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

[Rich Felker <dalias at libc dot org>]

On Thu, Nov 28, 2019 at 10:58:13AM -0500, Carlos O'Donell wrote:
> On 11/28/19 10:56 AM, Carlos O'Donell wrote:
> > 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.
> > 
> 
> I also think we should get changes into the linux man page project to call
> this out so that nobody thinks about changing this again and so the
> implementation is clear.
> 
> Have we asked Rich what musl does and what he thinks on the topic?

I missed this whole thread, and haven't had time to look back through
it yet. Is the claim that strnlen, etc. require a pointer to at least
n bytes? I do not think that matches the intent of these interfaces at
all. The language in POSIX is sloppy ("the number of bytes in the
array to which s points"?! I think they were just trying to avoid
saying "string" here because it's not necessarily a string, but they
botched it) but a function like this that requires a large array is
utterly useless. The whole point of strnlen is to be a bounded-time
strlen when lengths >n will be treated as errors (or otherwise
specially) after it returns.

Rich