On 1/23/19 2:09 AM, Torsten Duwe wrote:
> Hi Balbir!
>
Hi, Torsten!
> On Tue, Jan 22, 2019 at 02:39:32PM +1300, Singh, Balbir wrote:
>>
>> On 1/19/19 5:39 AM, Torsten Duwe wrote:
>>> + */
>>> +ftrace_common_return:
>>> + /* restore function args */
>>> + ldp x0, x1, [sp]
>>> + ldp x2, x3, [sp, #S_X2]
>>> + ldp x4, x5, [sp, #S_X4]
>>> + ldp x6, x7, [sp, #S_X6]
>>> + ldr x8, [sp, #S_X8]
>>> +
>>> + /* restore fp and x28 */
>>> + ldp x28, x29, [sp, #S_X28]
>>> +
>>> + ldr lr, [sp, #S_LR]
>>> + ldr x9, [sp, #S_PC]
>>
>> Is it fair to assume that we never modify registers beyond LR and PC as a
>> result of ftrace/livepatching? I presume it is, but just checking.
>
> These are either callee-save or scratch. Whatever is called, ftrace framework
> functions or replacement functions, must preserve the callee-saved regs; and
> the caller, who made a function call (sic!-) saves caller-saved and marks the
> rest dead on return. So it's the arguments that matter after all.
>
> As you can see, disabling IPA-RA is cruicial here.
>
> Or are you talking about deliberate argument manipulation?
I wonder if another use of ftrace via register_ftrace_ops can expect to modify
arguments, like we modify the PC and RA
>
>>> + 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);
>>> +
>>
>> Is this a branch or a call? Does addr always fit in the immediate limits?
>
> As Julien has now pointed out, the correct enum value AARCH64_INSN_BRANCH_LINK
> should clarify this. It will surely fit for the kernel proper, and the modules
> are handled with the trampolines.
OK, so there is an assumption of the size of vmlinux being in a certain range?
I suspect something like a allyesconfig with debug might be a worthy challenger
:) Yes, modules do get trampolines.
>
>>> + return ftrace_modify_code(pc, old, new, true);
>>
>> Can you talk to the semantics of whether this operation is atomic w.r.t
>> system? Will old and new return consistent values? Given the nature of
>> ftrace, I presume it's well isolated.
>
> aarch64_insn_patch_text_nosync() does a __flush_icache_range() on success.
> Mark wrote that this is already sufficient IIRC. (I had memory barriers
> there, when I was still trying to modify 2 insns every time).
>
>>
>>> + if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS) &&
>>> + addr == MCOUNT_ADDR) {
>>> + old = aarch64_insn_gen_nop();
>>> + new = MOV_X9_X30;
>>> + pc -= REC_IP_BRANCH_OFFSET;
>>> + return ftrace_modify_code(pc, old, new, validate);
>>
>> I presume all the icache flush and barrier handling is in
>> ftrace_modify_code()?
>
> Yes, see above.
>
Thanks!
>>> + }
>>> +
>>> if (offset < -SZ_128M || offset >= SZ_128M) {
>>> #ifdef CONFIG_ARM64_MODULE_PLTS
>>> u32 replaced;
>>> --- a/arch/arm64/include/asm/module.h
>>> +++ b/arch/arm64/include/asm/module.h
>>> @@ -32,7 +32,8 @@ struct mod_arch_specific {
>>> struct mod_plt_sec init;
>>>
>>> /* for CONFIG_DYNAMIC_FTRACE */
>>> - struct plt_entry *ftrace_trampoline;
>>> + struct plt_entry *ftrace_trampolines;
>>> +#define MOD_ARCH_NR_FTRACE_TRAMPOLINES 2
>>
>> I don't see the generation of ftrace_trampolines[1]
>>
>
> That was further up, install_ftrace_trampoline() in kernel/ftrace.c.
>
Thanks!
> + if (*addr == FTRACE_ADDR)
> + mod_trampoline = &mod->arch.ftrace_trampolines[0];
> + else if (*addr == FTRACE_REGS_ADDR)
> + mod_trampoline = &mod->arch.ftrace_trampolines[1];
> [...]
> + trampoline = get_plt_entry(*addr, mod_trampoline);
> +
> + if (!plt_entries_equal(mod_trampoline, &trampoline)) {
> [...]
>
> get_plt_entry() generates a small bunch of instructions that easily
> fit into the argument registers. Compare commit bdb85cd1d20669dfae8
> for the new trampoline insns.
>
> Hope I've covered all your concerns,
>
Yes, largely thank you
Balbir
