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Message-ID: <aQKpDIYwO9VTP7qM@meta.com>
Date: Wed, 29 Oct 2025 16:53:48 -0700
From: Ben Niu <BenNiu@...a.com>
To: Mark Rutland <mark.rutland@....com>
CC: <catalin.marinas@....com>, <will@...nel.org>, <tytso@....edu>,
<Jason@...c4.com>, <linux-arm-kernel@...ts.infradead.org>,
<niuben003@...il.com>, <linux-kernel@...r.kernel.org>,
<benniu@...a.com>
Subject: Re: [PATCH] tracing: Enable kprobe tracing for Arm64 asm functions
On Wed, Oct 29, 2025 at 02:33:26PM +0000, Mark Rutland wrote:
> On Mon, Oct 27, 2025 at 11:17:49AM -0700, Ben Niu wrote:
> > Currently, Arm64 assembly functions always have a bti c
> > instruction inserted before the prologue. When ftrace is enabled,
> > no padding nops are inserted at all.
> >
> > This breaks kprobe tracing for asm functions, which assumes that
> > proper nops are added before and within functions (when ftrace is
> > enabled) and bti c is only present when CONFIG_ARM64_BTI_KERNEL is
> > defined.
>
> What exactly do you mean by "breaks kprobe tracing"?
>
> The kprobes code knows NOTHING about those ftrace NOPs, so I cannot see
> how those are relevant.
>
> The patch adds entries to __patchable_function_entries, which is owned
> by ftrace, and has NOTHING to do with kprobes.
Thanks for reviewing my patch, Mark. This is the second ever Linux kernel patch
I've sent out so I'm still learning how much detail I should put in a patch for
reviewers. Clearly, not enough details were added for this patch, and sorry
about that.
kprobe and ftrace seem two seprate things, but kprobe actually checks if a
function is notrace based off whether the function is padded with proper nops
for ftrace when ftrace is enabled. See the following call stack for more
details (obtained by debugging centos 10 arm64 linux kernel):
#0 lookup_rec (start=18446603338384966592, end=18446603338384967175) at kernel/trace/ftrace.c:1591
#1 ftrace_location_range (start=18446603338384966592, end=18446603338384967175) at kernel/trace/ftrace.c:1626
#2 0xffff8000802bd070 [PAC] in __within_notrace_func (addr=<optimized out>, addr@...ry=18446603338384966592) at kernel/trace/trace_kprobe.c:456
#3 0xffff8000802bd13c [PAC] in within_notrace_func (tk=0xffff0000cb5b0400) at kernel/trace/trace_kprobe.c:464
#4 0xffff8000802bd534 [PAC] in __register_trace_kprobe (tk=tk@...ry=0xffff0000cb5b0400) at kernel/trace/trace_kprobe.c:496
#5 0xffff8000802bf7a0 [PAC] in __register_trace_kprobe (tk=0xffff0000cb5b0400) at kernel/trace/trace_kprobe.c:490
#6 create_local_trace_kprobe (func=func@...ry=0xffff0000cabb19c0 "__arch_copy_to_user", addr=<optimized out>, offs=<optimized out>, is_return=is_return@...ry=false)
at kernel/trace/trace_kprobe.c:1935
#7 0xffff80008029ef8c [PAC] in perf_kprobe_init (p_event=p_event@...ry=0xffff0000c6d89a40, is_retprobe=false) at kernel/trace/trace_event_perf.c:267
#8 0xffff800080364ec4 [PAC] in perf_kprobe_event_init (event=0xffff0000c6d89a40) at kernel/events/core.c:11102
#9 0xffff800080369740 [PAC] in perf_try_init_event (pmu=0xffff8000829347a8 <perf_kprobe>, event=0xffff0000c6d89a40) at kernel/events/core.c:12595
#10 0xffff8000803699dc [PAC] in perf_init_event (event=event@...ry=0xffff0000c6d89a40) at kernel/events/core.c:12693
#11 0xffff80008036d91c [PAC] in perf_event_alloc (attr=attr@...ry=0xffff80008785b970, cpu=cpu@...ry=0, task=task@...ry=0x0, group_leader=0xffff0000c6d89a40,
group_leader@...ry=0x0, parent_event=parent_event@...ry=0x0, overflow_handler=<optimized out>, overflow_handler@...ry=0x0, context=<optimized out>, context@...ry=0x0,
cgroup_fd=cgroup_fd@...ry=-1) at kernel/events/core.c:12968
#12 0xffff800080373244 [PAC] in __do_sys_perf_event_open (attr_uptr=<optimized out>, pid=<optimized out>, cpu=0, group_fd=<optimized out>, flags=<optimized out>)
at kernel/events/core.c:13485
#13 0xffff800080379aa0 [PAC] in __se_sys_perf_event_open (attr_uptr=<optimized out>, pid=<optimized out>, cpu=<optimized out>, group_fd=<optimized out>, flags=<optimized out>)
at kernel/events/core.c:13367
#14 __arm64_sys_perf_event_open (regs=<optimized out>) at kernel/events/core.c:13367
#15 0xffff80008004997c [PAC] in __invoke_syscall (regs=0xffff80008785beb0, syscall_fn=<optimized out>) at arch/arm64/kernel/syscall.c:35
#16 invoke_syscall (regs=regs@...ry=0xffff80008785beb0, scno=<optimized out>, sc_nr=sc_nr@...ry=463, syscall_table=<optimized out>) at arch/arm64/kernel/syscall.c:49
#17 0xffff800080049a88 [PAC] in el0_svc_common (sc_nr=463, syscall_table=<optimized out>, regs=0xffff80008785beb0, scno=<optimized out>) at arch/arm64/kernel/syscall.c:132
#18 do_el0_svc (regs=regs@...ry=0xffff80008785beb0) at arch/arm64/kernel/syscall.c:151
#19 0xffff800080fba654 [PAC] in el0_svc (regs=0xffff80008785beb0) at arch/arm64/kernel/entry-common.c:875
#20 0xffff800080fbab10 [PAC] in el0t_64_sync_handler (regs=<optimized out>) at arch/arm64/kernel/entry-common.c:894
#21 0xffff800080011684 [PAC] in el0t_64_sync () at arch/arm64/kernel/entry.S:600
within_notrace_func is only defined if defined(CONFIG_DYNAMIC_FTRACE) && \
!defined(CONFIG_KPROBE_EVENTS_ON_NOTRACE), which searches a list of records
about whether a function is ftrace-able or not. The list of records is built
by ftrace_init at boot time.
> > The patch fixes the bug by inserting nops and bti c in Arm64 asm
> > in the same way as compiled C code.
That's correct. I added __patchable_function_entries in assemblies so that
their addresses are added between __start_mcount_loc and __stop_mcount_loc
at link time, which is then picked up by ftrace_init. Proper nop and bti c
instructions are added to assemblies in the way as if the assemblies are
compiled from C code. This is crucial to satisfy ftrace requirements and nop
patching at boot time. Otherwise, ftrace_bug will be hit and the assembly
function won't be ftrace-able and also kprobe-traceable.
> As it stands, NAK to this change.
>
> I'm not averse to making (some) assembly functions traceable by ftrace,
> and hence giving those NOPs. However, that's not safe generally (e.g.
> due to noinstr requirements), and so special care will need to be taken.
Assemblies that are not traceable should use SYM_CODE_START or the new macro
SYM_FUNC_START_NOTRACE. If an assembly function starts with SYM_FUNC_START,
by default, it's assumed to be traceable. I dumped all assemblies under
arch/arm64, and majority of the assemblies are utility functions (e.g., crypto,
memset/memcpy, etc.), which seemed safe to trace. There are a few functions
in the following (and maybe others) that might not be traceable, so please
advise.
./kernel/entry-ftrace.S
./kernel/entry.S
./mm/proc.S
./kernel/head.S
We can either have SYM_FUNC_START be traceable by default, or flip the polarity
and introduce SYM_FUNC_START_TRACEABLE to explicitly mark those functions
deemed safe to trace.
> The rationale above does not make sense; it conflates distinct things,
> and I think a more complete explanation is necessary.
Please see my explanation above. If you have any questions, please let me know.
> Mark.
>
> > Note: although this patch unblocks kprobe tracing, fentry is still
> > broken because no BTF info gets generated from assembly files. A
> > separate patch is needed to fix that.
> >
> > I built this patch with different combos of the following features
> > and confirmed kprobe tracing for asm function __arch_copy_to_user
> > worked in all cases:
> >
> > CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
> > CONFIG_DYNAMIC_FTRACE_WITH_ARGS
> > CONFIG_ARM64_BTI_KERNEL
> >
> > Signed-off-by: Ben Niu <benniu@...a.com>
> > ---
> > arch/arm64/include/asm/linkage.h | 103 ++++++++++++++++-----
> > arch/arm64/kernel/vdso/vgetrandom-chacha.S | 2 +-
> > 2 files changed, 81 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
> > index d3acd9c87509..f3f3bc168162 100644
> > --- a/arch/arm64/include/asm/linkage.h
> > +++ b/arch/arm64/include/asm/linkage.h
> > @@ -5,8 +5,47 @@
> > #include <asm/assembler.h>
> > #endif
> >
> > -#define __ALIGN .balign CONFIG_FUNCTION_ALIGNMENT
> > -#define __ALIGN_STR ".balign " #CONFIG_FUNCTION_ALIGNMENT
> > +#define __ALIGN .balign CONFIG_FUNCTION_ALIGNMENT
> > +#define __ALIGN_STR ".balign " #CONFIG_FUNCTION_ALIGNMENT
> > +
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
> > +
> > +#define PRE_FUNCTION_NOPS \
> > + ALIGN; \
> > + nops CONFIG_FUNCTION_ALIGNMENT / 4 - 2; \
> > + .pushsection __patchable_function_entries, "awo", @progbits, .text; \
> > + .p2align 3; \
> > + .8byte 1f; \
> > + .popsection; \
> > + 1 :; \
> > + nops 2;
> > +
> > +#define PRE_PROLOGUE_NOPS nops 2;
> > +
> > +#elif defined(CONFIG_DYNAMIC_FTRACE_WITH_ARGS)
> > +
> > +#define PRE_FUNCTION_NOPS
> > +
> > +#define PRE_PROLOGUE_NOPS \
> > + .pushsection __patchable_function_entries, "awo", @progbits, .text; \
> > + .p2align 3; \
> > + .8byte 1f; \
> > + .popsection; \
> > + 1 :; \
> > + nops 2;
> > +
> > +#else
> > +
> > +#define PRE_FUNCTION_NOPS
> > +#define PRE_PROLOGUE_NOPS
> > +
> > +#endif
> > +
> > +#ifdef CONFIG_ARM64_BTI_KERNEL
> > +#define BTI_C bti c;
> > +#else
> > +#define BTI_C
> > +#endif
> >
> > /*
> > * When using in-kernel BTI we need to ensure that PCS-conformant
> > @@ -15,32 +54,50 @@
> > * everything, the override is done unconditionally so we're more
> > * likely to notice any drift from the overridden definitions.
> > */
> > -#define SYM_FUNC_START(name) \
> > - SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
> > - bti c ;
> > +#define SYM_FUNC_START(name) \
> > + PRE_FUNCTION_NOPS \
> > + SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
> > + BTI_C \
> > + PRE_PROLOGUE_NOPS
> > +
> > +#define SYM_FUNC_START_NOTRACE(name) \
> > + SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
> > + BTI_C
> >
> > -#define SYM_FUNC_START_NOALIGN(name) \
> > - SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE) \
> > - bti c ;
> > +#define SYM_FUNC_START_NOALIGN(name) \
> > + PRE_FUNCTION_NOPS \
> > + SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE) \
> > + BTI_C \
> > + PRE_PROLOGUE_NOPS
> >
> > -#define SYM_FUNC_START_LOCAL(name) \
> > - SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN) \
> > - bti c ;
> > +#define SYM_FUNC_START_LOCAL(name) \
> > + PRE_FUNCTION_NOPS \
> > + SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN) \
> > + BTI_C \
> > + PRE_PROLOGUE_NOPS
> >
> > -#define SYM_FUNC_START_LOCAL_NOALIGN(name) \
> > - SYM_START(name, SYM_L_LOCAL, SYM_A_NONE) \
> > - bti c ;
> > +#define SYM_FUNC_START_LOCAL_NOALIGN(name) \
> > + PRE_FUNCTION_NOPS \
> > + SYM_START(name, SYM_L_LOCAL, SYM_A_NONE) \
> > + BTI_C \
> > + PRE_PROLOGUE_NOPS
> >
> > -#define SYM_FUNC_START_WEAK(name) \
> > - SYM_START(name, SYM_L_WEAK, SYM_A_ALIGN) \
> > - bti c ;
> > +#define SYM_FUNC_START_WEAK(name) \
> > + PRE_FUNCTION_NOPS \
> > + SYM_START(name, SYM_L_WEAK, SYM_A_ALIGN) \
> > + BTI_C \
> > + PRE_PROLOGUE_NOPS
> >
> > -#define SYM_FUNC_START_WEAK_NOALIGN(name) \
> > - SYM_START(name, SYM_L_WEAK, SYM_A_NONE) \
> > - bti c ;
> > +#define SYM_FUNC_START_WEAK_NOALIGN(name) \
> > + PRE_FUNCTION_NOPS \
> > + SYM_START(name, SYM_L_WEAK, SYM_A_NONE) \
> > + BTI_C \
> > + PRE_PROLOGUE_NOPS
> >
> > -#define SYM_TYPED_FUNC_START(name) \
> > - SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
> > - bti c ;
> > +#define SYM_TYPED_FUNC_START(name) \
> > + PRE_FUNCTION_NOPS \
> > + SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN) \
> > + BTI_C \
> > + PRE_PROLOGUE_NOPS
> >
> > #endif
> > diff --git a/arch/arm64/kernel/vdso/vgetrandom-chacha.S b/arch/arm64/kernel/vdso/vgetrandom-chacha.S
> > index 67890b445309..21c27b64cf9f 100644
> > --- a/arch/arm64/kernel/vdso/vgetrandom-chacha.S
> > +++ b/arch/arm64/kernel/vdso/vgetrandom-chacha.S
> > @@ -40,7 +40,7 @@
> > * x2: 8-byte counter input/output
> > * x3: number of 64-byte block to write to output
> > */
> > -SYM_FUNC_START(__arch_chacha20_blocks_nostack)
> > +SYM_FUNC_START_NOTRACE(__arch_chacha20_blocks_nostack)
> >
> > /* copy0 = "expand 32-byte k" */
> > mov_q x8, 0x3320646e61707865
> > --
> > 2.47.3
> >
> >
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