Re: UB status of snprintf on invalid ptr+size combination?

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hello Vincent,

On 3/18/23 03:07, Vincent Lefevre wrote:
> On 2023-03-16 11:29:31 +0100, Stephan Bergmann wrote:
>> On 15/03/2023 13:39, Vincent Lefevre wrote:
>>> No, it is not obvious. If the C standard does not say that this is
>>> the size of the array, then it does not have to be the size of the
>>> array. The C standard just says:
>>>
>>>    Otherwise, output characters beyond the n-1st are discarded rather
>>>    than being written to the array, and a null character is written at
>>>    the end of the characters actually written into the array.
>>
>> But in 7.1.4 "Use of library functions" the standard also says
>>
>>> If a function argument is described as being an array, the pointer
>>> passed to the function shall have a value such that all address
>>> computations and accesses to objects (that would be valid if the
>>> pointer did point to the first element of such an array) are
>>> valid.
>>
>> which could be construed as meaning that the n-1st array element must always
>> be accessible, even if a given invocation is known to always generate less
>> then n output characters.
> 
> But the standard does not say that n is the size of the array.
> The size of the array could be the maximum of n and the size
> corresponding to the untruncated output string.

I guess you mean the minimum?  If it were the maximum, then it would
never truncate.

[assuming you meant minimum]:

As Andreas mentioned, that's valid for ISO C, but POSIX is more
restrictive.  Here's a quote from fprintf(3posix):

       The snprintf() function shall be equivalent to sprintf(),  with
       the  addition  of  the  n argument which states the size of the
       buffer referred to by s.

It clearly specifies that 'n' is the size of the buffer, so
implementations are free to assume that `s+n` is a valid pointer.

> 
> Similarly, for strncpy, I would not see n as the size of the arrays,
> i.e. it is not allowed for the implementation to read characters
> past a null character (possibly unless this does not have unwanted
> effects), even though such characters would be among the first n
> characters.

The size argument to strncpy(3) is the size of the destination buffer,
not the size of the input buffer.  The input buffer must be either
a string, or a character sequence at least as large as the destination
buffer.  Thus, in strncpy(3), reads are limited by
`strnlen(src, size)`, but writes are limited by `size`.

Cheers,
Alex

-- 
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