Re: [PATCH v25 12/12] LRNG - add interface for gathering of raw entropy

Nicolai Stange
2019-11-21T12:18:17Z

Hi Stephan,

two general remarks on debugfs usage below

Stephan Müller <smueller@chronox.de> writes:

> diff --git a/drivers/char/lrng/lrng_testing.c b/drivers/char/lrng/lrng_testing.c
> new file mode 100644
> index 000000000000..5c33d3bd2172
> --- /dev/null
> +++ b/drivers/char/lrng/lrng_testing.c

<snip>

> +/*
> + * This data structure holds the dentry's of the debugfs files establishing
> + * the interface to user space.
> + */
> +struct lrng_raw_debugfs {
> + struct dentry *lrng_raw_debugfs_root; /* root dentry */
> + struct dentry *lrng_raw_debugfs_lrng_raw; /* .../lrng_raw */
> +};
> +
> +static struct lrng_raw_debugfs lrng_raw_debugfs;
> +
> +/* DebugFS operations and definition of the debugfs files */
> +static ssize_t lrng_raw_read(struct file *file, char __user *to,
> + size_t count, loff_t *ppos)
> +{
> + loff_t pos = *ppos;
> + int ret;
> +
> + if (!count)
> + return 0;
> + lrng_raw_entropy_init();
> + ret = lrng_raw_extract_user(to, count);
> + lrng_raw_entropy_fini();
> + if (ret < 0)
> + return ret;
> + count -= ret;
> + *ppos = pos + count;
> + return ret;
> +}
> +
> +/* Module init: allocate memory, register the debugfs files */
> +static int lrng_raw_debugfs_init(void)
> +{
> + lrng_raw_debugfs.lrng_raw_debugfs_root =
> + debugfs_create_dir(KBUILD_MODNAME, NULL);
> + if (IS_ERR(lrng_raw_debugfs.lrng_raw_debugfs_root)) {
> + lrng_raw_debugfs.lrng_raw_debugfs_root = NULL;
> + return PTR_ERR(lrng_raw_debugfs.lrng_raw_debugfs_root);
> + }

I think pointers returned by the debugfs API are not supposed to get
checked for NULL/IS_ERR(), c.f commit ff9fb72bc077 ("debugfs: return
error values, not NULL") or the the output from

git log --pretty=oneline | grep 'no need to check return value of debugfs_create'

(Also the above code is dubious: you're effectively returning
PTR_ERR(NULL)).

> + return 0;
> +}
> +
> +static struct file_operations lrng_raw_name_fops = {
> + .owner = THIS_MODULE,
> + .read = lrng_raw_read,
> +};
> +
> +static int lrng_raw_debugfs_init_name(void)
> +{
> + lrng_raw_debugfs.lrng_raw_debugfs_lrng_raw =
> + debugfs_create_file("lrng_raw", 0400,
> + lrng_raw_debugfs.lrng_raw_debugfs_root,
> + NULL, &lrng_raw_name_fops);q

CONFIG_LRNG_TESTING is a bool and thus, this debugfs file can't ever get
removed. Even if it could, this inode hasn't got any data associated
with it and so file removal would not be a problem for lrng_raw_read().

Please consider using debugfs_create_file_unsafe() instead to save
debugfs from kmalloc()ing a proxy file_operations protecting your fops
against concurrent file removal.

> + if (IS_ERR(lrng_raw_debugfs.lrng_raw_debugfs_lrng_raw)) {
> + lrng_raw_debugfs.lrng_raw_debugfs_lrng_raw = NULL;
> + return PTR_ERR(lrng_raw_debugfs.lrng_raw_debugfs_lrng_raw);
> + }

Same comment regarding return value checking applies here.

Thanks,

Nicolai

> + return 0;
> +}
> +
> +static int __init lrng_raw_init(void)
> +{
> + int ret = lrng_raw_debugfs_init();
> +
> + if (ret < 0)
> + return ret;
> +
> + ret = lrng_raw_debugfs_init_name();
> + if (ret < 0)
> + debugfs_remove_recursive(
> + lrng_raw_debugfs.lrng_raw_debugfs_root);
> +
> + return ret;
> +}
> +
> +static void __exit lrng_raw_exit(void)
> +{
> + debugfs_remove_recursive(lrng_raw_debugfs.lrng_raw_debugfs_root);
> +}
> +
> +module_init(lrng_raw_init);
> +module_exit(lrng_raw_exit);
> +
> +MODULE_LICENSE("Dual BSD/GPL");
> +MODULE_AUTHOR("Stephan Mueller <smueller@chronox.de>");
> +MODULE_DESCRIPTION("Kernel module for gathering raw entropy");

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