Re: [RFC v5 6/7] sprintf: Add [v]sprintf_array()

[Alejandro Colomar <alx@kernel.org>]

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On Fri, Jul 11, 2025 at 01:23:56AM +0200, Alejandro Colomar wrote:
> Hi Linus,
> 
> [I'll reply to both of your emails at once]
> 
> On Thu, Jul 10, 2025 at 02:58:24PM -0700, Linus Torvalds wrote:
> > You took my suggestion, and then you messed it up.
> > 
> > Your version of sprintf_array() is broken. It evaluates 'a' twice.
> > Because unlike ARRAY_SIZE(), your broken ENDOF() macro evaluates the
> > argument.
> 
> An array has no issue being evaluated twice (unless it's a VLA).  On the
> other hand, I agree it's better to not do that in the first place.
> My bad for forgetting about it.  Sorry.
> 
> On Thu, Jul 10, 2025 at 03:08:29PM -0700, Linus Torvalds wrote:
> > If you want to return an error on truncation, do it right.  Not by
> > returning NULL, but by actually returning an error.
> 
> Okay.
> 
> > For example, in the kernel, we finally fixed 'strcpy()'. After about a
> > million different versions of 'copy a string' where every single
> > version was complete garbage, we ended up with 'strscpy()'. Yeah, the
> > name isn't lovely, but the *use* of it is:
> 
> I have implemented the same thing in shadow, called strtcpy() (T for
> truncation).  (With the difference that we read the string twice, since
> we don't care about threads.)
> 
> I also plan to propose standardization of that one in ISO C.
> 
> >  - it returns the length of the result for people who want it - which
> > is by far the most common thing people want
> 
> Agree.
> 
> >  - it returns an actual honest-to-goodness error code if something
> > overflowed, instead of the absoilutely horrible "source length" of the
> > string that strlcpy() does and which is fundamentally broken (because
> > it requires that you walk *past* the end of the source,
> > Christ-on-a-stick what a broken interface)
> 
> Agree.
> 
> >  - it can take an array as an argument (without the need for another
> > name - see my earlier argument about not making up new names by just
> > having generics)
> 
> We can't make the same thing with sprintf() variants because they're
> variadic, so you can't count the number of arguments.  And since the
> 'end' argument is of the same type as the formatted string, we can't
> do it with _Generic reliably either.
> 
> > Now, it has nasty naming (exactly the kind of 'add random character'
> > naming that I was arguing against), and that comes from so many
> > different broken versions until we hit on something that works.
> > 
> > strncpy is horrible garbage. strlcpy is even worse. strscpy actually
> > works and so far hasn't caused issues (there's a 'pad' version for the
> > very rare situation where you want 'strncpy-like' padding, but it
> > still guarantees NUL-termination, and still has a good return value).
> 
> Agree.
> 
> > Let's agree to *not* make horrible garbage when making up new versions
> > of sprintf.
> 
> Agree.  I indeed introduced the mistake accidentally in v4, after you
> complained of having too many functions, as I was introducing not one
> but two APIs: seprintf() and stprintf(), where seprintf() is what now
> we're calling sprintf_end(), and stprintf() we could call it
> sprintf_trunc().  So I did the mistake by trying to reduce the number of
> functions to just one, which is wrong.
> 
> So, maybe I should go back to those functions, and just give them good
> names.
> 
> What do you think of the following?
> 
> 	#define sprintf_array(a, ...)  sprintf_trunc(a, ARRAY_SIZE(a), __VA_ARGS__)
> 	#define vsprintf_array(a, ap)  vsprintf_trunc(a, ARRAY_SIZE(a), ap)
> 
> 	char *sprintf_end(char *p, const char end[0], const char *fmt, ...);
> 	char *vsprintf_end(char *p, const char end[0], const char *fmt, va_list args);
> 	int sprintf_trunc(char *buf, size_t size, const char *fmt, ...);
> 	int vsprintf_trunc(char *buf, size_t size, const char *fmt, va_list args);
> 
> 	char *sprintf_end(char *p, const char end[0], const char *fmt, ...)
> 	{
> 		va_list args;
> 
> 		va_start(args, fmt);
> 		p = vseprintf(p, end, fmt, args);

Typo here.  It's vsprintf_end().

> 		va_end(args);
> 
> 		return p;
> 	}
> 
> 	char *vsprintf_end(char *p, const char end[0], const char *fmt, va_list args)
> 	{
> 		int len;
> 
> 		if (unlikely(p == NULL))
> 			return NULL;
> 
> 		len = vsprintf_trunc(p, end - p, fmt, args);
> 		if (unlikely(len < 0))
> 			return NULL;
> 
> 		return p + len;
> 	}
> 
> 	int sprintf_trunc(char *buf, size_t size, const char *fmt, ...)
> 	{
> 		va_list args;
> 		int len;
> 
> 		va_start(args, fmt);
> 		len = vstprintf(buf, size, fmt, args);

Typo here.  It's vsprintf_trunc().

> 		va_end(args);
> 
> 		return len;
> 	}
> 
> 	int vsprintf_trunc(char *buf, size_t size, const char *fmt, va_list args)
> 	{
> 		int len;
> 
> 		if (WARN_ON_ONCE(size == 0 || size > INT_MAX))
> 			return -EOVERFLOW;
> 
> 		len = vsnprintf(buf, size, fmt, args);
> 		if (unlikely(len >= size))
> 			return -E2BIG;
> 
> 		return len;
> 	}
> 
> sprintf_trunc() is like strscpy(), but with a formatted string.  It
> could replace uses of s[c]nprintf() where there's a single call (no
> chained calls).
> 
> sprintf_array() is like the 2-argument version of strscpy().  It could
> replace s[c]nprintf() calls where there's no chained calls, where the
> input is an array.
> 
> sprintf_end() would replace the chained calls.
> 
> Does this sound good to you?
> 
> 
> Cheers,
> Alex
> 
> -- 
> <https://www.alejandro-colomar.es/>



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

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