On Wed, Aug 20, 2025 at 06:47:43PM -0400, Steven Rostedt wrote:

On Sun, 13 Jul 2025 10:17:30 +0300
Mike Rapoport [off-list ref] wrote:

quoted

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

For the most part ftrace uses text poking and can handle ROX memory.
The only place that requires writable memory is create_trampoline() that
updates the allocated memory and in the end makes it ROX.

Use execmem_alloc_rw() in x86::ftrace::alloc_tramp() and enable ROX cache
for EXECMEM_FTRACE when configuration and CPU features allow that.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---

The "ftrace=function" kernel command line started crashing with v6.17-rc1,
and I bisected it down to this commit:

 5d79c2be5081 ("x86/ftrace: enable EXECMEM_ROX_CACHE for ftrace allocations")

On boot I hit this:

[    0.159269] BUG: kernel NULL pointer dereference, address: 000000000000001c
[    0.160254] #PF: supervisor read access in kernel mode
[    0.160975] #PF: error_code(0x0000) - not-present page
[    0.161697] PGD 0 P4D 0
[    0.162055] Oops: Oops: 0000 [#1] SMP PTI
[    0.162619] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.17.0-rc2-test-00006-g48d06e78b7cb-dirty #9 PREEMPT(undef)
[    0.164141] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[    0.165439] RIP: 0010:kmem_cache_alloc_noprof (mm/slub.c:4237) 
[    0.177483] Call Trace:
[    0.177828]  <TASK>
[    0.178123] mas_alloc_nodes (lib/maple_tree.c:176 (discriminator 2) lib/maple_tree.c:1255 (discriminator 2)) 
[    0.178692] mas_store_gfp (lib/maple_tree.c:5468) 
[    0.179223] execmem_cache_add_locked (mm/execmem.c:207) 
[    0.179870] execmem_alloc (mm/execmem.c:213 mm/execmem.c:313 mm/execmem.c:335 mm/execmem.c:475) 
[    0.180397] ? ftrace_caller (arch/x86/kernel/ftrace_64.S:169) 
[    0.180922] ? __pfx_ftrace_caller (arch/x86/kernel/ftrace_64.S:158) 
[    0.181517] execmem_alloc_rw (mm/execmem.c:487) 
[    0.182052] arch_ftrace_update_trampoline (arch/x86/kernel/ftrace.c:266 arch/x86/kernel/ftrace.c:344 arch/x86/kernel/ftrace.c:474) 
[    0.182778] ? ftrace_caller_op_ptr (arch/x86/kernel/ftrace_64.S:182) 
[    0.183388] ftrace_update_trampoline (kernel/trace/ftrace.c:7947) 
[    0.184024] __register_ftrace_function (kernel/trace/ftrace.c:368) 
[    0.184682] ftrace_startup (kernel/trace/ftrace.c:3048) 
[    0.185205] ? __pfx_function_trace_call (kernel/trace/trace_functions.c:210) 
[    0.185877] register_ftrace_function_nolock (kernel/trace/ftrace.c:8717) 
[    0.186595] register_ftrace_function (kernel/trace/ftrace.c:8745) 
[    0.187254] ? __pfx_function_trace_call (kernel/trace/trace_functions.c:210) 
[    0.187924] function_trace_init (kernel/trace/trace_functions.c:170) 
[    0.188499] tracing_set_tracer (kernel/trace/trace.c:5916 kernel/trace/trace.c:6349) 
[    0.189088] register_tracer (kernel/trace/trace.c:2391) 
[    0.189642] early_trace_init (kernel/trace/trace.c:11075 kernel/trace/trace.c:11149) 
[    0.190204] start_kernel (init/main.c:970) 
[    0.190732] x86_64_start_reservations (arch/x86/kernel/head64.c:307) 
[    0.191381] x86_64_start_kernel (??:?) 
[    0.191955] common_startup_64 (arch/x86/kernel/head_64.S:419) 
[    0.192534]  </TASK>
[    0.192839] Modules linked in:
[    0.193267] CR2: 000000000000001c
[    0.193730] ---[ end trace 0000000000000000 ]---

maple tree is initialized after ftrace, so the patch below should fix it:

diff --git a/init/main.c b/init/main.c
index 0ee0ee7b7c2c..5753e9539ae6 100644
--- a/init/main.c
+++ b/init/main.c

@@ -956,6 +956,7 @@ void start_kernel(void)
 	sort_main_extable();
 	trap_init();
 	mm_core_init();
+	maple_tree_init();
 	poking_init();
 	ftrace_init();
 

@@ -973,7 +974,6 @@ void start_kernel(void)
 		 "Interrupts were enabled *very* early, fixing it\n"))
 		local_irq_disable();
 	radix_tree_init();
-	maple_tree_init();
 
 	/*
 	 * Set up housekeeping before setting up workqueues to allow the unbound
 

-- Steve

-- 
Sincerely yours,
Mike.