On Wed, Nov 27, 2013 at 9:10 AM, Noah Misch <noah@leadboat.com> wrote:
> The threat is that rounding the read size up to the next MAXALIGN would cross
> into an unreadable memory page, resulting in a SIGSEGV. Every palloc chunk
> has MAXALIGN'd size under the hood, so the excess read of "toDelete" cannot
> cause a SIGSEGV. For a stack variable, it depends on the ABI. I'm not aware
> of an ABI where the four bytes past the end of this stack variable could be
> unreadable, which is not to claim I'm well-read on the topic. We should fix
> this in due course on code hygiene grounds, but I would not back-patch it.
Attached patch silences the "Invalid read of size n" complaints of
Valgrind. I agree with your general thoughts around backpatching. Note
that the patch addresses a distinct complaint from Kevin's, as
Valgrind doesn't take issue with the invalid reads past the end of
spgxlogPickSplit variables on the stack.
--
Peter Geoghegan