Subject: Re: [PATCH v2 3/3] lib/string_helpers.c: Change semantics of string_escape_mem

From: Rasmus Villemoes <[email-masked]>
Date: Thu, 29 Jan 2015 15:29:21 +0100

On Thu, Jan 29 2015, Andy Shevchenko <[email-masked]> wrote:

>>   *
>>   * Return:
>> - * The amount of the characters processed to the destination buffer, or
>> - * %-ENOMEM if the size of buffer is not enough to put an escaped character is
>> - * returned.
>> - *
>> - * Even in the case of error @dst pointer will be updated to point to the byte
>> - * after the last processed character.
>> + * The total size of the escaped output that would be generated for
>> + * the given input and flags. To check whether the output was
>> + * truncated, compare the return value to osz. There is room left in
>> + * dst for a '\0' terminator if and only if ret < osz.
>>   */
>> -int string_escape_mem(const char *src, size_t isz, char **dst, size_t osz,
>> -		      unsigned int flags, const char *esc)
>> +size_t string_escape_mem(const char *src, size_t isz, char *dst, size_t osz,
>> +			 unsigned int flags, const char *esc)
>
> I prefer to leave the prototype the same. int for return is okay. dst
> should be updated accordingly.

Please explain exactly what you think the return value should be, and
what *dst should be set to.

>>  {
>> -	char *p = *dst;
>> +	char *p = dst;
>>  	char *end = p + osz;
>>  	bool is_dict = esc && *esc;
>> -	int ret;
>>  
>>  	while (isz--) {
>>  		unsigned char c = *src++;
>> @@ -466,13 +463,7 @@ int string_escape_mem(const char *src, size_t isz, char **dst, size_t osz,
>>  
>>  		escape_passthrough(c, &p, end);
>>  	}
>> -	if (p > end) {
>> -		*dst = end;
>> -		return -ENOMEM;
>> -	}
>>  
>> -	ret = p - *dst;
>> -	*dst = p;
>> -	return ret;
>> +	return p - dst;
>>  }
>>  EXPORT_SYMBOL(string_escape_mem);
>> diff --git a/lib/test-string_helpers.c b/lib/test-string_helpers.c
>> index ab0d30e1e18f..5f95114a2f86 100644
>> --- a/lib/test-string_helpers.c
>> +++ b/lib/test-string_helpers.c
>> @@ -264,12 +264,12 @@ static __init void test_string_escape(const char *name,
>>  				      const struct test_string_2 *s2,
>>  				      unsigned int flags, const char *esc)
>>  {
>> -	int q_real = 512;
>> -	char *out_test = kmalloc(q_real, GFP_KERNEL);
>> -	char *out_real = kmalloc(q_real, GFP_KERNEL);
>> +	size_t out_size = 512;
>> +	char *out_test = kmalloc(out_size, GFP_KERNEL);
>> +	char *out_real = kmalloc(out_size, GFP_KERNEL);
>>  	char *in = kmalloc(256, GFP_KERNEL);
>> -	char *buf = out_real;
>> -	int p = 0, q_test = 0;
>> +	size_t p = 0, q_test = 0;
>> +	size_t q_real;
>>  
>>  	if (!out_test || !out_real || !in)
>>  		goto out;
>> @@ -301,29 +301,26 @@ static __init void test_string_escape(const char *name,
>>  		q_test += len;
>>  	}
>>  
>> -	q_real = string_escape_mem(in, p, &buf, q_real, flags, esc);
>> +	q_real = string_escape_mem(in, p, out_real, out_size, flags, esc);
>>  
>>  	test_string_check_buf(name, flags, in, p, out_real, q_real, out_test,
>>  			      q_test);
>> +
>> +	memset(out_real, 'Z', out_size);
>> +	q_real = string_escape_mem(in, p, out_real, 0, flags, esc);
>> +	if (q_real != q_test)
>> +		pr_warn("Test '%s' failed: flags = %u, osz = 0, expected %zu, got %zu\n",
>> +			name, flags, q_test, q_real);
>> +	if (memchr_inv(out_real, 'Z', out_size))
>> +		pr_warn("Test '%s' failed: osz = 0 but string_escape_mem wrote to the buffer\n",
>> +			name);
>
> Could it be a part of nomem test still?

What nomem test? string_escape_mem with snprintf-like semantics cannot
return an error; that has to be checked by the caller. 

>> +
>>  out:
>>  	kfree(in);
>>  	kfree(out_real);
>>  	kfree(out_test);
>>  }
>>  
>> -static __init void test_string_escape_nomem(void)
>> -{
>> -	char *in = "\eb \\C\007\"\x90\r]";
>> -	char out[64], *buf = out;
>> -	int rc = -ENOMEM, ret;
>> -
>> -	ret = string_escape_str_any_np(in, &buf, strlen(in), NULL);
>> -	if (ret == rc)
>> -		return;
>> -
>> -	pr_err("Test 'escape nomem' failed: got %d instead of %d\n", ret, rc);
>> -}
>> -
>>  static int __init test_string_helpers_init(void)
>>  {
>>  	unsigned int i;
>> @@ -342,8 +339,6 @@ static int __init test_string_helpers_init(void)
>>  	for (i = 0; i < (ESCAPE_ANY_NP | ESCAPE_HEX) + 1; i++)
>>  		test_string_escape("escape 1", escape1, i, TEST_STRING_2_DICT_1);
>>  
>> -	test_string_escape_nomem();
>> -
>>  	return -EINVAL;
>>  }
>>  module_init(test_string_helpers_init);
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>> index 3568e3906777..d02c394b5b58 100644
>> --- a/lib/vsprintf.c
>> +++ b/lib/vsprintf.c
>> @@ -1160,7 +1160,7 @@ char *escaped_string(char *buf, char *end, u8 *addr, struct printf_spec spec,
>>  	len = spec.field_width < 0 ? 1 : spec.field_width;
>>  
>>  	/* Ignore the error. We print as many characters as we can */
>> -	string_escape_mem(addr, len, &buf, end - buf, flags, NULL);
>> +	buf += string_escape_mem(addr, len, buf, buf < end ? end - buf : 0, flags, NULL);
>
> So, the problem is when we have end < buf, right?
> How about to move this check out of the call parameters?
>
> [Keep in might the original prototype]
>
> if (buf < end)
>  string_escape_mem(addr, len, &buf, end - buf, flags, NULL);
> else
>  string_escape_mem(addr, len, &buf, 0, flags, NULL);

In that case, I just did the same as is done for %pV, and prefer to keep
it that way.

>> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
>> index 33fb105d4352..22c4418057f4 100644
>> --- a/net/sunrpc/cache.c
>> +++ b/net/sunrpc/cache.c
>> @@ -1068,12 +1068,14 @@ void qword_add(char **bpp, int *lp, char *str)
>>  {
>>  	char *bp = *bpp;
>>  	int len = *lp;
>> -	int ret;
>> +	int ret, written;
>>  
>>  	if (len < 0) return;
>>  
>> -	ret = string_escape_str(str, &bp, len, ESCAPE_OCTAL, "\\ \n\t");
>> -	if (ret < 0 || ret == len)
>> +	ret = string_escape_str(str, bp, len, ESCAPE_OCTAL, "\\ \n\t");
>> +	written = min(ret, len);
>> +	bp += written;
>> +	if (ret >= len)
>>  		len = -1;
>>  	else {
>>  		len -= ret;
>
> For this part the comment from J. Bruce is needed.
>
> There is one more user, i.e. fs/proc/array.c::task_name().
>
> In all of them we have to amend a prepend commentary. Like changing
> 'Ignore the error' to 'Ignore the overflow'.

I hadn't looked for users in -next. I'll leave it to you to amend that
patch before it hits mainline.

Rasmus