ssize_t: was: Re: [PATCH] of: Add printf '%pOFm' for generating modalias

[Petr Mladek]

On Tue 2024-12-17 12:37:09, Rob Herring (Arm) wrote:
> The callers for of_modalias() generally need the module alias as part of
> some larger string. That results in some error prone manipulation of the
> buffer prepend/append the module alias string. In fact,
> of_device_uevent_modalias() has several issues. First, it's off by one
> too few characters in utilization of the full buffer. Second, the error
> paths leave OF_MODALIAS with a truncated value when in the end nothing
> should be added to the buffer. It is also fragile because it needs
> internal details of struct kobj_uevent_env. add_uevent_var() really
> wants to write the env variable and value in one shot which would need
> either a temporary buffer for value or a format specifier.
> 
> Fix these issues by adding a new printf format specifier, "%pOFm". With
> the format specifier in place, simplify all the callers of
> of_modalias(). of_modalias() can also be simplified with vsprintf()
> being the only caller as it avoids the error conditions.
> 
> --- a/drivers/of/module.c
> +++ b/drivers/of/module.c
> @@ -8,21 +8,14 @@
>  #include <linux/slab.h>
>  #include <linux/string.h>
>  
> -ssize_t of_modalias(const struct device_node *np, char *str, ssize_t len)
> +/* Do not use directly, use %pOFm format specifier instead */
> +size_t of_modalias(const struct device_node *np, char *str, size_t len)

We should keep ssize_t.

"end - buf" passed from device_node_string() in vprintf.c might be
negative. The "buf" pointer is used to count the number of characters
which might be written when the buffer is big enough.

>  {
>  	const char *compat;
>  	char *c;
>  	struct property *p;
> -	ssize_t csize;
> -	ssize_t tsize;
> -
> -	/*
> -	 * Prevent a kernel oops in vsnprintf() -- it only allows passing a
> -	 * NULL ptr when the length is also 0. Also filter out the negative
> -	 * lengths...
> -	 */
> -	if ((len > 0 && !str) || len < 0)
> -		return -EINVAL;

There is later called

		csize = snprintf(str, len, "C%s", compat);

and snprintf() uses size_t for the len attribute. It would go wild
when we pass a negative len.


> +	size_t csize;
> +	size_t tsize;
>  
>  	/* Name & Type */
>  	/* %p eats all alphanum characters, so %c must be used here */

Best Regards,
Petr