Re: [PATCH RFC 09/13] arm64: ftrace: Maintain Tasks RCU trampoline nesting in ftrace_caller
From: sashiko-bot@kernel.org
Date: 2026-09-10 19:04:37
Also in:
bpf
Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Build failure due to missing `ftrace_static_tramp_end` symbol when `CONFIG_DYNAMIC_FTRACE_WITH_ARGS` is disabled.
--
commit 459a25e6b720c08167da420eb83180b2856610e3
Author: Josef Bacik [off-list ref]
arm64: ftrace: Maintain Tasks RCU trampoline nesting in ftrace_caller
Bracket the call out to the ftrace_ops callback in ftrace_caller with an
increment/decrement of current->rcu_tramp_nesting, using x12/w13 which
are scratch there. The read-modify-write is not atomic, but only current
modifies the count and every interrupting user is balanced, so nothing is
lost.
quoted hunk ↗ jump to hunk
diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S index 025140caafe74..46a102e7199ac 100644 --- a/arch/arm64/kernel/entry-ftrace.S +++ b/arch/arm64/kernel/entry-ftrace.S@@ -14,6 +14,33 @@ #include <asm/insn.h> #ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
[ ... ]
quoted hunk ↗ jump to hunk
@@ -178,6 +209,10 @@ SYM_CODE_START(ftrace_stub_direct_tramp) SYM_CODE_END(ftrace_stub_direct_tramp) #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */ +/* End of [ftrace_caller, ...) for arch_rcu_tasks_ip_in_trampoline(). */ +SYM_CODE_START(ftrace_static_tramp_end) +SYM_CODE_END(ftrace_static_tramp_end) + #else /* CONFIG_DYNAMIC_FTRACE_WITH_ARGS */ /*
[Severity: Low] Will this cause a linker error when compiling with CONFIG_RCU_TASKS_PREEMPT_QS=y but CONFIG_DYNAMIC_FTRACE_WITH_ARGS=n? The ftrace_static_tramp_end label is defined here exclusively inside the CONFIG_DYNAMIC_FTRACE_WITH_ARGS block. The legacy mcount-based ftrace path in the #else block lacks this definition.
quoted hunk ↗ jump to hunk
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c index e1a3c0b3a0514..1b7ac2afed0df 100644 --- a/arch/arm64/kernel/ftrace.c +++ b/arch/arm64/kernel/ftrace.c@@ -17,6 +17,22 @@ #include <asm/insn.h> #include <asm/text-patching.h> +#ifdef CONFIG_RCU_TASKS_PREEMPT_QS +extern void ftrace_static_tramp_end(void); + +/* + * See rcu_tasks_ip_in_trampoline(). ftrace_caller and ftrace_stub_direct_tramp + * are core kernel text but must be treated as trampolines: a task preempted in + * them may be carrying an ops pointer (x11) or a direct-call BPF trampoline + * address (x17) whose lifetime is guarded only by Tasks RCU. + */ +bool arch_rcu_tasks_ip_in_trampoline(unsigned long ip) +{ + return ip >= (unsigned long)ftrace_caller && + ip < (unsigned long)ftrace_static_tramp_end; +} +#endif
[Severity: Low] Since arch_rcu_tasks_ip_in_trampoline() unconditionally references ftrace_static_tramp_end here under CONFIG_RCU_TASKS_PREEMPT_QS, this might lead to an undefined reference at link time if CONFIG_DYNAMIC_FTRACE_WITH_ARGS is disabled. -- Sashiko AI review · https://sashiko.dev/#/patchset/20260910-b4-rcu-tasks-preempt-qs-v1-0-d4469f4cc101@toxicpanda.com?part=9