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From: Tom Lane <[email protected]>
To: Michael Paquier <[email protected]>
Cc: Xing Guo <[email protected]>
Cc: [email protected]
Subject: Re: Fix an incorrect assertion condition in mdwritev().
Date: Sat, 25 May 2024 23:59:05 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <CACpMh+BM-VgKeO7suPG-VHTtpzJ+zsbDPwVHu42PLp-iTk0z+A@mail.gmail.com>
	<[email protected]>

Michael Paquier <[email protected]> writes:
> On Sat, May 25, 2024 at 11:52:22PM +0800, Xing Guo wrote:
>> #ifdef CHECK_WRITE_VS_EXTEND
>> -	Assert(blocknum < mdnblocks(reln, forknum));
>> +	Assert(blocknum + nblocks <= mdnblocks(reln, forknum));
>> #endif

> Yes, it looks like you're right that this can be made stricter,
> computing the number of blocks we're adding in the number calculated
> (aka adding one block to this number fails immediately at initdb).

Hmm ... I agree that this is better normally.  But there's an
edge case where it would fail to notice a problem that the
existing code does notice: if blocknum is close to UINT32_MAX
and adding nblocks causes it to wrap around to a small value.
Is there an inexpensive way to catch that?  (If not, it's
not a reason to block this patch; but let's think about it
while we're here.)

			regards, tom lane

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