Subject: Re: [PATCH 03/12] powerpc/module: Create a special stub for ftrace_caller()

From: Balbir Singh <[email-masked]>
Date: Thu, 25 Feb 2016 11:08:54 +1100

On 25/02/16 01:28, Michael Ellerman wrote:
> In order to support the new -mprofile-kernel ABI, we need to be able to
> call from the module back to ftrace_caller() (in the kernel) without
> using the module's r2. That is because the function in this module which
> is calling ftrace_caller() may not have setup r2, if it doesn't
> otherwise need it (ie. it accesses no globals).
>
> To make that work we add a new stub which is used for calling
> ftrace_caller(), which uses the kernel toc instead of the module toc.
>
> Signed-off-by: Michael Ellerman <[email-masked]>
> ---
>  arch/powerpc/kernel/module_64.c | 48 ++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index 9629966e614b..e711d40a3b8f 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -671,10 +671,56 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
>  }
>  
>  #ifdef CONFIG_DYNAMIC_FTRACE
> +
> +#define PACATOC offsetof(struct paca_struct, kernel_toc)
> +
> +static unsigned long create_ftrace_stub(const Elf64_Shdr *sechdrs, struct module *me)
> +{
> +	struct ppc64_stub_entry *entry;
> +	unsigned int i, num_stubs;
How about some comments on r2
r2 is still pointing to the module's toc, will be saved by ftrace_caller and restored by the instruction following bl ftrace_caller (after patching _mcount/nop)

> +	static u32 stub_insns[] = {
> +		0xe98d0000 | PACATOC, 	/* ld      r12,PACATOC(r13)	*/
> +		0x3d8c0000,		/* addis   r12,r12,<high>	*/
> +		0x398c0000, 		/* addi    r12,r12,<low>	*/
> +		0x7d8903a6, 		/* mtctr   r12			*/
> +		0x4e800420, 		/* bctr				*/
> +	};
> +	long reladdr;
> +
> +	num_stubs = sechdrs[me->arch.stubs_section].sh_size / sizeof(*entry);
> +
> +	/* Find the next available stub entry */
> +	entry = (void *)sechdrs[me->arch.stubs_section].sh_addr;
> +	for (i = 0; i < num_stubs && stub_func_addr(entry->funcdata); i++, entry++);
> +
> +	if (i >= num_stubs) {
> +		pr_err("%s: Unable to find a free slot for ftrace stub.\n", me->name);
> +		return 0;
> +	}
> +
> +	memcpy(entry->jump, stub_insns, sizeof(stub_insns));
> +
> +	/* Stub uses address relative to kernel_toc */
> +	reladdr = (unsigned long)ftrace_caller - get_paca()->kernel_toc;
> +	if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {
> +		pr_err("%s: Address of ftrace_caller out of range of kernel_toc.\n", me->name);
> +		return 0;
> +	}
> +
> +	entry->jump[1] |= PPC_HA(reladdr);
> +	entry->jump[2] |= PPC_LO(reladdr);
> +
> +	/* Eventhough we don't use funcdata in the stub, it's needed elsewhere. */
> +	entry->funcdata = func_desc((unsigned long)ftrace_caller);
> +	entry->magic = STUB_MAGIC;
> +
> +	return (unsigned long)entry;
> +}
> +
>  int module_finalize_ftrace(struct module *mod, const Elf_Shdr *sechdrs)
>  {
>  	mod->arch.toc = my_r2(sechdrs, mod);
> -	mod->arch.tramp = stub_for_addr(sechdrs, (unsigned long)ftrace_caller, mod);
> +	mod->arch.tramp = create_ftrace_stub(sechdrs, mod);
>  
>  	if (!mod->arch.tramp)
>  		return -ENOENT;
Reviewed-by: Balbir Singh <[email-masked]>