On 09/10/25 4:57 pm, Naveen N Rao wrote:
On Thu, Oct 09, 2025 at 11:19:45AM +0530, Hari Bathini wrote:
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.
Does it? I'm referring to BPF_TRAMP_F_CALL_ORIG handling in
__arch_prepare_bpf_trampoline().
LR at BPF trampoline entry points at
the ftrace OOL stub. We recover the "real LR" pointing to the function
being traced from there so that we can call into it from within the BPF
trampoline.
Naveen, from the snippet in livepatch_handler code shared below,
the LR at BPF trmapoline entry points at the 'nop' after the call
to trampoline with 'bnectrl cr1' in the updated livepatch_handler.
Mimic'ing ftrace OOL branch instruction in livepatch_handler
with 'b 1f' (the instruction after nop) to ensure the trmapoline
derives the real LR to '1f' and jumps back into the livepatch_handler..
+ /* 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
I should probably improve my comments for better readability..
- Hari
