Subject: Re: [PATCH v7 1/3] modules: Support extended MODVERSIONS info

From: Michael Ellerman <[email-masked]>
Date: Fri, 25 Oct 2024 22:14:06 +1100

Matthew Maurer <[email-masked]> writes:
> Adds a new format for MODVERSIONS which stores each field in a separate
> ELF section. This initially adds support for variable length names, but
> could later be used to add additional fields to MODVERSIONS in a
> backwards compatible way if needed. Any new fields will be ignored by
> old user tooling, unlike the current format where user tooling cannot
> tolerate adjustments to the format (for example making the name field
> longer).
>
> Since PPC munges its version records to strip leading dots, we reproduce
> the munging for the new format. Other architectures do not appear to
> have architecture-specific usage of this information.
>
> Signed-off-by: Matthew Maurer <[email-masked]>
> ---
>  arch/powerpc/kernel/module_64.c | 24 ++++++++++-
>  kernel/module/internal.h        | 11 +++++
>  kernel/module/main.c            | 92 +++++++++++++++++++++++++++++++++++++----
>  kernel/module/version.c         | 45 ++++++++++++++++++++
>  4 files changed, 162 insertions(+), 10 deletions(-)
>
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index e9bab599d0c2745e4d2b5cae04f2c56395c24654..02ada0b057cef6b2f29fa7519a5d52acac740ee5 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -355,6 +355,24 @@ static void dedotify_versions(struct modversion_info *vers,
>  		}
>  }
>  
> +/* Same as normal versions, remove a leading dot if present. */
> +static void dedotify_ext_version_names(char *str_seq, unsigned long size)
> +{
> +	unsigned long out = 0;
> +	unsigned long in;
> +	char last = '\0';
> +
> +	for (in = 0; in < size; in++) {
> +		/* Skip one leading dot */
> +		if (last == '\0' && str_seq[in] == '.')
> +			in++;
> +		last = str_seq[in];
> +		str_seq[out++] = last;
> +	}
> +	/* Zero the trailing portion of the names table for robustness */
> +	memset(&str_seq[out], 0, size - out);
> +}

Sorry I realise it's version 7, but although the above looks correct it's
kind of dense.

I think the below would also work and is (I think) easier to follow, and
is more obviously similar to the existing code. I'm sure your version is
faster, but I don't think it's that performance critical.

static void dedotify_ext_version_names(char *str_seq, unsigned long size)
{
	char *end = str_seq + size;
	char *p = str_seq;

	while (p < end) {
		if (*p == '.')
			memmove(p, p + 1, end - p - 1);

		p += strlen(p) + 1;
	}
}

The tail of str_seq will be filled with nulls as long as the last string
was null terminated.

cheers