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 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().

> 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

> 
>  passt-repair.c | 26 +++++++++++++++++++-------
>  pasta.c        | 20 +++++++++++++-------
>  2 files changed, 32 insertions(+), 14 deletions(-)
> 
> diff --git a/passt-repair.c b/passt-repair.c
> index 120f7aa..86581c6 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) * 128]
> +		   __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 +140,21 @@ 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 + strlen(ev->name) -
> +					    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);
>  		if ((stat(path, &sb))) {
> diff --git a/pasta.c b/pasta.c
> index fa3e7de..017fa32 100644
> --- a/pasta.c
> +++ b/pasta.c
> @@ -498,17 +498,23 @@ void pasta_netns_quit_init(const struct ctx *c)
>   */
>  void pasta_netns_quit_inotify_handler(struct ctx *c, int inotify_fd)
>  {
> -	char buf[sizeof(struct inotify_event) + NAME_MAX + 1];
> -	const struct inotify_event *in_ev = (struct inotify_event *)buf;
> +	char buf[sizeof(struct inotify_event) + NAME_MAX + 1]
> +		__attribute__ ((aligned(__alignof__(struct inotify_event))));
> +	const struct inotify_event *ev;
> +	ssize_t n;
> +	char *p;
>  
> -	if (read(inotify_fd, buf, sizeof(buf)) < (ssize_t)sizeof(*in_ev))
> +	if ((n = read(inotify_fd, buf, sizeof(buf))) < (ssize_t)sizeof(*ev))
>  		return;
>  
> -	if (strncmp(in_ev->name, c->netns_base, sizeof(c->netns_base)))
> -		return;
> +	for (p = buf; p < buf + n; p += sizeof(*ev) + ev->len) {
> +		ev = (const struct inotify_event *)p;
>  
> -	info("Namespace %s is gone, exiting", c->netns_base);
> -	_exit(EXIT_SUCCESS);
> +		if (!strncmp(ev->name, c->netns_base, sizeof(c->netns_base))) {
> +			info("Namespace %s is gone, exiting", c->netns_base);
> +			_exit(EXIT_SUCCESS);
> +		}
> +	}
>  }
>  
>  /**

-- 
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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