From: Rich Felker <dalias@libc.org>
To: Carlos O'Donell <codonell@redhat.com>
Cc: Florian Weimer <fw@deneb.enyo.de>, libc-alpha@sourceware.org
Subject: Re: [review] manual: Clarify strnlen, wcsnlen, strndup null termination behavior
Date: Thu, 28 Nov 2019 18:23:00 -0000 [thread overview]
Message-ID: <20191128182251.GM16318@brightrain.aerifal.cx> (raw)
In-Reply-To: <4312fc25-9b4c-9fab-25b1-495a618ab132@redhat.com>
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 âarrayâ, 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
next prev parent reply other threads:[~2019-11-28 18:23 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-30 10:25 Florian Weimer (Code Review)
2019-10-30 10:44 ` Andreas Schwab
2019-10-30 10:55 ` Florian Weimer
2019-10-30 11:00 ` Andreas Schwab
2019-10-30 11:03 ` Florian Weimer
2019-10-30 11:10 ` Andreas Schwab
2019-10-30 12:01 ` Zack Weinberg
2019-10-30 16:20 ` Andreas Schwab
2019-10-30 16:31 ` Zack Weinberg
2019-10-30 16:47 ` Andreas Schwab
2019-10-30 16:58 ` Zack Weinberg
2019-10-30 17:26 ` Andreas Schwab
2019-10-30 18:12 ` Zack Weinberg
2019-10-30 18:36 ` Florian Weimer
2019-10-30 17:24 ` Joseph Myers
2019-11-28 9:43 ` Florian Weimer
2019-11-28 15:56 ` Carlos O'Donell
2019-11-28 15:58 ` Carlos O'Donell
2019-11-28 18:23 ` Rich Felker [this message]
2019-11-28 18:38 ` Szabolcs Nagy
2019-11-29 18:20 ` Martin Sebor
2019-11-27 19:08 ` Carlos O'Donell (Code Review)
2019-11-27 19:14 ` Florian Weimer (Code Review)
2019-11-27 22:11 ` Carlos O'Donell (Code Review)
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