Re: [PATCH v2] pasta, passt-repair: Support multiple events per read() in inotify handlers

[Stefano Brivio <sbrivio@redhat.com>]

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):

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

-- 
Stefano