Re: Making type Datum be 8 bytes everywhere

Em qua., 10 de set. de 2025 às 17:35, Tom Lane <tgl@sss.pgh.pa.us> escreveu:

> Tomas Vondra <tomas@vondra.me> writes:
> > While testing a different patch, I tried running with address sanitizer
> > on rpi5, running the 32-bit OS (which AFAIK is 64-bit kernel and 32-bit
> > user space). With that, stats_ext regression tests fail like this:
>
> > extended_stats.c:1082:27: runtime error: store to misaligned address
> > 0x036671dc for type 'Datum', which requires 8 byte alignment
> > 0x036671dc: note: pointer points here
> >   00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 7e
> > 7f  08 00 00 00 7f 7f 7f 7f
> >               ^
>
> > This happens because build_sorted_items() does palloc(), and then
> > accesses the pointer as array of structs, with a Datum field. And it
> > apparently expects the pointer to be a multiple of 8 bytes. Isn't that a
> > bit strange, with 32-bit user space? The pointer is indeed a multiple of
> > 4B, so maybe the expected alignment is wrong?
>
> I think build_sorted_items is plainly at fault here, where it does
>
>     /* Compute the total amount of memory we need (both items and values).
> */
>     len = data->numrows * sizeof(SortItem) + nvalues * (sizeof(Datum) +
> sizeof(bool));
>
>     /* Allocate the memory and split it into the pieces. */
>     ptr = palloc0(len);
>
>     /* items to sort */
>     items = (SortItem *) ptr;
>     ptr += data->numrows * sizeof(SortItem);
>
>     /* values and null flags */
>     values = (Datum *) ptr;
>     ptr += nvalues * sizeof(Datum);
>
> This is silently assuming that sizeof(SortItem) is a multiple of
> alignof(Datum), which on a 32-bit-pointer platform is not true
> any longer.  We ought to MAXALIGN the two occurrences of
> data->numrows * sizeof(SortItem).
>
We possibly have two more instances?

1. Function ndistinct_for_combination (src/backend/statistics/mvdistinct.c)
- items = (SortItem *) palloc(numrows * sizeof(SortItem));
+ items = (SortItem *) palloc(MAXALIGN(numrows * sizeof(SortItem)));

2. Function build_distinct_groups (src/backend/statistics/mcv.c)
- SortItem   *groups = (SortItem *) palloc(ngroups * sizeof(SortItem));
+ SortItem   *groups = (SortItem *) palloc(MAXALIGN(ngroups *
sizeof(SortItem)));

best regards,
Ranier Vilela