On Tue, Jun 25, 2019 at 11:52:28PM -0400, Tom Lane wrote:
I'm seeing a reproducible bus error here:
#0 0x00417420 in statext_mcv_serialize (mcvlist=0x62223450, stats=Variable
"stats" is not available.
)
at mcv.c:785
785 memcpy(ITEM_BASE_FREQUENCY(item, ndims),
&mcvitem->base_frequency, sizeof(double));
What appears to be happening is that since ITEM_BASE_FREQUENCY is defined as
#define ITEM_BASE_FREQUENCY(item,ndims) ((double *) (ITEM_FREQUENCY(item,
ndims) + 1))
the compiler is assuming that the first argument to memcpy is
double-aligned, and it is generating code that depends on that being
true, and of course it isn't true and kaboom.
You can *not* cast something to an aligned pointer type if it's not
actually certain to be aligned suitably for that type. In this example,
even if you wrote "(char *)" in front of this, it wouldn't save you;
the compiler would still be entitled to believe that the intermediate
cast value meant something. The casts in the underlying macros
ITEM_FREQUENCY and so on are equally unsafe.
OK. So the solution is to ditch the casts altogether, and then do plain
pointer arithmetics like this:
#define ITEM_INDEXES(item) (item)
#define ITEM_NULLS(item,ndims) (ITEM_INDEXES(item) + (ndims))
#define ITEM_FREQUENCY(item,ndims) (ITEM_NULLS(item, ndims) + (ndims))
#define ITEM_BASE_FREQUENCY(item,ndims) (ITEM_FREQUENCY(item, ndims) +
sizeof(double))
Or is that still relying on alignment, somehow?
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
