kern/163076: It is not possible to read in chunks from linprocfs and procfs.

[Petr Salinger]

The following reply was made to PR kern/163076; it has been noted by GNATS.

From: Petr Salinger <Petr.Salinger at seznam.cz>
To: mdf at FreeBSD.org
Cc: Jaakko Heinonen <jh at FreeBSD.org>,
        =?ISO-8859-15?Q?Dag-Erling_Sm=F8rgrav?= <des at des.no>,
        Poul-Henning Kamp <phk at phk.freebsd.dk>, bug-followup at FreeBSD.org
Subject: Re: kern/163076: It is not possible to read in chunks from linprocfs
 and procfs.
Date: Fri, 9 Dec 2011 17:18:00 +0100 (CET)

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 >> As I wrote existing code depends on sbuf_len() to return the actual
 >> length regardless of the error status after sbuf_finish(). I am not
 >> willing to through all code using sbufs to check where it causes
 >> problems. phk@ asserts that r222004 is correct.
 >
 > What is causing sbuf to have an error in the first place?  The size of
 > flies in /proc are generally small and malloc(3) errors are rare.
 
 In this particular case, the userland wants to read first n bytes,
 the sbuf is allocated as fixed for n bytes.
 The first n bytes are generated and correctly stored in sbuf,
 for the rest bytes, there is no space in sbuf, but these bytes are not 
 needed later anyway.
 
 In stable-8,
 sbuf_data(sb) point to data, the sbuf_len(sb) returns number of stored 
 bytes.
 
 In stable-9,
 sbuf_data(sb) point to data, the sbuf_len(sb) returns -1.
 
 Petr