Re: [PATCH v7 2/3] arm64: implement ftrace with regs

[Julien Thierry <julien.thierry@arm.com>]

Hi Torsten,

On 18/01/2019 16:39, Torsten Duwe wrote:
> Once gcc8 adds 2 NOPs at the beginning of each function, replace the
> first NOP thus generated with a quick LR saver (move it to scratch reg
> x9), so the 2nd replacement insn, the call to ftrace, does not clobber
> the value. Ftrace will then generate the standard stack frames.
> 
> Note that patchable-function-entry in GCC disables IPA-RA, which means
> ABI register calling conventions are obeyed *and* scratch registers
> such as x9 are available.
> 
> Introduce and handle an ftrace_regs_trampoline for module PLTs, right
> after ftrace_trampoline, and double the size of this special section.
> 
> Signed-off-by: Torsten Duwe <duwe@suse.de>
> 
> ---
> 
> Mark, if you see your ftrace entry macro code being represented correctly
> here, please add your sign-off, As I've initially copied it from your mail.
> 
> ---
>  arch/arm64/include/asm/ftrace.h  |   17 ++++-
>  arch/arm64/include/asm/module.h  |    3 
>  arch/arm64/kernel/entry-ftrace.S |  125 +++++++++++++++++++++++++++++++++++++--
>  arch/arm64/kernel/ftrace.c       |  114 ++++++++++++++++++++++++++---------
>  arch/arm64/kernel/module-plts.c  |    3 
>  arch/arm64/kernel/module.c       |    2 
>  6 files changed, 227 insertions(+), 37 deletions(-)

[...]

> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -133,17 +163,45 @@ int ftrace_make_call(struct dyn_ftrace *
>  	return ftrace_modify_code(pc, old, new, true);
>  }
>  
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
> +			unsigned long addr)
> +{
> +	unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
> +	u32 old, new;
> +
> +	old = aarch64_insn_gen_branch_imm(pc, old_addr, true);
> +	new = aarch64_insn_gen_branch_imm(pc, addr, true);
> +
> +	return ftrace_modify_code(pc, old, new, true);
> +}
> +#endif
> +
>  /*
>   * Turn off the call to ftrace_caller() in instrumented function
>   */
>  int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>  		    unsigned long addr)
>  {
> -	unsigned long pc = rec->ip;
> +	unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;

Sorry to come back on this patch again, but I was looking at the ftrace
code a bit, and I see that when processing the ftrace call locations,
ftrace calls ftrace_call_adjust() on every ip registered as mcount
caller (or in our case patchable entries). This ftrace_call_adjust() is
arch specific, so I was thinking we could place the offset in here once
and for all so we don't have to worry about it in the future.

Also, I'm unsure whether it would be safe, but we could patch the "mov
x9, lr" there as well. In theory, this would be called at init time
(before secondary CPUs are brought up) and when loading a module (so I'd
expect no-one is executing that code *yet*.

If this is possible, I think it would make things a bit cleaner.

Let me know if that would work.

Thanks,

-- 
Julien Thierry