Re: [RFC PATCH] pasta, passt-repair: Support multiple events per read() in inotify handlers

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Mar 28, 2025 at 10:23:40AM +0100, Stefano Brivio wrote:
> On Fri, 28 Mar 2025 10:36:39 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Thu, Mar 27, 2025 at 10:28:22PM +0100, Stefano Brivio wrote:
> > > The current code assumes that we'll get one event per read() on
> > > inotify descriptors, but that's not the case, not from documentation,
> > > and not from reports.
> > > 
> > > Add loops in the two inotify handlers we have, in pasta-specific code
> > > and passt-repair, to go through all the events we receive.
> > > 
> > > While in pasta the the inotify watch is in an epoll list, and we'll  
> > 
> > "the the"
> > 
> > > get epoll wakeups as long as there's data to read, in passt-repair
> > > that's not the case. So, in pasta we can simply size the buffer for
> > > a single event and try to read one, but in passt-repair, we'll need
> > > to size the buffer to a safer, reasonable amount of events.  
> > 
> > I'm not following the reasoning here.  In passt-repair we're just
> > looping on the read() until we find what we want, so we'll still
> > eventually get what we want.  AFAICT it's a stream-like interface, so
> > we won't lose events just because we didn't get them in the first
> > read().
> 
> Under the assumption that message (event) boundaries are preserved
> across read() calls, yes.

Right.  My observation was that was the case, and inotify(7) also
seems to state so outright:

| Each successful read(2) returns a buffer containing one or more of the following structures:
|
|	struct inotify_event {
|		...
|	};


> And... this assumption actually holds (I had forgotten about that), so
> expanding the buffer as I did makes no sense, because by reading
> sizeof(struct inotify_event) + NAME_MAX + 1) bytes we always read one
> or more events, but never a partial buffer.
> 
> So, yes, never mind.
> 
> > > Link: https://bugs.passt.top/show_bug.cgi?id=119
> > > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>  
> > 
> > Otherwise, LGTM.
> > 
> > > ---
> > > I'm posting this as RFC because, while it seems to do the job and I
> > > tested all the code paths, Coverity isn't amused by the fact that
> > > we assume that inotify 'name' attributes (ev->name) are
> > > NULL-terminated. They actually are, but the code is not very robust.
> > > 
> > > Addressing that is kind of trivial but I keep getting it wrong, so
> > > I'll start posting this and fix that up later (direct fixes/edits of
> > > this patch are also welcome, of course).  
> > 
> > Ok, I might try to polish this up today
> 
> Thanks!
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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