Re: [PATCH v3 2/6] powerpc/module: Handle caller-saved TOC in module linker

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 05/10/2022 à 07:32, Benjamin Gray a écrit :
> The callee may set a field in st_other to 1 to indicate r2 should be
> treated as caller-saved. This means a trampoline must be used to save
> the current TOC before calling it and restore it afterwards, much like
> external calls.

The 'callee', what is that here in that context ?

Don't you mean the 'linker' instead ?

> 
> This is necessary for supporting V2 ABI static calls that do not
> preserve the TOC.
> 
> Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
> ---
>   arch/powerpc/kernel/module_64.c | 14 +++++++++++++-
>   1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index 7e45dc98df8a..4d816f7785b4 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -55,6 +55,12 @@ static unsigned int local_entry_offset(const Elf64_Sym *sym)
>   	 * of function and try to derive r2 from it). */
>   	return PPC64_LOCAL_ENTRY_OFFSET(sym->st_other);
>   }
> +
> +static bool need_r2save_stub(unsigned char st_other)
> +{
> +	return ((st_other & STO_PPC64_LOCAL_MASK) >> STO_PPC64_LOCAL_BIT) == 1;

I would have writen :

	return st_other & STO_PPC64_LOCAL_MASK == 1 << STO_PPC64_LOCAL_BIT;

But it is just a matter of preference, so up to you.

> +}
> +
>   #else
>   
>   static func_desc_t func_desc(unsigned long addr)
> @@ -66,6 +72,11 @@ static unsigned int local_entry_offset(const Elf64_Sym *sym)
>   	return 0;
>   }
>   
> +static bool need_r2save_stub(unsigned char st_other)
> +{
> +	return false;
> +}
> +
>   void *dereference_module_function_descriptor(struct module *mod, void *ptr)
>   {
>   	if (ptr < (void *)mod->arch.start_opd ||
> @@ -632,7 +643,8 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
>   		case R_PPC_REL24:
>   			/* FIXME: Handle weak symbols here --RR */
>   			if (sym->st_shndx == SHN_UNDEF ||
> -			    sym->st_shndx == SHN_LIVEPATCH) {
> +			    sym->st_shndx == SHN_LIVEPATCH ||
> +			    need_r2save_stub(sym->st_other)) {
>   				/* External: go via stub */
>   				value = stub_for_addr(sechdrs, value, me,
>   						strtab + sym->st_name);