Date: Wed, 18 Jun 2003 01:18:59 +0100 From: Ian Dowse <iedowse@maths.tcd.ie> To: Don Lewis <truckman@FreeBSD.org> Cc: freebsd-arch@FreeBSD.org Subject: Re: Message buffer and printf reentrancy patch Message-ID: <200306180119.aa03806@salmon.maths.tcd.ie> In-Reply-To: Your message of "Mon, 16 Jun 2003 21:10:33 PDT." <200306170410.h5H4AXM7050537@gw.catspoiler.org>
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In message <200306170410.h5H4AXM7050537@gw.catspoiler.org>, Don Lewis writes: >Since MSGBUF_SEQSUB() calls MSGBUF_SEQNORM() on the difference between >the sequence numbers, a negative value will never be returned. If you >want a signed result, you'll probably want to do something more like: > tmp = MSGBUF_SEQNORM(mbp, (seq1) - (seq2) + (mbp)->seqmod); > return (tmp < ((mbp)->seqmod / 2)) ? tmp : (tmp - (mbp)->seqmod)); > >and you'll have to use a slightly different function if you are >comparing indexes. Oops, you're quite right - MSGBUF_SEQSUB was intended to return negative values, but got broken somewhere along the way. This appears not to affect the code that uses it, so I guess that means that the sequence numbers might as well be unsigned after all. >> 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.: > >Wouldn't it be better to have assertions to detect obviously bogus >sequence numbers rather than using them to generate a valid pointer to a >random location in the message buffer? It would if the assertion didn't trigger a panic that gets written to the message buffer via the same macros :-) Ian
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