Re: [RFC v2 3/4] modules, x86: use vmalloc_exec for module core

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Fri, 2022-10-07 at 16:43 -0700, Song Liu wrote:
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index a4e4d84b6f4e..b44806e31a56 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -53,6 +53,7 @@
>  #include <linux/bsearch.h>
>  #include <linux/dynamic_debug.h>
>  #include <linux/audit.h>
> +#include <linux/bpf.h>
>  #include <uapi/linux/module.h>
>  #include "internal.h"
>  
> @@ -1203,7 +1204,7 @@ static void free_module(struct module *mod)
>         lockdep_free_key_range(mod->data_layout.base, mod-
> >data_layout.size);
>  
>         /* Finally, free the core (containing the module structure)
> */
> -       module_memfree(mod->core_layout.base);
> +       vfree_exec(mod->core_layout.base);
>  #ifdef CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC
>         vfree(mod->data_layout.base);
>  #endif
> @@ -1321,7 +1322,8 @@ static int simplify_symbols(struct module *mod,
> const struct load_info *info)
>                         ksym = resolve_symbol_wait(mod, info, name);
>                         /* Ok if resolved.  */
>                         if (ksym && !IS_ERR(ksym)) {
> -                               sym[i].st_value =
> kernel_symbol_value(ksym);
> +                               unsigned long val =
> kernel_symbol_value(ksym);
> +                               bpf_arch_text_copy(&sym[i].st_value,
> &val, sizeof(val));

Why bpf_arch_text_copy()? This of course won't work for other
architectures. So there needs to be fallback method. That RFC broke the
operation into two stages: Loading and finalized. When loading, on non-
x86 the writes would simply be to the allocation mapped as writable.
When it was finalized it changed it to it's final permission (RO, etc).
Then for x86 it does text_pokes() for the writes and has it RO from the
beginning.

I ended up needing a staging buffer for modules too, so that the code
could operate on it directly. I can't remember why that was, it might
be unneeded now since you moved data out of the core allocation.