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To: Bruce Evans <bde@zeta.org.au>
In-Reply-To: Your message of "Mon, 16 Jun 2003 21:13:13 +1000."
             <20030616205631.F28116@gamplex.bde.org> 
Date: Tue, 17 Jun 2003 01:59:34 +0100
From: Ian Dowse <iedowse@maths.tcd.ie>
Message-ID: <200306170159.aa26127@salmon.maths.tcd.ie>
cc: Don Lewis <truckman@FreeBSD.org>
cc: iedowse@maths.tcd.ie
cc: freebsd-arch@FreeBSD.org
Subject: Re: Message buffer and printf reentrancy patch 
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In message <20030616205631.F28116@gamplex.bde.org>, Bruce Evans writes:
>On Mon, 16 Jun 2003, Don Lewis wrote:
>> It looks like MSGBUF_SEQNORM() could avoid the conditional code and any
>> questions about signed remainders if it was defined like this:
>>
>> #define MSGBUF_SEQNORM(mbp, seq) (((seq) + (mbp)->msg_seqmod) % \
>>     (mbp)->msg_seqmod)
>>
>> as long as msg_seqmod < INT_MAX/2.  MSGBUF_SEQNORM() could be simplified
>> further if msg_seqmod was added by the caller (such as MSGBUF_SEQSUB())
>> if the argument could be negative.
>
>Yes.  The negative numbers of interest seem to be limited to at most
>differences of sequence numbers (or maybe differeces of indexes, which
>are smaller), so they are larger than -msg_seqmod.  MSGBUF_SEQSUB()
>shouldn't add the bias, however,  since it is used in contexts where
>we really want to see the negative values.

The only minor problem I see with the above is that it is fragile
with respect to arbitrary input sequence numbers, in that it could
return a negative value. However, the property of guaranteeing to
return a normalised sequence number can be achieved by forcing an
unsigned division like in MSGBUF_SEQ_TO_POS, i.e.:

#define MSGBUF_SEQNORM(mbp, seq) ((int)((u_int)((seq) + \
    (mbp)->msg_seqmod) % (mbp)->msg_seqmod))
    
This should do the right thing for the expected ranges, but also
ensures that the macro itself can never return an out-of-range
sequence number, whatever the input value.

Ian