Обсуждение: hash_create(nelem = 0) does invalid memory accesses

Hi,

debugging a citus valgrind bleat I noticed that hash_create() accesses
the result of palloc(0) as an hash element:
HTAB *
hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
{
...if ((flags & HASH_SHARED_MEM) ||    nelem < hctl->nelem_alloc){    if (!element_alloc(hashp, (int) nelem))
ereport(ERROR,               (errcode(ERRCODE_OUT_OF_MEMORY),                 errmsg("out of memory")));}
 
...}

I.e. e call element_alloc with nelem = 0. There we then do:
static bool
element_alloc(HTAB *hashp, int nelem)
{
...firstElement = (HASHELEMENT *) hashp->alloc(nelem * elementSize);
...firstElement->link = hctlv->freeList;
}

which means we'll write to the result of palloc(0).

Do we consider this an API usage error that we want to fix?

Greetings,

Andres Freund

Andres Freund <andres@anarazel.de> writes:
> debugging a citus valgrind bleat I noticed that hash_create() accesses
> the result of palloc(0) as an hash element:
> Do we consider this an API usage error that we want to fix?

I think Assert(nelem > 0) would be an appropriate response.
There are probably issues in sizing the hashtable quite aside
from this one.
        regards, tom lane