Re: [PATCH v7 07/10] vsprintf: Consolidate handling of unknown pointer specifiers

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

On (04/17/19 13:53), Petr Mladek wrote:
> A reasonable compromise seems to be writing the unknown format specifier
> into the original string with a question mark, for example (%pC?).
> It should be self-explaining enough. Note that it is in brackets
> to follow the (null) style.

Hmm, seems that error string now sometimes try to `guess' what was the
error, but the guess can be misleading.

A very small example.
flags_string() can have a number of fmt specifiers - p, v, g.

        switch (fmt[1]) {
        case 'p':
                flags = *(unsigned long *)flags_ptr;
                /* Remove zone id */
                flags &= (1UL << NR_PAGEFLAGS) - 1;
                names = pageflag_names;
                break;
        case 'v':
                flags = *(unsigned long *)flags_ptr;
                names = vmaflag_names;
                break;
        case 'g':
                flags = *(gfp_t *)flags_ptr;
                names = gfpflag_names;
                break;
        default:
                WARN_ONCE(1, "Unsupported flags modifier: %c\n", fmt[1]);
                return buf;
        }

The new error message, however, will hint '%pG', which may or may not
be helpful.

> -char *flags_string(char *buf, char *end, void *flags_ptr, const char *fmt)
> +char *flags_string(char *buf, char *end, void *flags_ptr,
> +		   struct printf_spec spec, const char *fmt)
>  {
>  	unsigned long flags;
>  	const struct trace_print_flags *names;
> @@ -1760,8 +1767,7 @@ char *flags_string(char *buf, char *end, void *flags_ptr, const char *fmt)
>  		names = gfpflag_names;
>  		break;
>  	default:
> -		WARN_ONCE(1, "Unsupported flags modifier: %c\n", fmt[1]);
> -		return buf;
> +		return string_nocheck(buf, end, "(%pG?)", spec);
>  	}

Wouldn't it be better to use fmt[1] instead?

	-ss