linux-kernel - Re: [PATCH v5 1/3] lib/ucs2_string: Add UCS-2 strscpy function

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Date:   Fri, 4 Aug 2023 21:23:43 +0200
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Bjorn Andersson <andersson@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Johan Hovold <johan@...nel.org>,
        Steev Klimaszewski <steev@...i.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/3] lib/ucs2_string: Add UCS-2 strscpy function

On 8/4/23 10:18, Kees Cook wrote:
> On Sun, Jul 30, 2023 at 06:19:02PM +0200, Maximilian Luz wrote:
>> Add a ucs2_strscpy() function for UCS-2 strings. The behavior is
>> equivalent to the standard strscpy() function, just for 16-bit character
>> UCS-2 strings.
>>
>> Signed-off-by: Maximilian Luz <luzmaximilian@...il.com>
>> ---
>>
>> Changes in v5:
>>   - Add ucs2_strscpy() instead of ucs2_strlcpy()
>>
>> Patch introduced in v4.
>>
>> ---
>>   include/linux/ucs2_string.h |  1 +
>>   lib/ucs2_string.c           | 35 +++++++++++++++++++++++++++++++++++
>>   2 files changed, 36 insertions(+)
>>
>> diff --git a/include/linux/ucs2_string.h b/include/linux/ucs2_string.h
>> index cf3ada3e820e..c499ae809c7d 100644
>> --- a/include/linux/ucs2_string.h
>> +++ b/include/linux/ucs2_string.h
>> @@ -10,6 +10,7 @@ typedef u16 ucs2_char_t;
>>   unsigned long ucs2_strnlen(const ucs2_char_t *s, size_t maxlength);
>>   unsigned long ucs2_strlen(const ucs2_char_t *s);
>>   unsigned long ucs2_strsize(const ucs2_char_t *data, unsigned long maxlength);
>> +ssize_t ucs2_strscpy(ucs2_char_t *dst, const ucs2_char_t *src, size_t count);
>>   int ucs2_strncmp(const ucs2_char_t *a, const ucs2_char_t *b, size_t len);
>>   
>>   unsigned long ucs2_utf8size(const ucs2_char_t *src);
>> diff --git a/lib/ucs2_string.c b/lib/ucs2_string.c
>> index 0a559a42359b..b608129fcbdc 100644
>> --- a/lib/ucs2_string.c
>> +++ b/lib/ucs2_string.c
>> @@ -32,6 +32,41 @@ ucs2_strsize(const ucs2_char_t *data, unsigned long maxlength)
>>   }
>>   EXPORT_SYMBOL(ucs2_strsize);
>>   
>> +ssize_t ucs2_strscpy(ucs2_char_t *dst, const ucs2_char_t *src, size_t count)
>> +{
>> +	long res;
>> +
>> +	/*
>> +	 * Ensure that we have a valid amount of space. We need to store at
>> +	 * least one NUL-character.
>> +	 */
>> +	if (count == 0 || WARN_ON_ONCE(count > INT_MAX))
> 
> Is "count" a measure of bytes or characters? It seems to be characters.
> can you please add some kern-doc for this function to clarify this.
> Also, I wonder if the above check should be "count > INT_MAX / 2" since
> the INT_MAX is, generally, done in byte counts.

Count is a measure of characters. I'll add a doc-comment.

Regarding INT_MAX / 2: I'm fine with either. I'd change it to
INT_MAX / sizeof(*dst) if you say it's generally enforced in bytes.

>> +		return -E2BIG;
>> +
>> +	/*
>> +	 * Copy at most 'count' bytes, return early if we find a
> 
> If "count" is characters, this comment should not say "bytes". :)

Correct. Will fix this.
  
>> +	 * NUL-terminator.
>> +	 */
>> +	for (res = 0; res < count; res++) {
>> +		ucs2_char_t c;
>> +
>> +		c = src[res];
>> +		dst[res] = c;
>> +
>> +		if (!c)
>> +			return res;
>> +	}
>> +
>> +	/*
>> +	 * The loop above terminated without finding a NUL-terminator,
>> +	 * exceeding the 'count': Enforce proper NUL-termination and return
>> +	 * error.
>> +	 */
>> +	dst[count - 1] = 0;
>> +	return -E2BIG;
>> +}
>> +EXPORT_SYMBOL(ucs2_strscpy);
>> +
>>   int
>>   ucs2_strncmp(const ucs2_char_t *a, const ucs2_char_t *b, size_t len)
>>   {
>> -- 
>> 2.41.0
>>
> 
> Otherwise looks good to me!

Thanks!

Regards
Max