From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3xtDmt1zfbzDr2L for ; Thu, 14 Sep 2017 20:10:22 +1000 (AEST) Date: Thu, 14 Sep 2017 03:10:19 -0700 From: Masami Hiramatsu To: "Naveen N. Rao" Cc: Michael Ellerman , linuxppc-dev@lists.ozlabs.org, Ananth N Mavinakayanahalli , Kamalesh Babulal Subject: Re: [PATCH 3/5] powerpc/kprobes: Fix warnings from __this_cpu_read() on preempt kernels Message-Id: <20170914031019.6660f4533c10691816dd27d1@kernel.org> In-Reply-To: <20170914064720.u36di37zsascsqtp@naverao1-tp.localdomain> References: <2bc413d679c563d3ee338c318066777318577ab2.1505336870.git.naveen.n.rao@linux.vnet.ibm.com> <63b0f9f3fd3d95d758678a892b87b7c561545c99.1505336870.git.naveen.n.rao@linux.vnet.ibm.com> <20170913173655.1ada3c799d98cab104c45cae@kernel.org> <20170914064720.u36di37zsascsqtp@naverao1-tp.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, 14 Sep 2017 12:17:20 +0530 "Naveen N. Rao" wrote: > On 2017/09/13 05:36PM, Masami Hiramatsu wrote: > > On Thu, 14 Sep 2017 02:50:34 +0530 > > "Naveen N. Rao" wrote: > > > > > Kamalesh pointed out that we are getting the below call traces with > > > livepatched functions when we enable CONFIG_PREEMPT: > > > > > > [ 495.470721] BUG: using __this_cpu_read() in preemptible [00000000] code: cat/8394 > > > [ 495.471167] caller is is_current_kprobe_addr+0x30/0x90 > > > [ 495.471171] CPU: 4 PID: 8394 Comm: cat Tainted: G K 4.13.0-rc7-nnr+ #95 > > > [ 495.471173] Call Trace: > > > [ 495.471178] [c00000008fd9b960] [c0000000009f039c] dump_stack+0xec/0x160 (unreliable) > > > [ 495.471184] [c00000008fd9b9a0] [c00000000059169c] check_preemption_disabled+0x15c/0x170 > > > [ 495.471187] [c00000008fd9ba30] [c000000000046460] is_current_kprobe_addr+0x30/0x90 > > > [ 495.471191] [c00000008fd9ba60] [c00000000004e9a0] ftrace_call+0x1c/0xb8 > > > [ 495.471195] [c00000008fd9bc30] [c000000000376fd8] seq_read+0x238/0x5c0 > > > [ 495.471199] [c00000008fd9bcd0] [c0000000003cfd78] proc_reg_read+0x88/0xd0 > > > [ 495.471203] [c00000008fd9bd00] [c00000000033e5d4] __vfs_read+0x44/0x1b0 > > > [ 495.471206] [c00000008fd9bd90] [c0000000003402ec] vfs_read+0xbc/0x1b0 > > > [ 495.471210] [c00000008fd9bde0] [c000000000342138] SyS_read+0x68/0x110 > > > [ 495.471214] [c00000008fd9be30] [c00000000000bc6c] system_call+0x58/0x6c > > > > > > Commit c05b8c4474c030 ("powerpc/kprobes: Skip livepatch_handler() for > > > jprobes") introduced a helper is_current_kprobe_addr() to help determine > > > if the current function has been livepatched or if it has a jprobe > > > installed, both of which modify the NIP. > > > > > > In the case of a jprobe, kprobe_ftrace_handler() disables pre-emption > > > before calling into setjmp_pre_handler() which returns without disabling > > > pre-emption. This is done to ensure that the jprobe han dler won't > > > disappear beneath us if the jprobe is unregistered between the > > > setjmp_pre_handler() and the subsequent longjmp_break_handler() called > > > from the jprobe handler. Due to this, we can use __this_cpu_read() in > > > is_current_kprobe_addr() with the pre-emption check as we know that > > > pre-emption will be disabled. > > > > > > However, if this function has been livepatched, we are still doing this > > > check and when we do so, pre-emption won't necessarily be disabled. This > > > results in the call trace shown above. > > > > > > Fix this by only invoking is_current_kprobe_addr() when pre-emption is > > > disabled. And since we now guard this within a pre-emption check, we can > > > instead use raw_cpu_read() to get the current_kprobe value skipping the > > > check done by __this_cpu_read(). > > > > Hmm, can you disable preempt temporary at this function and read it? > > Yes, but I felt this approach is more optimal specifically for live > patching. We don't normally expect preemption to be disabled while > handling a livepatched function, so the simple 'if (!preemptible())' > check helps us bypass looking at current_kprobe. Ah, I see. this is for checking "jprobes", since kprobes/kretprobes usually already done there. > > The check also ensures we can use raw_cpu_read() safely in other > scenarios. Do you see any other concerns with this approach? Yes, there are 2 reasons. At first, if user's custom kprobe pre-handler changes NIP for their custom handwriting livepatching, such handler will enable preemption before return. We don't prohibit it. I think this function can not detect it. And also, the function "name" is very confusing :) Especially, since this symbol is exported, if you are checking jprobes context, it should be renamed, at least it starts with "__" so that show it as local use. Thank you, > > Thanks, > Naveen > > > > > Thank you, > > > > > > > > Fixes: c05b8c4474c030 ("powerpc/kprobes: Skip livepatch_handler() for jprobes") > > > Reported-by: Kamalesh Babulal > > > Signed-off-by: Naveen N. Rao > > > --- > > > arch/powerpc/kernel/kprobes.c | 8 ++++++-- > > > 1 file changed, 6 insertions(+), 2 deletions(-) > > > > > > diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c > > > index e848fe2c93fb..db40b13fd3d1 100644 > > > --- a/arch/powerpc/kernel/kprobes.c > > > +++ b/arch/powerpc/kernel/kprobes.c > > > @@ -45,8 +45,12 @@ struct kretprobe_blackpoint kretprobe_blacklist[] = {{NULL, NULL}}; > > > > > > int is_current_kprobe_addr(unsigned long addr) > > > { > > > - struct kprobe *p = kprobe_running(); > > > - return (p && (unsigned long)p->addr == addr) ? 1 : 0; > > > + if (!preemptible()) { > > > + struct kprobe *p = raw_cpu_read(current_kprobe); > > > + return (p && (unsigned long)p->addr == addr) ? 1 : 0; > > > + } > > > + > > > + return 0; > > > } > > > > > > bool arch_within_kprobe_blacklist(unsigned long addr) > > > -- > > > 2.14.1 > > > > > > > > > -- > > Masami Hiramatsu > > > -- Masami Hiramatsu