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Message-ID: <YzWgnyCNO4tEl4JT@FVFF77S0Q05N>
Date: Thu, 29 Sep 2022 14:41:51 +0100
From: Mark Rutland <mark.rutland@....com>
To: Li Huafei <lihuafei1@...wei.com>
Cc: catalin.marinas@....com, will@...nel.org, rostedt@...dmis.org,
mingo@...hat.com, Julia.Lawall@...ia.fr, akpm@...ux-foundation.org,
andreyknvl@...il.com, elver@...gle.com, wangkefeng.wang@...wei.com,
zhouchengming@...edance.com, ardb@...nel.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] arm64: module/ftrace: Fix mcount-based ftrace
initialization failure
On Thu, Sep 29, 2022 at 08:26:17PM +0800, Li Huafei wrote:
>
>
> On 2022/9/29 19:59, Mark Rutland wrote:
> > On Thu, Sep 29, 2022 at 12:26:52PM +0100, Mark Rutland wrote:
> >> On Thu, Sep 29, 2022 at 05:41:34PM +0800, Li Huafei wrote:
> >>> The commit a6253579977e ("arm64: ftrace: consistently handle PLTs.")
> >>> makes ftrace_make_nop() always validate the 'old' instruction that will
> >>> be replaced. However, in the mcount-based implementation,
> >>> ftrace_init_nop() also calls ftrace_make_nop() to do the initialization,
> >>> and the 'old' target address is MCOUNT_ADDR at this time. with
> >>> CONFIG_MODULE_PLT support, the distance between MCOUNT_ADDR and callsite
> >>> may exceed 128M, at which point ftrace_find_callable_addr() will fail
> >>> because it cannot find an available PLT.
> >>
> >> Ah, sorry about this.
> >>
> >>> We can reproduce this problem by forcing the module to alloc memory away
> >>> from the kernel:
> >>>
> >>> ftrace_test: loading out-of-tree module taints kernel.
> >>> ftrace: no module PLT for _mcount
> >>> ------------[ ftrace bug ]------------
> >>> ftrace failed to modify
> >>> [<ffff800029180014>] 0xffff800029180014
> >>> actual: 44:00:00:94
> >>> Initializing ftrace call sites
> >>> ftrace record flags: 2000000
> >>> (0)
> >>> expected tramp: ffff80000802eb3c
> >>> ------------[ cut here ]------------
> >>> WARNING: CPU: 3 PID: 157 at kernel/trace/ftrace.c:2120 ftrace_bug+0x94/0x270
> >>> Modules linked in:
> >>> CPU: 3 PID: 157 Comm: insmod Tainted: G O 6.0.0-rc6-00151-gcd722513a189-dirty #22
> >>> Hardware name: linux,dummy-virt (DT)
> >>> pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> >>> pc : ftrace_bug+0x94/0x270
> >>> lr : ftrace_bug+0x21c/0x270
> >>> sp : ffff80000b2bbaf0
> >>> x29: ffff80000b2bbaf0 x28: 0000000000000000 x27: ffff0000c4d38000
> >>> x26: 0000000000000001 x25: ffff800009d7e000 x24: ffff0000c4d86e00
> >>> x23: 0000000002000000 x22: ffff80000a62b000 x21: ffff8000098ebea8
> >>> x20: ffff0000c4d38000 x19: ffff80000aa24158 x18: ffffffffffffffff
> >>> x17: 0000000000000000 x16: 0a0d2d2d2d2d2d2d x15: ffff800009aa9118
> >>> x14: 0000000000000000 x13: 6333626532303830 x12: 3030303866666666
> >>> x11: 203a706d61727420 x10: 6465746365707865 x9 : 3362653230383030
> >>> x8 : c0000000ffffefff x7 : 0000000000017fe8 x6 : 000000000000bff4
> >>> x5 : 0000000000057fa8 x4 : 0000000000000000 x3 : 0000000000000001
> >>> x2 : ad2cb14bb5438900 x1 : 0000000000000000 x0 : 0000000000000022
> >>> Call trace:
> >>> ftrace_bug+0x94/0x270
> >>> ftrace_process_locs+0x308/0x430
> >>> ftrace_module_init+0x44/0x60
> >>> load_module+0x15b4/0x1ce8
> >>> __do_sys_init_module+0x1ec/0x238
> >>> __arm64_sys_init_module+0x24/0x30
> >>> invoke_syscall+0x54/0x118
> >>> el0_svc_common.constprop.4+0x84/0x100
> >>> do_el0_svc+0x3c/0xd0
> >>> el0_svc+0x1c/0x50
> >>> el0t_64_sync_handler+0x90/0xb8
> >>> el0t_64_sync+0x15c/0x160
> >>> ---[ end trace 0000000000000000 ]---
> >>> ---------test_init-----------
> >>>
> >>> In fact, in .init.plt or .plt or both of them, we have the mcount PLT.
> >>> If we save the mcount PLT entry address, we can determine what the 'old'
> >>> instruction should be when initializing the nop instruction.
> >>>
> >>> Fixes: a6253579977e ("arm64: ftrace: consistently handle PLTs.")
> >>> Signed-off-by: Li Huafei <lihuafei1@...wei.com>
> >>> ---
> >>> arch/arm64/include/asm/module.h | 7 +++++++
> >>> arch/arm64/kernel/ftrace.c | 29 ++++++++++++++++++++++++++++-
> >>> arch/arm64/kernel/module-plts.c | 16 ++++++++++++++++
> >>> arch/arm64/kernel/module.c | 11 +++++++++++
> >>> 4 files changed, 62 insertions(+), 1 deletion(-)
> >>
> >> Since this only matters for the initalization of a module callsite, I'd rather
> >> we simply didn't check in this case, so that we don't have to go scanning for
> >> the PLTs and keep that information around forever.
> >>
> >> To be honest, I'd rather we simply didn't check when initializing an mcount
> >> call-site for a module, as we used to do prior to commit a6253579977e.
>
> Yes, I agree. If it's just for the initialization phase validation, my patch does make a bit of a fuss.
>
> >>
> >> Does the below work for you?
> >
> > Thinking some more, that's probably going to warn in the insn code when
> > unconditionally generating the 'old' branch; I'll spin a new version after some
> > testing.
> >
>
> I see it. And ftrace_find_callable_addr() would still fail.
Ah, yes, since that points to the `_mcount` stub, but we'll generate the
address of the module's ftrace PLT.
>
> With a slight modification, it worked for me:
>
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index ea5dc7c90f46..621c62238d96 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -216,14 +216,28 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
> {
> unsigned long pc = rec->ip;
> u32 old = 0, new;
> + bool validate = true;
> +
> + /*
> + * When using mcount, calls can be indirected via a PLT generated by
> + * the toolchain. Ignore this when initializing the callsite.
> + *
> + * Note: `mod` is only set at module load time.
> + */
> + if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS) &&
> + IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) && mod) {
> + validate = false;
> + goto make_nop;
> + }
>
> if (!ftrace_find_callable_addr(rec, mod, &addr))
> return -EINVAL;
>
> old = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
> +make_nop:
> new = aarch64_insn_gen_nop();
>
> - return ftrace_modify_code(pc, old, new, true);
> + return ftrace_modify_code(pc, old, new, validate);
> }
Great; I'll clean this up a bit and post as a patch shortly.
Thanks,
Mark.
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