
>> +    float-cast-overflow
>> +    # NULL checking is disabled because this is not a UB and
>> +    # raises lots of false-positive fails.
>> +    null
> Maybe it is worth to add also "nonnull-attribute" to the ignore options:
>
> ```
> LSAN_OPTIONS="abort_on_error=1" src/luajit -e 'error("bad usage", 3)'
> /home/burii/builds_workspace/luajit/gh-8473-ubsan/src/lj_buf.h:75:25: runtime error: null pointer passed as argument 1, which is declared to never be null
> /usr/include/string.h:44:28: note: nonnull attribute specified here
> ```
>
> Here, `memcpy()` gets the NULL pointer as the first argument and the
> `len` == 0. So there are no problems here. Also, the nullability
> violation is not a UB, as mentioned in the documentation.
>
> Thoughts?

I agree with arguments provided by Maxim - it's UB, the standard states 
it clearly,

and it should be fixed. I propose to do the following:

- suppress the check (add it to UBSAN_IGNORE_OPTIONS) with appropriate 
comment

(something like "we know it is bad, but it is unfixed in upstream, we 
will wait a fix").

Probably we should suppress it per file like you did with other checks.

- send a fix to upstream or submit an issue to the upstream (I'll not 
insist, but it would be desirable)

>
>> +    # Not interested in checking arithmetic with NULL.
>> +    pointer-overflow
>> +    # Shifts of negative numbers are widely used in parsing ULEB,
>> +    # cdata arithmetic, vmevent hash calculation, etc.
>> +    shift-base
>> +  )

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