From: "Stefan Kanthak" <stefan.kanthak@nexgo.de>
To: "Alejandro Colomar \(man-pages\)" <alx.manpages@gmail.com>
Cc: <linux-man@vger.kernel.org>, <mtk.manpages@gmail.com>
Subject: Re: wcstok(3) code sample
Date: Mon, 26 Jul 2021 15:13:29 +0200 [thread overview]
Message-ID: <400F0D31E91143D3A0DBA2335E4D18BB@H270> (raw)
In-Reply-To: <69a547a9-7d25-73bf-e1f4-7cfbfdcb79aa@gmail.com>
Hi Alex,
you wrote Sunday, July 25, 2021 10:25 PM:
> Hi Stefan,
>
> On 7/12/21 11:07 AM, Stefan Kanthak wrote:
>> Hi,
>>
>> the examples section of wcstok(3) shows the following code
>> which exhibits undefined behaviour and typically segfaults:
>>
>> <https://man7.org/linux/man-pages/man3/wcstok.3.html#EXAMPLES>
>>
>> | wchar_t *wcs = ...;
>> | wchar_t *token;
>> | wchar_t *state;
>> | for (token = wcstok(wcs, " \t\n", &state);
>> | token != NULL;
>> | token = wcstok(NULL, " \t\n", &state)) {
>> | ...
>> | }
>>
>> The string literal pointed to by wcs is read-only, and an
>> attempt to modify a string literal results in undefined
>> behaviour; wcstok() but writes NULs into its input string.
>>
>> FIX: replace the first line with either
>>
>> | wchar_t *wcs = strdup(...);
>>
>> or
>>
>> | wchar_t wcs[] = ...;
>
> That code is a bit unfortunate.
Yes.
> It is not a complete program, so it can be interpreted in different ways,
> one of them the one you said, which results in UB.
Correct: I interpret such snippets in their worst case and notice olny/first
their vulnerabilities or UB.
> I guess the intent of the code was that wcs was assigned a pointer to a
> wchar_t * (not a literal), and therefore, it would be correct.
Yes.
> The code predates version control, so we'll never know...
>
> Would you mind sending a complete example?
Just add a comment which tells that the string pointed to by wcs must not
be a literal (or read-only).
| wchar_t *wcs[] = ...; // must not point to a literal or read-only memory
regards
Stefan
prev parent reply other threads:[~2021-07-26 13:17 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-12 9:07 wcstok(3) code sample Stefan Kanthak
2021-07-12 16:36 ` Jakub Wilk
2021-07-25 20:25 ` Alejandro Colomar (man-pages)
2021-07-26 13:13 ` Stefan Kanthak [this message]
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