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

[Stefano Brivio <sbrivio@redhat.com>]

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.

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!

-- 
Stefano