Re: [PATCH bpf-next 5/7] selftests/bpf: Add netdevsim helper library

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Busy loop in `recv_device_ifindex()` when `recv()` returns 0
--

commit a319bb58b65c3ac30499b9e1c997fd27ae962cff
Author: Anton Protopopov <a.s.protopopov@gmail.com>

selftests/bpf: Add netdevsim helper library
    
This patch adds a minimal netdevsim helper library for BPF selftests,
handling the creation of netdevsim devices with one port and one queue.

> diff --git a/tools/testing/selftests/bpf/netdevsim_helpers.c b/tools/testing/selftests/bpf/netdevsim_helpers.c
> new file mode 100644
> index 0000000000000..fbed7442b2e8d
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/netdevsim_helpers.c
[ ... ]
> +static int recv_device_ifindex(int fd, unsigned int id, unsigned int *ifindex)
> +{
[ ... ]
> +	for (timeout = remaining_timeout_ms(&deadline); timeout > 0;
> +	     timeout = remaining_timeout_ms(&deadline)) {
> +		ret = poll(&pfd, 1, timeout);
> +		if (ret < 0) {
> +			if (errno == EINTR)
> +				continue;
> +			return -errno;
> +		}
> +		if (!ret)
> +			return -ETIMEDOUT;
> +		if (!(pfd.revents & POLLIN))
> +			return -EIO;
> +
> +		len = recv(fd, buf, sizeof(buf), 0);
> +		if (len < 0)
> +			return -errno;
> +
> +		for (nlh = (struct nlmsghdr *)buf; NLMSG_OK(nlh, len);

[Severity: Medium]
What happens when recv() returns 0 (e.g., on EOF)?

It looks like the len < 0 check is bypassed, and NLMSG_OK(nlh, 0) evaluates
to false. This means the code skips the message processing loop entirely.

Since the outer loop is driven by poll() which immediately returns POLLIN on
EOF, will this cause recv() to continually return 0?

Does this result in a tight busy-loop that consumes 100% CPU until the
5-second timeout expires?

> +		     nlh = NLMSG_NEXT(nlh, len)) {
> +			struct ifinfomsg *ifm;
> +			struct rtattr *attr;
[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260831110934.241898-1-a.s.protopopov@gmail.com?part=5