Re: [PATCH v8 0/3] ARM: Implement MODULE_PLT support in FTRACE

[Qais Yousef]

Adding Linus Walleij back to CC

On 03/30/21 13:40, Alexander A Sverdlin wrote:
> From: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxx>
> 
> FTRACE's function tracer currently doesn't always work on ARM with
> MODULE_PLT option enabled. If the module is loaded too far, FTRACE's
> code modifier cannot cope with introduced veneers and turns the
> function tracer off globally.
> 
> ARM64 already has a solution for the problem, refer to the following
> patches:
> 
> arm64: ftrace: emit ftrace-mod.o contents through code
> arm64: module-plts: factor out PLT generation code for ftrace
> arm64: ftrace: fix !CONFIG_ARM64_MODULE_PLTS kernels
> arm64: ftrace: fix building without CONFIG_MODULES
> arm64: ftrace: add support for far branches to dynamic ftrace
> arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()
> 
> But the presented ARM variant has just a half of the footprint in terms of
> the changed LoCs. It also retains the code validation-before-modification
> instead of switching it off.
> 
> Changelog:
> v8:
> * Add warn suppress parameter to arm_gen_branch_link()

I still think the ifdefery in patch 3 is ugly. Any reason my suggestion didn't
work out for you? I struggle to see how this is better and why it was hard to
incorporate my suggestion.

For example

	-       old = ftrace_call_replace(ip, adjust_address(rec, addr));
	+#ifdef CONFIG_ARM_MODULE_PLTS
	+       /* mod is only supplied during module loading */
	+       if (!mod)
	+               mod = rec->arch.mod;
	+       else
	+               rec->arch.mod = mod;
	+#endif
	+
	+       old = ftrace_call_replace(ip, aaddr,
	+                                 !IS_ENABLED(CONFIG_ARM_MODULE_PLTS) || !mod);
	+#ifdef CONFIG_ARM_MODULE_PLTS
	+       if (!old && mod) {
	+               aaddr = get_module_plt(mod, ip, aaddr);
	+               old = ftrace_call_replace(ip, aaddr, true);
	+       }
	+#endif
	+

There's an ifdef, followed by a code that embeds
!IS_ENABLED(CONFIG_ARM_MODULE_PLTS) followed by another ifdef :-/

And there was no need to make the new warn arg visible all the way to
ftrace_call_repalce() and all of its users.

FWIW

Tested-by: Qais Yousef <qais.yousef@xxxxxxx>

If this gets accepted as-is, I'll send a patch to improve on this.


Thanks

--
Qais Yousef