Re: [PATCH 30/32] strlen.3, wcslen.3: Add recommendations for safer variants

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Alex,

On 8/8/21 2:16 AM, Alejandro Colomar (man-pages) wrote:
> Hi, Michael!
> 
> On 8/8/21 1:45 AM, Michael Kerrisk (man-pages) wrote:
>> Hello Alex,
>>
>> I see there was a rather long mail thread that led
>> to this patch. The patch definitely deserves a commit
>> message.
>>
>> See also below.
>> On 7/28/21 10:20 PM, Alejandro Colomar wrote:
>>> Reported-by: Jonny Grant <jg@jguk.org>
>>> Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
>>> ---
>>>  man3/strlen.3 | 6 ++++++
>>>  man3/wcslen.3 | 9 ++++++++-
>>>  2 files changed, 14 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/man3/strlen.3 b/man3/strlen.3
>>> index dea4c1050..78783c446 100644
>>> --- a/man3/strlen.3
>>> +++ b/man3/strlen.3
>>> @@ -66,6 +66,12 @@ T}	Thread safety	MT-Safe
>>>  .sp 1
>>>  .SH CONFORMING TO
>>>  POSIX.1-2001, POSIX.1-2008, C89, C99, C11, SVr4, 4.3BSD.
>>> +.SH NOTES
>>> +.SS strnlen(3)
>>> +If the input buffer size is known,
>>> +it is probably better to use
>>> +.BR strnlen (3),
>>> +which can prevent reading past the end of the array.
>>
>> I hesitate slightly about this. strlen() is in the C standard, but
>> strnlen() is not. What do you think; do we need to care?
> 
> I have some doubts about this patch, but in a completely different sense:
> 
> I don't know if I'm being a bit paranoid in treating user input.  I've
> always been taught that I should program deffensively, but as time
> passes by, I realize myself that I was programming too much
> deffensively, and even introducing bugs in the error handling code.  And
> in many cases, strings will always be NUL-terminated, so maybe I'm just
> passing around a wrong recommendation.
> 
> What do you think about this?

How about a sentence something like:

[[
In cases where the input buffer may not contain a terminating null byte,
.BR strnlen (3)
should be used instead.
]]

What do you think?

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/