Re: [PATCH v2 17/23] bpf: selftests: introduce read_cgroup_file() helper

[bot+bpf-ci]

```
commit 712137918bba1b0a9c7c670bab9200b0f0d6a7e8
Author: Roman Gushchin <roman.gushchin@xxxxxxxxx>

bpf: selftests: introduce read_cgroup_file() helper

This patch adds a new helper function read_cgroup_file() to read from
cgroup control files in BPF selftests. The function opens a file,
reads into a buffer, and returns the result.

> diff --git a/tools/testing/selftests/bpf/cgroup_helpers.c b/tools/testing/selftests/bpf/cgroup_helpers.c
> index 20cede4db..8fb02fe4c 100644
> --- a/tools/testing/selftests/bpf/cgroup_helpers.c
> +++ b/tools/testing/selftests/bpf/cgroup_helpers.c
> @@ -126,6 +126,45 @@ int enable_controllers(const char *relative_path, const char *controllers)
>  	return __enable_controllers(cgroup_path, controllers);
>  }
>
> +static size_t __read_cgroup_file(const char *cgroup_path, const char *file,
> +				 char *buf, size_t size)
> +{
> +	char file_path[PATH_MAX + 1];
> +	size_t ret;
> +	int fd;
> +
> +	snprintf(file_path, sizeof(file_path), "%s/%s", cgroup_path, file);
> +	fd = open(file_path, O_RDONLY);
> +	if (fd < 0) {
> +		log_err("Opening %s", file_path);
> +		return -1;

Can __read_cgroup_file() return -1 when its return type is size_t?

The function signature declares size_t (unsigned), but returns -1 here.
When -1 is implicitly converted to size_t, it becomes SIZE_MAX
(18446744073709551615 on 64-bit systems). Callers checking for errors
with "if (ret < 0)" will fail because size_t is unsigned, and callers
checking "if (ret == -1)" will also fail after the cast.

> +	}
> +
> +	ret = read(fd, buf, size);
> +	close(fd);
> +	return ret;

Does this handle read() errors correctly?

The read() syscall returns ssize_t which can be negative on error.
Assigning this to size_t (line 143) converts negative error codes into
large positive numbers. For example, read() returning -1 becomes SIZE_MAX
when stored in ret.

Should the return type be ssize_t instead of size_t to properly propagate
both success (byte count) and error (negative values) to callers?

> +}
> +
> +/**
> + * read_cgroup_file() - Read to a cgroup file
> + * @relative_path: The cgroup path, relative to the workdir
> + * @file: The name of the file in cgroupfs to read to
> + * @buf: Buffer to read from the file
> + * @size: Size of the buffer
> + *
> + * Read to a file in the given cgroup's directory.
> + *
> + * If successful, the number of read bytes is returned.
> + */
> +size_t read_cgroup_file(const char *relative_path, const char *file,
> +			char *buf, size_t size)
> +{
> +	char cgroup_path[PATH_MAX - 24];
> +
> +	format_cgroup_path(cgroup_path, relative_path);
> +	return __read_cgroup_file(cgroup_path, file, buf, size);
> +}
> +

[ ... ]

> diff --git a/tools/testing/selftests/bpf/cgroup_helpers.h b/tools/testing/selftests/bpf/cgroup_helpers.h
> index 3857304be..9f9bb6b5d 100644
> --- a/tools/testing/selftests/bpf/cgroup_helpers.h
> +++ b/tools/testing/selftests/bpf/cgroup_helpers.h
> @@ -11,6 +11,8 @@
>
>  /* cgroupv2 related */
>  int enable_controllers(const char *relative_path, const char *controllers);
> +size_t read_cgroup_file(const char *relative_path, const char *file,
> +			char *buf, size_t size);
>  int write_cgroup_file(const char *relative_path, const char *file,
>  		      const char *buf);

[ ... ]


```

---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

In-Reply-To-Subject: `bpf: selftests: introduce read_cgroup_file() helper`
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/18859027430