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From: Daniel Axtens <dja@axtens.net>
To: Michael Ellerman <mpe@ellerman.id.au>, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 1/2] powerpc/prom_init: Convert prom_strcpy() into
 prom_strscpy_pad()
In-Reply-To: <20210621064938.2021419-1-mpe@ellerman.id.au>
References: <20210621064938.2021419-1-mpe@ellerman.id.au>
Date: Mon, 21 Jun 2021 22:57:29 +1000
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hi

> -static char __init *prom_strcpy(char *dest, const char *src)
> +static ssize_t __init prom_strscpy_pad(char *dest, const char *src, size_t n)
>  {
> -	char *tmp = dest;
> +	ssize_t rc;
> +	size_t i;
>  
> -	while ((*dest++ = *src++) != '\0')
> -		/* nothing */;
> -	return tmp;
> +	if (n == 0 || n > INT_MAX)
> +		return -E2BIG;
> +
> +	// Copy up to n bytes
> +	for (i = 0; i < n && src[i] != '\0'; i++)
> +		dest[i] = src[i];
> +
> +	rc = i;
> +
> +	// If we copied all n then we have run out of space for the nul
> +	if (rc == n) {
> +		// Rewind by one character to ensure nul termination
> +		i--;
> +		rc = -E2BIG;
> +	}
> +
> +	for (; i < n; i++)
> +		dest[i] = '\0';
> +
> +	return rc;
>  }
>  

This implementation seems good to me.

I copied it into a new C file and added the following:

int main() {
	char longstr[255]="abcdefghijklmnopqrstuvwxyz";
	char shortstr[5];
	assert(prom_strscpy_pad(longstr, "", 0) == -E2BIG);
	assert(prom_strscpy_pad(longstr, "hello", 255) == 5);
	assert(prom_strscpy_pad(shortstr, "hello", 5) == -E2BIG);
	assert(memcmp(shortstr, "hell", 5) == 0);
	assert(memcmp(longstr, "hello\0\0\0\0\0\0\0\0\0", 6) == 0);
	return 0;
}

All the assertions pass. I believe this covers all the conditions from
the strscpy_pad docstring.

Reviewed-by: Daniel Axtens <dja@axtens.net>

Kind regards,
Daniel

>  static int __init prom_strncmp(const char *cs, const char *ct, size_t count)
> @@ -2701,7 +2719,7 @@ static void __init flatten_device_tree(void)
>  
>  	/* Add "phandle" in there, we'll need it */
>  	namep = make_room(&mem_start, &mem_end, 16, 1);
> -	prom_strcpy(namep, "phandle");
> +	prom_strscpy_pad(namep, "phandle", sizeof("phandle"));
>  	mem_start = (unsigned long)namep + prom_strlen(namep) + 1;
>  
>  	/* Build string array */
> -- 
> 2.25.1