Re: [PATCH] Input: mouse: vsxxxaa: Replace strlcat() with strscpy()

[Alejandro Colomar]

Hi David,

> Date: 2026-11-05 20:53:21+0000
> From: David Laight <david.laight.linux@xxxxxxxxx>
>
> On Thu, 5 Nov 2026 12:49:59 +0100
> Alejandro Colomar <alx+linux-hardening@xxxxxxxxxx> wrote:
> 
> > > Personally I think that while there are issues with strlcat()
> > > (mostly to do with its return value), and that there are often
> > > much better (and faster - so sprintf is out) ways of getting
> > > the same result, the deprecation really needs to propose a
> > > sensible alternative (not inlined strlen() + strscpy()).  
> > 
> > One alternative would be to add strscat(), with the obvious internal
> > strlen(3).  This would be a drop-in replacement, and wouldn't have the
> > issues of strlcat(3) --mainly, the DoS vulnerability which is
> > consequence of the return value, and also the more difficult error
> > handling of doing '>=sizeof(...)' than '<0'--.
> 
> I've had thoughts of a more general function - but I've not tried to
> write it.
> 
> int str_append(char *dest, size_t dest_sz, size_t dest_off, const char *src, size_t src_max_len);
> 
> Where dest_sz is usually sizeof(*dest) and the header can substitute that for
>   a constant zero.
> dest_off would similarly default to strlen(dest), if -ve the function would
>   return that value immediately.
> src_max_len would be used for an effective strnlen(src, src_max_len).
> 
> The idea would be that calls like:
> 	char tgt[100];
> 	len = str_append(tgt, 0, strscpy(tgt, src_1), src_2, 0);
> would append src_1 and src_2 in tgt returning -ve on overflow.
> (Further chaining would then work.)
> 
> But, as I said, I've not tried to write it or see what it would look like in use.

I think it's too complex for what it does.  See how stpecpy() works:

	char  tgt[100];
	char  *p, *e;

	e = tgt + _Countof(tgt);
	p = tgt;
	p = stpecpy(p, e, src_1);
	p = stpecpy(p, e, src_2);
	p = stpecpy(p, e, src_3);
	p = stpecpy(p, e, src_4);
	p = stpecpy(p, e, src_5);
	...
	p = stpecpy(p, e, src_9);
	if (p == NULL)
		goto truncated;

	// 'p - tgt' is the length of the string.
	// In other words, 'p' points to the '\0'.

It copies all the strings into tgt, with truncation.  If any of the 5
calls resulted in truncation, it will be reported as a NULL.

You can also do the chain in a single line:

	p = stpecpy(stpecpy(stpecpy(p, e, src_1), e, src_2), e, src_3);

But IMO it reads better one per line.

Then, strscpy(9) is trivially implementable as:

	ssize_t
	strscpy(char *dst, const char *src, size_t dsize)
	{
		char  *p;

		BUG_ON(dsize == 0);
		p = stpecpy(dst, dst + dsize, src);
		return p ? p - dst : -E2BIG;
	}

And a hypothetical strscat() would be:

	ssize_t
	strscat(char *dst, const char *src, size_t dsize)
	{
		char  *p;

		BUG_ON(dsize == 0);
		p = stpecpy(dst + strlen(dst), dst + dsize, src);
		return p ? p - dst : -E2BIG;
	}

I have been working literally years on this API, and believe it has
reached perfection.  It includes some feedback I got from Doug McIlroy
about the design, which improved the usability and safety.


Have a lovely night!
Alex

> 
> David

-- 
<https://www.alejandro-colomar.es>

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