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From: Tom Lane <[email protected]>
To: David Rowley <[email protected]>
Cc: PostgreSQL Developers <[email protected]>
Subject: Re: Small and unlikely overflow hazard in bms_next_member()
Date: Thu, 02 Apr 2026 00:22:48 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAApHDvq0T=iJ0Sf5TNE9yyWwfOeVjmrBt0wSywDnGD9Y4YJQBA@mail.gmail.com>
References: <CAApHDvq0T=iJ0Sf5TNE9yyWwfOeVjmrBt0wSywDnGD9Y4YJQBA@mail.gmail.com>

David Rowley <[email protected]> writes:
> When testing some ERROR code I added to ensure we don't get an
> excessively large left shift value and end up with members higher than
> INT32_MAX, I discovered that bms_next_member() can't handle that
> value, as "prevbit++" will wrap to INT32_MIN and then we'll try to
> access a negative array index, i.e. seg fault.

> I appreciate that such a large member is quite unlikely,

I think it's impossible, and if it's not then this is not the
only place in bitmapset.c that could theoretically overflow.
As an example, bms_prev_member does

	Assert(prevbit <= a->nwords * BITS_PER_BITMAPWORD);

but if the bitmapset were large enough to accommodate INT_MAX
as a member then a->nwords * BITS_PER_BITMAPWORD must overflow.

I don't think we should add cycles here for this purpose.
If it makes you feel better, maybe add Asserts to
bms_make_singleton and bms_add_member to constrain the
maximum member value to somewhat less than INT_MAX?

			regards, tom lane

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