Re: [PATCH v6 4/5] modpost: Produce extended MODVERSIONS information

[Luis Chamberlain]

On Tue, Oct 15, 2024 at 11:18:59PM +0000, Matthew Maurer wrote:
> Generate both the existing modversions format and the new extended one
> when running modpost. Presence of this metadata in the final .ko is
> guarded by CONFIG_EXTENDED_MODVERSIONS.
> 
> We no longer generate an error on long symbols in modpost if
> CONFIG_EXTENDED_MODVERSIONS is set, as they can now be appropriately
> encoded in the extended section. These symbols will be skipped in the
> previous encoding. An error will still be generated if
> CONFIG_EXTENDED_MODVERSIONS is not set.
> 
> Signed-off-by: Matthew Maurer <mmaurer@xxxxxxxxxx>
> ---
>  kernel/module/Kconfig |  8 ++++++++
>  scripts/mod/modpost.c | 41 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 49 insertions(+)
> 
> diff --git a/kernel/module/Kconfig b/kernel/module/Kconfig
> index 7c6588148d42..a5de2b7f2758 100644
> --- a/kernel/module/Kconfig
> +++ b/kernel/module/Kconfig
> @@ -177,6 +177,14 @@ config ASM_MODVERSIONS
>  	  assembly. This can be enabled only when the target architecture
>  	  supports it.
>  
> +config EXTENDED_MODVERSIONS
> +	bool "Extended Module Versioning Support"
> +	depends on MODVERSIONS
> +	help
> +	  This enables extended MODVERSIONs support, allowing long symbol
> +	  names to be versioned. The most likely reasons you would enable
> +	  this are for Rust usage or aggressive LTO configurations.

What is "aggressive LTO configurations" please elaborate. Can we infer
on that through configuration?

> +
>  config MODULE_SRCVERSION_ALL
>  	bool "Source checksum for all modules"
>  	help
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index 107393a8c48a..d18ff8a1109a 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -1840,15 +1840,56 @@ static void add_versions(struct buffer *b, struct module *mod)
>  			continue;
>  		}
>  		if (strlen(s->name) >= MODULE_NAME_LEN) {
> +#ifdef CONFIG_EXTENDED_MODVERSIONS
> +			/* this symbol will only be in the extended info */
> +			continue;
> +#else
>  			error("too long symbol \"%s\" [%s.ko]\n",
>  			      s->name, mod->name);
>  			break;
> +#endif
>  		}

Using #ifdefs on a loop like this seems fragile, even if its more code
make the code clearer and use separate routines for both worlds. Make
the code easy to review and maintain.

>  		buf_printf(b, "\t{ %#8x, \"%s\" },\n",
>  			   s->crc, s->name);
>  	}
>  
>  	buf_printf(b, "};\n");
> +
> +	buf_printf(b, "#ifdef CONFIG_EXTENDED_MODVERSIONS\n");

Why not *in-code* rather than the output? And if possible why not
two routines as above.

  Luis