Re: [PATCH] powerpc64/bpf: support direct_call on livepatch function

[Hari Bathini <hbathini@linux.ibm.com>]

On 08/10/25 1:43 pm, Naveen N Rao wrote:
> On Mon, Oct 06, 2025 at 06:50:20PM +0530, Hari Bathini wrote:
>>
>>
>> On 06/10/25 1:22 pm, Naveen N Rao wrote:
>>> On Fri, Oct 03, 2025 at 12:57:54AM +0530, Hari Bathini wrote:
>>>> Today, livepatch takes precedence over direct_call. Instead, save the
>>>> state and make direct_call before handling livepatch.
>>>
>>> If we call into the BPF trampoline first and if we have
>>> BPF_TRAMP_F_CALL_ORIG set, does this result in the BPF trampoline
>>> calling the new copy of the live-patched function or the old one?
>>
>> Naveen, calls the new copy of the live-patched function..
> 
> Hmm... I'm probably missing something.
> 
> With ftrace OOL stubs, what I recall is that BPF trampoline derives the
> original function address from the OOL stub (which would be associated
> with the original function, not the livepatch one).

Trampoline derives the address from LR. The below snippet
in livepatch_handler ensures the trampoline jumps to '1f'
label instead of the original function with LR updated:

+	/* Jump to the direct_call */
+	bnectrl	cr1
+
+	/*
+	 * The address to jump after direct call is deduced based on ftrace 
OOL stub sequence.
+	 * The seemingly insignificant couple of instructions below is to 
mimic that here to
+	 * jump back to the livepatch handler code below.
+	 */
+	nop
+	b	1f
+
+	/*
+	 * Restore the state for livepatching from the livepatch stack.
+	 * Before that, check if livepatch stack is intact. Use r0 for it.
+	 */
+1:	mtctr	r0


- Hari