From: Masami Hiramatsu <mhiramat@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
Andy Lutomirski <luto@amacapital.net>,
Andy Lutomirski <luto@kernel.org>, Yonghong Song <yhs@fb.com>,
Jann Horn <jannh@google.com>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andriin@fb.com>, Martin KaFai Lau <kafai@fb.com>,
bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
kernel-team <kernel-team@fb.com>, X86 ML <x86@kernel.org>,
KP Singh <kpsingh@kernel.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
systemtap@sourceware.org
Subject: Re: [PATCH bpf] x86/bpf: handle bpf-program-triggered exceptions properly
Date: Fri, 29 Jan 2021 22:57:07 +0900 [thread overview]
Message-ID: <20210129225707.225ee83f7d204c0f119dca5f@kernel.org> (raw)
In-Reply-To: <YBPeh9VYq+YdkMNg@hirez.programming.kicks-ass.net>
On Fri, 29 Jan 2021 11:08:07 +0100
Peter Zijlstra <peterz@infradead.org> wrote:
> On Fri, Jan 29, 2021 at 10:44:48AM +0100, Peter Zijlstra wrote:
> > There is one case where it hijacks the fault entirely, and I'm tempted
> > to rip that out, that's just gross. Also, it seems entirely unused in-kernel.
>
> Masami, please explain why the below isn't appropriate.
>
> ---
> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> index df776cdca327..86cd8f15a978 100644
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -949,9 +949,13 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
> * if handler tries to access user space by
> * copy_from_user(), get_user() etc. Let the
> * user-specified handler try to fix it first.
> - */
> + *
> + * Which is a bloody stupid thing to do from non-preemptible code
> + * so why should we support idiocy like that.
> + *
> if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
> return 1;
> + */
> }
Hmm, good point. This is a fail-safe code which, as far as I know, systemtap
uses this hook to count faults and notify user an error (e.g. guru-mode).
I just maintained it to preserve the use case. Actually, in the kernel there
is no fault handler user. e.g. kprobe tracer uses non-fault (safe) kernel
memory access functions.
>
> return 0;
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 106b22d1d189..817a93da794e 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1186,7 +1186,7 @@ do_kern_addr_fault(struct pt_regs *regs, unsigned long hw_error_code,
> return;
>
> /* kprobes don't want to hook the spurious faults: */
> - if (kprobe_page_fault(regs, X86_TRAP_PF))
> + if (WARN_ON_ONCE(kprobe_page_fault(regs, X86_TRAP_PF)))
> return;
>
> /*
> @@ -1217,7 +1217,7 @@ void do_user_addr_fault(struct pt_regs *regs,
> mm = tsk->mm;
>
> /* kprobes don't want to hook the spurious faults: */
> - if (unlikely(kprobe_page_fault(regs, X86_TRAP_PF)))
> + if (WARN_ON_ONCE(kprobe_page_fault(regs, X86_TRAP_PF)))
> return;
>
> /*
--
Masami Hiramatsu <mhiramat@kernel.org>
parent reply other threads:[~2021-01-29 13:57 UTC|newest]
Thread overview: expand[flat|nested] mbox.gz Atom feed
[parent not found: <YBPeh9VYq+YdkMNg@hirez.programming.kicks-ass.net>]
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