Re: [review] manual: Clarify strnlen, wcsnlen, strndup null termination behavior

[Carlos O'Donell <codonell@redhat.com>]

On 11/28/19 4:43 AM, Florian Weimer wrote:
> * Florian Weimer:
> 
>> * Andreas Schwab:
>>
>>> On Okt 30 2019, Florian Weimer wrote:
>>>
>>>> * Andreas Schwab:
>>>>
>>>>> On Okt 30 2019, Florian Weimer (Code Review) wrote:
>>>>>
>>>>>> +Note that @var{s} must be an array of at least @var{maxlen} bytes.  It
>>>>>> +is undefined to call @code{strnlen} on a shorter array, even if it is
>>>>>> +known that the shorter array contains a null terminator.
>>>>>
>>>>> This is not true.  strnlen _always_ stops before the null byte.
>>>>
>>>> This is not how it is specified in POSIX.
>>>
>>> Yes, it is.
>>>
>>>     The strnlen() function shall return the number of bytes preceding
>>>     the first null byte in the array to which s points, if s contains a
>>>     null byte within the first maxlen bytes; otherwise, it shall return
>>>     maxlen.
>>>
>>> There is nothing undefined here.  Your interpretation would be
>>> completely useless anyway.
>>
>> It says “array”, which implies a length.  Admittedly, it does not say
>> that maxlen corresponds to the arrray length.  POSIX also says this:
>>
>> | The strnlen() function shall never examine more than maxlen bytes of
>> | the array pointed to by s.
>>
>> But it does NOT say that reading stops after the first null terminator.
> 
> I have built glibc with --disable-multi-arch and this patch on x86-64:
> 
> diff --git a/string/strnlen.c b/string/strnlen.c
> index 0b3a12e8b1..d5781dbb6f 100644
> --- a/string/strnlen.c
> +++ b/string/strnlen.c
> @@ -33,6 +33,10 @@
>  size_t
>  __strnlen (const char *str, size_t maxlen)
>  {
> +  /* Assert that the entire input is readable.  */
> +  for (size_t i = 0; i < maxlen; ++i)
> +    asm volatile ("" :: "r" (str[i]));
> +
>    const char *char_ptr, *end_ptr = str + maxlen;
>    const unsigned long int *longword_ptr;
>    unsigned long int longword, himagic, lomagic;
> diff --git a/sysdeps/x86_64/strnlen.S b/sysdeps/x86_64/strnlen.S
> deleted file mode 100644
> index d3c43ac482..0000000000
> --- a/sysdeps/x86_64/strnlen.S
> +++ /dev/null
> @@ -1,6 +0,0 @@
> -#define AS_STRNLEN
> -#define strlen __strnlen
> -#include "strlen.S"
> -
> -weak_alias (__strnlen, strnlen);
> -libc_hidden_builtin_def (strnlen)
> diff --git a/wcsmbs/wcsnlen.c b/wcsmbs/wcsnlen.c
> index 17e004dcc0..0d3709ac91 100644
> --- a/wcsmbs/wcsnlen.c
> +++ b/wcsmbs/wcsnlen.c
> @@ -26,6 +26,10 @@
>  size_t
>  __wcsnlen (const wchar_t *s, size_t maxlen)
>  {
> +  /* Assert that the entire input is readable.  */
> +  for (size_t i = 0; i < maxlen; ++i)
> +    asm volatile ("" :: "r" (s[i]));
> +
>    const wchar_t *ret = __wmemchr (s, L'\0', maxlen);
>    if (ret)
>      maxlen = ret - s;
> 
> The resulting crashes demonstrate that the test suite verifies that we
> do not treat the input as an array (to some degree; there might be
> scopes in coverage).
> 
> I think we should document this as a GNU extension.  Thoughts?

We should absolutely document this. It's an implementation-dependent detail
that we choose to interpret the standard in this way.

-- 
Cheers,
Carlos.