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Message-Id: <20250921130647.9bd0cba7d49b15d0b0ebe6f7@kernel.org>
Date: Sun, 21 Sep 2025 13:06:47 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Menglong Dong <menglong.dong@...ux.dev>
Cc: Peter Zijlstra <peterz@...radead.org>, Steven Rostedt
<rostedt@...nel.org>, Menglong Dong <menglong8.dong@...il.com>,
jolsa@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
kees@...nel.org, samitolvanen@...gle.com, rppt@...nel.org, luto@...nel.org,
ast@...nel.org, andrii@...nel.org, linux-kernel@...r.kernel.org,
bpf@...r.kernel.org
Subject: Re: [PATCH] tracing: fgraph: Protect return handler from recursion
loop
On Sat, 20 Sep 2025 21:39:25 +0800
Menglong Dong <menglong.dong@...ux.dev> wrote:
> On 2025/9/19 19:57, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> >
> > function_graph_enter_regs() prevents itself from recursion by
> > ftrace_test_recursion_trylock(), but __ftrace_return_to_handler(),
> > which is called at the exit, does not prevent such recursion.
> > Therefore, while it can prevent recursive calls from
> > fgraph_ops::entryfunc(), it is not able to prevent recursive calls
> > to fgraph from fgraph_ops::retfunc(), resulting in a recursive loop.
> > This can lead an unexpected recursion bug reported by Menglong.
> >
> > is_endbr() is called in __ftrace_return_to_handler -> fprobe_return
> > -> kprobe_multi_link_exit_handler -> is_endbr.
> >
> > To fix this issue, acquire ftrace_test_recursion_trylock() in the
> > __ftrace_return_to_handler() after unwind the shadow stack to mark
> > this section must prevent recursive call of fgraph inside user-defined
> > fgraph_ops::retfunc().
> >
> > This is essentially a fix to commit 4346ba160409 ("fprobe: Rewrite
> > fprobe on function-graph tracer"), because before that fgraph was
> > only used from the function graph tracer. Fprobe allowed user to run
> > any callbacks from fgraph after that commit.
> >
> > Reported-by: Menglong Dong <menglong8.dong@...il.com>
> > Closes: https://lore.kernel.org/all/20250918120939.1706585-1-dongml2@chinatelecom.cn/
> > Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > ---
> > kernel/trace/fgraph.c | 12 ++++++++++++
> > 1 file changed, 12 insertions(+)
> >
> > diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> > index 1e3b32b1e82c..08dde420635b 100644
> > --- a/kernel/trace/fgraph.c
> > +++ b/kernel/trace/fgraph.c
> > @@ -815,6 +815,7 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
> > unsigned long bitmap;
> > unsigned long ret;
> > int offset;
> > + int bit;
> > int i;
> >
> > ret_stack = ftrace_pop_return_trace(&trace, &ret, frame_pointer, &offset);
> > @@ -829,6 +830,15 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
> > if (fregs)
> > ftrace_regs_set_instruction_pointer(fregs, ret);
> >
> > + bit = ftrace_test_recursion_trylock(trace.func, ret);
> > + /*
> > + * This must be succeeded because the entry handler returns before
> > + * modifying the return address if it is nested. Anyway, we need to
> > + * avoid calling user callbacks if it is nested.
> > + */
> > + if (WARN_ON_ONCE(bit < 0))
> > + goto out;
>
> Hi, the logic seems right, but the warning is triggered when
> I try to run the bpf bench testing:
Hmm, this is strange. Let me check why this happens.
Thank you,
>
> $ ./benchs/run_bench_trigger.sh kretprobe-multi-all
> [ 20.619642] NOTICE: Automounting of tracing to debugfs is deprecated and will be removed in 2030
> [ 139.509036] ------------[ cut here ]------------
> [ 139.509180] WARNING: CPU: 2 PID: 522 at kernel/trace/fgraph.c:839 ftrace_return_to_handler+0x2b9/0x2d0
> [ 139.509411] Modules linked in: virtio_net
> [ 139.509514] CPU: 2 UID: 0 PID: 522 Comm: bench Not tainted 6.17.0-rc5-g1fe6d652bfa0 #106 NONE
> [ 139.509720] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.17.0-1-1 04/01/2014
> [ 139.509948] RIP: 0010:ftrace_return_to_handler+0x2b9/0x2d0
> [ 139.510086] Code: e8 0c 08 0e 00 0f 0b 49 c7 c1 00 73 20 81 e9 d1 fe ff ff 40 f6 c6 10 75 11 49 c7 c3 ef ff ff ff ba 10 00 00 00 e9 57 fe ff ff <0f> 0b e9 a5 fe ff ff e8 1b 72 0d 01 66 66 2e 0f 1f 84 00 00 00 00
> [ 139.510536] RSP: 0018:ffffc9000012cef8 EFLAGS: 00010002
> [ 139.510664] RAX: ffff88810f709800 RBX: ffffc900007c3678 RCX: 0000000000000003
> [ 139.510835] RDX: 0000000000000008 RSI: 0000000000000018 RDI: 0000000000000000
> [ 139.511007] RBP: 0000000000000000 R08: 0000000000000034 R09: ffffffff82550319
> [ 139.511184] R10: ffffc9000012cf50 R11: fffffffffffffff7 R12: 0000000000000000
> [ 139.511357] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> [ 139.511532] FS: 00007fe58276fb00(0000) GS:ffff8884ab3b8000(0000) knlGS:0000000000000000
> [ 139.511724] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 139.511865] CR2: 0000562a28314b67 CR3: 00000001143f9000 CR4: 0000000000750ef0
> [ 139.512038] PKRU: 55555554
> [ 139.512106] Call Trace:
> [ 139.512177] <IRQ>
> [ 139.512232] ? irq_exit_rcu+0x4/0xb0
> [ 139.512322] return_to_handler+0x1e/0x50
> [ 139.512422] ? idle_cpu+0x9/0x50
> [ 139.512506] ? sysvec_apic_timer_interrupt+0x69/0x80
> [ 139.512638] ? idle_cpu+0x9/0x50
> [ 139.512731] ? irq_exit_rcu+0x3a/0xb0
> [ 139.512833] ? ftrace_stub_direct_tramp+0x10/0x10
> [ 139.512961] ? sysvec_apic_timer_interrupt+0x69/0x80
> [ 139.513101] </IRQ>
> [ 139.513168] <TASK>
>
> > +
> > #ifdef CONFIG_FUNCTION_GRAPH_RETVAL
> > trace.retval = ftrace_regs_get_return_value(fregs);
> > #endif
> > @@ -852,6 +862,8 @@ __ftrace_return_to_handler(struct ftrace_regs *fregs, unsigned long frame_pointe
> > }
> > }
> >
> > + ftrace_test_recursion_unlock(bit);
> > +out:
> > /*
> > * The ftrace_graph_return() may still access the current
> > * ret_stack structure, we need to make sure the update of
> >
> >
> >
>
>
>
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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