Re: [Minios-devel] [UNIKRAFT PATCH] lib/nolibc: Fix vsnprintf when size is zero

[Florian Schmidt <Florian.Schmidt@xxxxxxxxx>]

Hi Razvan,

[adding Wei as a heads-up, as the code came from Minios, and this bug 
most likely appears there, too]
after looking into this a bit more, making my own test case and 
debugging, I understand now what the problem is.
This is triggered by line 376 in lib/nolibc/stdio.c, where PCHAR is 
called with a postdecrement. However, the PCHAR macro does not evaluate 
its input if there is nothing to write any more, so the postdecrement is 
never executed, leading to the infinite loop.
The patch looks good, but could you write a bit more of a verbose commit 
message, so it's clear what was going on half a year from now? Basically 
that a macro should always evaluate its input variables exactly once, 
hence we need this temporary variable outside the if-clause, otherwise a 
postdecrement such as in line 376 might not happen if we're already 
outside the writable area of the output buffer and are only doing this 
to calculate the size we would've returned if there was enough space 
available, etc. etc. something like this.
Thanks!
Florian

On 08/17/2018 04:35 PM, Florian Schmidt wrote:
> Hi Razvan,
>
> On 08/16/2018 04:43 PM, Razvan Rotaru wrote:
> > Modified PCHAR macro so that it evaluates its parameter and updates
> > their values every time. Without this, when size is zero, vsnprintf
> > blocks in an infinite loop whenever the format string contains '%d'.
> I've had a look at your patch, but I'm not sure I exactly understand the 
> issue, and how your patch solves it. Can you provide a test case to make 
> this clearer?
> Thanks,
> Florian
--
Dr. Florian Schmidt
フローリアン・シュミット
Research Scientist,
Systems and Machine Learning Group
NEC Laboratories Europe
Kurfürsten-Anlage 36, D-69115 Heidelberg
Tel.     +49 (0)6221 4342-265
Fax:     +49 (0)6221 4342-155
e-mail:  florian.schmidt@xxxxxxxxx
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