linux-kernel - Re: [PATCH v7 1/3] modules: Support extended MODVERSIONS info

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Message-ID: <874j50juyp.fsf@mail.lhotse>
Date: Fri, 25 Oct 2024 22:14:06 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: Matthew Maurer <mmaurer@...gle.com>, Nicholas Piggin
 <npiggin@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>,
 Naveen N Rao <naveen@...nel.org>, Madhavan Srinivasan
 <maddy@...ux.ibm.com>, Luis Chamberlain <mcgrof@...nel.org>, Petr Pavlu
 <petr.pavlu@...e.com>, Sami Tolvanen <samitolvanen@...gle.com>, Daniel
 Gomez <da.gomez@...sung.com>, Masahiro Yamada <masahiroy@...nel.org>,
 Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy
 Baron <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl
 <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>
Cc: linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
 linux-modules@...r.kernel.org, linux-kbuild@...r.kernel.org,
 rust-for-linux@...r.kernel.org, Matthew Maurer <mmaurer@...gle.com>
Subject: Re: [PATCH v7 1/3] modules: Support extended MODVERSIONS info

Matthew Maurer <mmaurer@...gle.com> 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 <mmaurer@...gle.com>
> ---
>  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