Re: [PATCH v4 next 09/23] tools/nolibc: Implement strerror() in terms of strerror_r()

[Willy Tarreau]

Hi David,

On Mon, Mar 02, 2026 at 10:18:01AM +0000, david.laight.linux@xxxxxxxxx wrote:
> From: David Laight <david.laight.linux@xxxxxxxxx>
> 
> strerror() can be the only part of a program that has a .data section.
> This requres 4k in the program file.

Thanks for handling this one! Indeed, I saw a trivial hello world program
take 4kB once %m got supported, which is a shame.

> Add a simple implementation of strerror_r() (ignores buflen) and use
> that in strerror() so that the "errno=" string is copied at run-time.
> Use __builtin_memcpy() because that optimises away the input string
> and just writes the required constants to the target buffer.
> 
> Ignoring buflen is unlikely to be a problem given that the output is
> always short.

On this point it's not necessarily true, as we can overflow too short
an output, e.g. when calling strerror_r() on a single-byte buffer:

> +static __attribute__((unused,))
> +int strerror_r(int errnum, char *buf, size_t buflen __attribute__((unused)))
> +{

Here I think we can simply do this to comply with the man page:

	if (buflen < 18) {
		errno = ERANGE;
		return -1;
	}

(and we can safely ignore it for strerror()).

> +	__builtin_memcpy(buf, "errno=", 6);
> +	return 6 + i64toa_r(errnum, buf + 6);
> +}
> +
(...)

Thanks,
Willy