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

[Maxim Kokryashkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Hi, Sergey!
See my thoughts below.

On Thu, May 16, 2024 at 01:14:14PM UTC, 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.

It is UB, though: https://en.cppreference.com/w/cpp/string/byte/memcpy
Even with the zero len it may still cause issues, so I don't think we
should disable this check.

> Thoughts?
>
> > +    # 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
> > +  )
>
> --
> Best regards,
> Sergey Kaplun