From: Peter Zijlstra <firstname.lastname@example.org> To: Steven Rostedt <email@example.com> Cc: Alexei Starovoitov <firstname.lastname@example.org>, Masami Hiramatsu <email@example.com>, Nikolay Borisov <firstname.lastname@example.org>, LKML <email@example.com>, Alexei Starovoitov <firstname.lastname@example.org>, bpf <email@example.com>, Josh Poimboeuf <firstname.lastname@example.org> Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()") Date: Tue, 2 Feb 2021 17:45:47 +0100 Message-ID: <YBmBu0c24RjNYFet@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <email@example.com> On Tue, Feb 02, 2021 at 09:52:49AM -0500, Steven Rostedt wrote: > But from a handler, you could do: > > if (in_nmi()) > return; > local_irq_save(flags); > /* Now you are safe from being re-entrant. */ But that's an utter crap thing to do. That's like saying I don't care about my events, at which point you might as well not bother at all. And you can still do that, you just get less coverage today than you used to. You used to throw things under the bus, now you throw more under the bus. If you didn't care, I can't seem to find myself caring either. > Where as there's no equivalent in a NMI handler. That's what makes > kprobe/ftrace handlers different than NMI handlers. I don't see how. > > Also, given how everything can nest, it had better all be lockless > > anyway. You can get your regular function trace interrupted, which can > > hit a #DB, which can function trace, which can #BP which can function > > trace again which can get #NMI etc.. Many wonderfun nestings possible. > > I would call #DB an #BP handlers very special. They are, just like NMI is special, which is why they're classed together. > Question: Do #DB and #BP set "in_interrupt()"? Because the function tracer > has infrastructure to prevent recursion in the same context. Sure we _could_ do that, but then we get into the 'fun' problem of getting a breakpoint/int3 at random places and calling random code and having deadlocks because they take the same lock. There was very little that stopped that from happening. > That is, a > ftrace handler calls something that gets traced, the recursion protection > will detect that and prevent the handler from being called again. But the > recursion protection is interrupt context aware and lets the handler get > called again if the recursion happens from a different context: > If #DB and #BP do not change the in_interrupt() context, then the above > still will protect the ftrace handlers from recursion due to them. But it doesn't help with: spin_lock_irq(&foo); // task context #DB spin_lock_irq(&foo); // interrupt context per your above All you need to do is put a breakpoint on a piece of code that holds a spinlock and a handler that takes the same spinlock. There was very little from stopping that. > That would require refactoring all the code that's been around since 2008. Because I couldn't tell why/if any of that was correct at all. #DB/#BP don't play by the normal rules. They're _far_ more NMI-like than they're IRQ-like due to ignoring IF.
next prev parent reply index Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <firstname.lastname@example.org> [not found] ` <email@example.com> [not found] ` <firstname.lastname@example.org> [not found] ` <email@example.com> [not found] ` <firstname.lastname@example.org> 2021-01-28 3:38 ` Masami Hiramatsu 2021-01-28 7:11 ` Nikolay Borisov 2021-01-28 16:12 ` Nikolay Borisov 2021-01-28 16:45 ` Nikolay Borisov 2021-01-28 16:50 ` Josh Poimboeuf 2021-01-28 21:52 ` [PATCH] x86: Disable CET instrumentation in the kernel Josh Poimboeuf 2021-01-29 6:23 ` Nikolay Borisov 2021-01-29 10:21 ` Borislav Petkov [not found] ` <20210129151034.iba4eaa2fuxsipqa@treble> 2021-01-29 16:30 ` Borislav Petkov 2021-01-29 16:49 ` Josh Poimboeuf 2021-01-29 16:54 ` Nikolay Borisov 2021-01-29 17:03 ` Josh Poimboeuf 2021-01-29 17:07 ` Borislav Petkov 2021-01-29 17:58 ` Seth Forshee 2021-01-28 18:24 ` kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()") Peter Zijlstra 2021-01-29 1:34 ` Alexei Starovoitov 2021-01-29 6:36 ` Nikolay Borisov [not found] ` <YBPNyRyrkzw2echi@hirez.programming.kicks-ass.net> [not found] ` <email@example.com> [not found] ` <firstname.lastname@example.org> 2021-01-29 16:24 ` Peter Zijlstra 2021-01-29 17:45 ` Alexei Starovoitov 2021-01-29 17:59 ` Peter Zijlstra 2021-01-29 19:01 ` Steven Rostedt 2021-01-29 21:05 ` Alexei Starovoitov 2021-01-30 1:41 ` Masami Hiramatsu 2021-01-29 21:24 ` Steven Rostedt 2021-01-30 8:28 ` Peter Zijlstra 2021-01-30 12:44 ` Steven Rostedt 2021-02-02 10:45 ` Peter Zijlstra 2021-02-02 14:52 ` Steven Rostedt 2021-02-02 16:45 ` Peter Zijlstra [this message] 2021-02-02 16:56 ` Steven Rostedt 2021-02-02 18:30 ` Peter Zijlstra 2021-02-02 21:05 ` Steven Rostedt 2021-02-03 13:33 ` Masami Hiramatsu 2021-02-03 13:52 ` Steven Rostedt 2021-01-30 2:02 ` Masami Hiramatsu 2021-01-30 3:08 ` Alexei Starovoitov 2021-01-30 12:10 ` Masami Hiramatsu
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=YBmBu0c24RjNYFet@hirez.programming.kicks-ass.net \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
BPF Archive on lore.kernel.org Archives are clonable: git clone --mirror https://lore.kernel.org/bpf/0 bpf/git/0.git # If you have public-inbox 1.1+ installed, you may # initialize and index your mirror using the following commands: public-inbox-init -V2 bpf bpf/ https://lore.kernel.org/bpf \ firstname.lastname@example.org public-inbox-index bpf Example config snippet for mirrors Newsgroup available over NNTP: nntp://nntp.lore.kernel.org/org.kernel.vger.bpf AGPL code for this site: git clone https://public-inbox.org/public-inbox.git