Re: [PATCH v2] 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 03:29:10PM +0100, Stefano Brivio wrote:
> On Fri, 28 Mar 2025 11:39:58 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > From: Stefano Brivio <sbrivio@redhat.com>
> > 
> > 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.
> > 
> > Link: https://bugs.passt.top/show_bug.cgi?id=119
> > 
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > [dwg: Remove unnecessary buffer expansion, use strnlen instead of strlen
> >  to make Coverity happier]
> 
> I have to admit I didn't know about strnlen(). POSIX.1-2008, that's
> bleeding edge stuff. :)
> 
> That wasn't actually enough though: in my Coverity scan, it was also
> complaining about:
> 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  passt-repair.c | 27 ++++++++++++++++++++-------
> >  pasta.c        | 20 +++++++++++++-------
> >  2 files changed, 33 insertions(+), 14 deletions(-)
> > 
> > diff --git a/passt-repair.c b/passt-repair.c
> > index 120f7aa7..d36c9c94 100644
> > --- a/passt-repair.c
> > +++ b/passt-repair.c
> > @@ -111,14 +111,14 @@ int main(int argc, char **argv)
> >  	}
> >  
> >  	if ((sb.st_mode & S_IFMT) == S_IFDIR) {
> > -		char buf[sizeof(struct inotify_event) + NAME_MAX + 1];
> > +		char buf[sizeof(struct inotify_event) + NAME_MAX + 1]
> > +		   __attribute__ ((aligned(__alignof__(struct inotify_event))));
> >  		const struct inotify_event *ev;
> >  		char path[PATH_MAX + 1];
> > +		bool found = false;
> >  		ssize_t n;
> >  		int fd;
> >  
> > -		ev = (struct inotify_event *)buf;
> > -
> >  		if ((fd = inotify_init1(IN_CLOEXEC)) < 0) {
> >  			fprintf(stderr, "inotify_init1: %i\n", errno);
> >  			_exit(1);
> > @@ -130,6 +130,8 @@ int main(int argc, char **argv)
> >  		}
> >  
> >  		do {
> > +			char *p;
> > +
> >  			n = read(fd, buf, sizeof(buf));
> >  			if (n < 0) {
> >  				fprintf(stderr, "inotify read: %i", errno);
> > @@ -138,11 +140,22 @@ int main(int argc, char **argv)
> >  
> >  			if (n < (ssize_t)sizeof(*ev)) {
> >  				fprintf(stderr, "Short inotify read: %zi", n);
> > -				_exit(1);
> > +				continue;
> > +			}
> > +
> > +			for (p = buf; p < buf + n; p += sizeof(*ev) + ev->len) {
> > +				ev = (const struct inotify_event *)p;
> > +
> > +				if (ev->len >= REPAIR_EXT_LEN &&
> > +				    !memcmp(ev->name +
> > +					    strnlen(ev->name, ev->len) -
> > +					    REPAIR_EXT_LEN,
> > +					    REPAIR_EXT, REPAIR_EXT_LEN)) {
> > +					found = true;
> > +					break;
> > +				}
> >  			}
> > -		} while (ev->len < REPAIR_EXT_LEN ||
> > -			 memcmp(ev->name + strlen(ev->name) - REPAIR_EXT_LEN,
> > -				REPAIR_EXT, REPAIR_EXT_LEN));
> > +		} while (!found);
> >  
> >  		snprintf(path, sizeof(path), "%s/%s", argv[1], ev->name);
> 
> ...ev->name not being necessarily NULL-terminated and passed to
> snprintf() here. So I applied this with an additional (and useless, unless
> the kernel is unrealistically buggy):

Ah, yeah, I saw that error but wasn't sure how to fix it.

> 		if (ev->len > NAME_MAX + 1 || ev->name[ev->len] != '\0') {
> 			fprintf(stderr, "Invalid filename from inotify\n");
> 			_exit(1);
> 		}

That should do it.

-- 
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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