lkml.org 
[lkml]   [2020]   [Nov]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v5 14/21] kprobes: Remove NMI context check
    On Mon, 2 Nov 2020 09:27:26 -0500
    Steven Rostedt <rostedt@goodmis.org> wrote:

    >
    > [ Peter Z, please take a look a this ]
    >
    > On Mon, 2 Nov 2020 16:02:34 +0900
    > Masami Hiramatsu <mhiramat@kernel.org> wrote:
    >
    > > >From 509b27efef8c7dbf56cab2e812916d6cd778c745 Mon Sep 17 00:00:00 2001
    > > From: Masami Hiramatsu <mhiramat@kernel.org>
    > > Date: Mon, 2 Nov 2020 15:37:28 +0900
    > > Subject: [PATCH] kprobes: Disable lockdep for kprobe busy area
    > >
    > > Since the code area in between kprobe_busy_begin()/end() prohibits
    > > other kprobs to call probe handlers, we can avoid inconsitent
    > > locks there. But lockdep doesn't know that, so it warns rp->lock
    > > or kretprobe_table_lock.
    > >
    > > To supress those false-positive errors, disable lockdep while
    > > kprobe_busy is set.
    > >
    > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
    > > ---
    > > kernel/kprobes.c | 2 ++
    > > 1 file changed, 2 insertions(+)
    > >
    > > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
    > > index 8a12a25fa40d..c7196e583600 100644
    > > --- a/kernel/kprobes.c
    > > +++ b/kernel/kprobes.c
    > > @@ -1295,10 +1295,12 @@ void kprobe_busy_begin(void)
    > > __this_cpu_write(current_kprobe, &kprobe_busy);
    > > kcb = get_kprobe_ctlblk();
    > > kcb->kprobe_status = KPROBE_HIT_ACTIVE;
    > > + lockdep_off();
    > > }
    > >
    > > void kprobe_busy_end(void)
    > > {
    > > + lockdep_on();
    > > __this_cpu_write(current_kprobe, NULL);
    > > preempt_enable();
    > > }
    > > --
    >
    > No, this is not the correct workaround (too big of a hammer). You could do
    > the following:
    >
    > From 4139d9c8437b0bd2262e989ca4eb0a83b7e7bb72 Mon Sep 17 00:00:00 2001
    > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
    > Date: Mon, 2 Nov 2020 09:17:49 -0500
    > Subject: [PATCH] kprobes: Tell lockdep about kprobe nesting
    >
    > Since the kprobe handlers have protection that prohibits other handlers from
    > executing in other contexts (like if an NMI comes in while processing a
    > kprobe, and executes the same kprobe, it will get fail with a "busy"
    > return). Lockdep is unaware of this protection. Use lockdep's nesting api to
    > differentiate between locks taken in NMI context and other context to
    > supress the false warnings.

    Ah, OK. This looks good to me.

    BTW, in_nmi() in pre_handler_kretprobe() always be true because
    now int3 is treated as an NMI. So you can always pass 1 there.

    Acked-by: Masami Hiramatsu <mhiramat@kernel.org>

    Thank you,

    >
    > Link: https://lore.kernel.org/r/20201102160234.fa0ae70915ad9e2b21c08b85@kernel.org
    >
    > Cc: Peter Zijlstra <peterz@infradead.org>
    > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    > ---
    > kernel/kprobes.c | 24 ++++++++++++++++++++----
    > 1 file changed, 20 insertions(+), 4 deletions(-)
    >
    > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
    > index 8a12a25fa40d..ccb285867059 100644
    > --- a/kernel/kprobes.c
    > +++ b/kernel/kprobes.c
    > @@ -1249,7 +1249,12 @@ __acquires(hlist_lock)
    >
    > *head = &kretprobe_inst_table[hash];
    > hlist_lock = kretprobe_table_lock_ptr(hash);
    > - raw_spin_lock_irqsave(hlist_lock, *flags);
    > + /*
    > + * Nested is a workaround that will soon not be needed.
    > + * There's other protections that make sure the same lock
    > + * is not taken on the same CPU that lockdep is unaware of.
    > + */
    > + raw_spin_lock_irqsave_nested(hlist_lock, *flags, !!in_nmi());
    > }
    > NOKPROBE_SYMBOL(kretprobe_hash_lock);
    >
    > @@ -1258,7 +1263,12 @@ static void kretprobe_table_lock(unsigned long hash,
    > __acquires(hlist_lock)
    > {
    > raw_spinlock_t *hlist_lock = kretprobe_table_lock_ptr(hash);
    > - raw_spin_lock_irqsave(hlist_lock, *flags);
    > + /*
    > + * Nested is a workaround that will soon not be needed.
    > + * There's other protections that make sure the same lock
    > + * is not taken on the same CPU that lockdep is unaware of.
    > + */
    > + raw_spin_lock_irqsave_nested(hlist_lock, *flags, !!in_nmi());
    > }
    > NOKPROBE_SYMBOL(kretprobe_table_lock);
    >
    > @@ -2025,10 +2035,16 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
    > struct kretprobe *rp = container_of(p, struct kretprobe, kp);
    > unsigned long hash, flags = 0;
    > struct kretprobe_instance *ri;
    > + int nmi = !!in_nmi();
    >
    > /* TODO: consider to only swap the RA after the last pre_handler fired */
    > hash = hash_ptr(current, KPROBE_HASH_BITS);
    > - raw_spin_lock_irqsave(&rp->lock, flags);
    > + /*
    > + * Nested is a workaround that will soon not be needed.
    > + * There's other protections that make sure the same lock
    > + * is not taken on the same CPU that lockdep is unaware of.
    > + */
    > + raw_spin_lock_irqsave_nested(&rp->lock, flags, nmi);
    > if (!hlist_empty(&rp->free_instances)) {
    > ri = hlist_entry(rp->free_instances.first,
    > struct kretprobe_instance, hlist);
    > @@ -2039,7 +2055,7 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
    > ri->task = current;
    >
    > if (rp->entry_handler && rp->entry_handler(ri, regs)) {
    > - raw_spin_lock_irqsave(&rp->lock, flags);
    > + raw_spin_lock_irqsave_nested(&rp->lock, flags, nmi);
    > hlist_add_head(&ri->hlist, &rp->free_instances);
    > raw_spin_unlock_irqrestore(&rp->lock, flags);
    > return 0;
    > --
    > 2.25.4
    >


    --
    Masami Hiramatsu <mhiramat@kernel.org>

    \
     
     \ /
      Last update: 2020-11-03 06:41    [W:4.430 / U:1.884 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site