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

[Florian Weimer]

* 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?