[PATCH v2] riscv: entry: Mark ret_from_fork_kernel() and ret_from_fork_user() noinstr

[Karl Mehltretter <kmehltretter@gmail.com>]

ret_from_fork_kernel() and ret_from_fork_user() are traceable and
return after syscall_exit_to_user_mode() enters the RCU extended
quiescent state via user_enter_irqoff() on NO_HZ_FULL CPUs. When
function_graph tracing is enabled with
FTRACE_VALIDATE_RCU_IS_WATCHING, the return hook runs after RCU stops
watching and triggers the ftrace_return_to_handler() warning.

Mark ret_from_fork_kernel() and ret_from_fork_user() noinstr.

Fixes: f955aa8723a6 ("riscv: entry: Convert ret_from_fork() to C")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Assisted by an LLM.

v2:
 - Drop instrumentation_begin()/instrumentation_end() around the kernel
   thread callback. They only feed objtool's noinstr validation, which
   riscv does not have (Jisheng Zhang). ret_from_fork_kernel() and
   ret_from_fork_user() disassemble identically to v1 (Clang, defconfig
   with NO_HZ_FULL and FUNCTION_GRAPH_TRACER).

v1: https://lore.kernel.org/r/20261107194931.69176-1-kmehltretter@gmail.com/

Tested with v1 on mainline d48723711bae (after v7.4-rc1) under QEMU
riscv64 (-smp 4) with NO_HZ_FULL, FTRACE_VALIDATE_RCU_IS_WATCHING,
PROVE_LOCKING, nohz_full=1-3 rcu_nocbs=1-3 on the kernel command line,
and function_graph tracing during parallel fork bursts across CPUs 0-3.

 arch/riscv/kernel/process.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 34c1d498d1cad..6f2b46d1a8e98 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -225,14 +225,15 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 	return 0;
 }
 
-asmlinkage void ret_from_fork_kernel(void *fn_arg, int (*fn)(void *), struct pt_regs *regs)
+asmlinkage void noinstr ret_from_fork_kernel(void *fn_arg, int (*fn)(void *),
+					     struct pt_regs *regs)
 {
 	fn(fn_arg);
 
 	syscall_exit_to_user_mode(regs);
 }
 
-asmlinkage void ret_from_fork_user(struct pt_regs *regs)
+asmlinkage void noinstr ret_from_fork_user(struct pt_regs *regs)
 {
 	syscall_exit_to_user_mode(regs);
 }

base-commit: d48723711bae9b8cdc56719c448ba42fef28cbee
-- 
2.39.5 (Apple Git-154)