Re: [PATCH] lkdtm: use kmalloc() instead of __get_free_page

[David Laight <david.laight.linux@gmail.com>]

On Wed, 22 Jul 2026 23:02:46 +0000
Mahad Ibrahim <mahad.ibrahim.dev@gmail.com> wrote:

> lkdtm_debugfs_entry and direct_entry use __get_free_page to allocate a
> temporary buffer, perform copy_from_user to get the crashtype name,
> strim() to strip whitespace and find_crashtype to find the corresponding
> crashtype that is being requested.
> 
> The lkdtm_debugfs_read uses __get_free_page to allocate a temporary
> buffer to store all the available crashtypes, and then copy it to
> userspace.
> 
> The buffers that are allocated can be allocated with kmalloc as there is
> nothing special that requires a struct page, or the page allocator.
> 
> kmalloc() additionally provides a better API that doesn't require ugly
> casts which obfuscate the code and kfree does not need to know the size
> of the freed object.
> 
> Replace use of __get_free_page() with kmalloc().

None of those buffers need to be PAGE_SIZE.
Even the sanity limit for overlong requests could be 4k.
The longest 'crashtype->name' is probably about 32 characters, so could
probably go on stack.

And can't this code use the sysfs/kernfs wrappers?

	David

> 
> Signed-off-by: Mahad Ibrahim <mahad.ibrahim.dev@gmail.com>
> ---
>  drivers/misc/lkdtm/core.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index ededa32d6744..01bebcb33bd4 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -236,11 +236,11 @@ static ssize_t lkdtm_debugfs_entry(struct file *f,
>  	if (count >= PAGE_SIZE)
>  		return -EINVAL;
>  
> -	buf = (char *)__get_free_page(GFP_KERNEL);
> +	buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
>  	if (!buf)
>  		return -ENOMEM;
>  	if (copy_from_user(buf, user_buf, count)) {
> -		free_page((unsigned long) buf);
> +		kfree(buf);
>  		return -EFAULT;
>  	}
>  	/* NULL-terminate and remove enter */
> @@ -248,7 +248,7 @@ static ssize_t lkdtm_debugfs_entry(struct file *f,
>  	strim(buf);
>  
>  	crashtype = find_crashtype(buf);
> -	free_page((unsigned long)buf);
> +	kfree(buf);
>  
>  	if (!crashtype)
>  		return -EINVAL;
> @@ -271,7 +271,7 @@ static ssize_t lkdtm_debugfs_read(struct file *f, char __user *user_buf,
>  	ssize_t out;
>  	char *buf;
>  
> -	buf = (char *)__get_free_page(GFP_KERNEL);
> +	buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
>  	if (buf == NULL)
>  		return -ENOMEM;
>  
> @@ -290,7 +290,7 @@ static ssize_t lkdtm_debugfs_read(struct file *f, char __user *user_buf,
>  
>  	out = simple_read_from_buffer(user_buf, count, off,
>  				      buf, n);
> -	free_page((unsigned long) buf);
> +	kfree(buf);
>  
>  	return out;
>  }
> @@ -313,11 +313,11 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf,
>  	if (count < 1)
>  		return -EINVAL;
>  
> -	buf = (char *)__get_free_page(GFP_KERNEL);
> +	buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
>  	if (!buf)
>  		return -ENOMEM;
>  	if (copy_from_user(buf, user_buf, count)) {
> -		free_page((unsigned long) buf);
> +		kfree(buf);
>  		return -EFAULT;
>  	}
>  	/* NULL-terminate and remove enter */
> @@ -325,7 +325,7 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf,
>  	strim(buf);
>  
>  	crashtype = find_crashtype(buf);
> -	free_page((unsigned long) buf);
> +	kfree(buf);
>  	if (!crashtype)
>  		return -EINVAL;
>