[PATCH v3] powerpc/64: Option to use ELFv2 ABI for big-endian kernels

[Michal Suchánek]

On Wed, May 05, 2021 at 10:07:29PM +1000, Michael Ellerman wrote:
> Michal Suchánek <msuchanek at suse.de> writes:
> > On Mon, May 03, 2021 at 01:37:57PM +0200, Andreas Schwab wrote:
> >> Should this add a tag to the module vermagic?
> >
> > Would the modues link even if the vermagic was not changed?
> 
> Most modules will require some symbols from the kernel, and those will
> be dot symbols, which won't resolve.
> 
> But there are a few small modules that don't rely on any kernel symbols,
> which can load.
> 
> > I suppose something like this might do it.
> 
> It would, but I feel like we should be handling this at the ELF level.
> ie. we don't allow loading modules with a different ELF machine type, so
> neither should we allow loading a module with the wrong ELF ABI.
> 
> And you can build the kernel without MODVERSIONS, so relying on
> MODVERSIONS still leaves a small exposure (same kernel version
> with/without ELFv2).
> 
> I don't see an existing hook that would do what we want. There's
> elf_check_arch(), but that also applies to userspace binaries, which is
> not what we want.
> 
> Maybe something like below.
Yes, that looks better.

Thanks

Michal
> 
> cheers
> 
> 
> diff --git a/arch/powerpc/include/asm/module.h b/arch/powerpc/include/asm/module.h
> index 857d9ff24295..d0e9368982d8 100644
> --- a/arch/powerpc/include/asm/module.h
> +++ b/arch/powerpc/include/asm/module.h
> @@ -83,5 +83,28 @@ static inline int module_finalize_ftrace(struct module *mod, const Elf_Shdr *sec
>  }
>  #endif
>  
> +#ifdef CONFIG_PPC64
> +static inline bool elf_check_module_arch(Elf_Ehdr *hdr)
> +{
> +	unsigned long flags;
> +
> +	if (!elf_check_arch(hdr))
> +		return false;
> +
> +	flags = hdr->e_flags & 0x3;
> +
> +#ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
> +	if (flags == 2)
> +		return true;
> +#else
> +	if (flags < 2)
> +		return true;
> +#endif
> +	return false;
> +}
> +
> +#define elf_check_module_arch elf_check_module_arch
> +#endif /* CONFIG_PPC64 */
> +
>  #endif /* __KERNEL__ */
>  #endif	/* _ASM_POWERPC_MODULE_H */
> diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
> index 9e09d11ffe5b..fdc042a84562 100644
> --- a/include/linux/moduleloader.h
> +++ b/include/linux/moduleloader.h
> @@ -13,6 +13,11 @@
>   * must be implemented by each architecture.
>   */
>  
> +// Allow arch to optionally do additional checking of module ELF header
> +#ifndef elf_check_module_arch
> +#define elf_check_module_arch elf_check_arch
> +#endif
> +
>  /* Adjust arch-specific sections.  Return 0 on success.  */
>  int module_frob_arch_sections(Elf_Ehdr *hdr,
>  			      Elf_Shdr *sechdrs,
> diff --git a/kernel/module.c b/kernel/module.c
> index b5dd92e35b02..c71889107226 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2941,7 +2941,7 @@ static int elf_validity_check(struct load_info *info)
>  
>  	if (memcmp(info->hdr->e_ident, ELFMAG, SELFMAG) != 0
>  	    || info->hdr->e_type != ET_REL
> -	    || !elf_check_arch(info->hdr)
> +	    || !elf_check_module_arch(info->hdr)
>  	    || info->hdr->e_shentsize != sizeof(Elf_Shdr))
>  		return -ENOEXEC;
>