Re: [PATCH v2 17/18] modules: check arch and block load on mismatch

[Daniel P . Berrangé]

On Thu, Jun 10, 2021 at 07:57:54AM +0200, Gerd Hoffmann wrote:
> Add module_allow_arch() to set the target architecture.
> In case a module is limited to some arch verify arches
> match and ignore the module if not.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  include/qemu/module.h |  1 +
>  softmmu/vl.c          |  3 +++
>  util/module.c         | 15 +++++++++++++++
>  3 files changed, 19 insertions(+)
> 
> diff --git a/include/qemu/module.h b/include/qemu/module.h
> index d3cab3c25a2f..7825f6d8c847 100644
> --- a/include/qemu/module.h
> +++ b/include/qemu/module.h
> @@ -72,6 +72,7 @@ void module_call_init(module_init_type type);
>  bool module_load_one(const char *prefix, const char *lib_name, bool mayfail);
>  void module_load_qom_one(const char *type);
>  void module_load_qom_all(void);
> +void module_allow_arch(const char *arch);
>  
>  /*
>   * macros to store module metadata in a .modinfo section.
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index ba26a042b284..96316774fcc9 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -126,6 +126,8 @@
>  #include "sysemu/iothread.h"
>  #include "qemu/guest-random.h"
>  
> +#include "config-host.h"
> +
>  #define MAX_VIRTIO_CONSOLES 1
>  
>  typedef struct BlockdevOptionsQueueEntry {
> @@ -2723,6 +2725,7 @@ void qemu_init(int argc, char **argv, char **envp)
>      error_init(argv[0]);
>      qemu_init_exec_dir(argv[0]);
>  
> +    module_allow_arch(TARGET_NAME);
>      qemu_init_subsystems();
>  
>      /* first pass of option parsing */
> diff --git a/util/module.c b/util/module.c
> index 6e4199169c41..564b8e3da760 100644
> --- a/util/module.c
> +++ b/util/module.c
> @@ -122,6 +122,12 @@ void module_call_init(module_init_type type)
>  static Modules *modinfo;
>  static char *module_dirs[5];
>  static int module_ndirs;
> +static const char *module_arch;
> +
> +void module_allow_arch(const char *arch)
> +{
> +    module_arch = arch;
> +}
>  
>  static void module_load_path_init(void)
>  {
> @@ -295,6 +301,14 @@ bool module_load_one(const char *prefix, const char 
> *lib_name, bool mayfail)
>      module_load_modinfo();
>  
>      for (modlist = modinfo->list; modlist != NULL; modlist = modlist->next) {
> +        if (modlist->value->has_arch) {
> +            if (strcmp(modlist->value->name, module_name) == 0) {
> +                if (!module_arch ||
> +                    strcmp(modlist->value->arch, module_arch) != 0) {
> +                    return false;
> +                }
> +            }
> +        }

I have a little hard time following the code paths, but IIUC, with this
change, instead of "module.so" we would have multiple copies of something
like "module-$arch.so" ?  Then we load them all, read their modinfo section
and discard the ones with non-matching arch ?

If that is a correct understanding, then I wonder about the scalability of
it. We have 30 system emulator targets right now, so if we get even a few
arch specific modues, that's going to be alot of modules we're loading,
checking and discarding.

Wouldn't it be simpler if we just made use of the directory layout
/usr/lib64/qemu/$mod.so for common modules and /usr/lib64/qemu/$arch/$mod.so
for arch specific modules. That would let us load only the modules we know
are applicable for the system target arch and not need this post-load
filtering from metadata.

Regards,
Daniel
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