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Date:   Mon, 11 May 2020 09:11:49 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Rafael Aquini <aquini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kexec@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        dyoung@...hat.com, corbet@....net, mcgrof@...nel.org,
        keescook@...omium.org, akpm@...ux-foundation.org, cai@....pw,
        rdunlap@...radead.org, tytso@....edu, bunk@...nel.org,
        torvalds@...ux-foundation.org, gregkh@...uxfoundation.org,
        labbott@...hat.com, jeffm@...e.com, jikos@...nel.org, jeyu@...e.de,
        tiwai@...e.de, AnDavis@...e.com, rpalethorpe@...e.de
Subject: Re: [PATCH v3] kernel: add panic_on_taint

On 05/10/20 at 02:22pm, Rafael Aquini wrote:
> > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > index 7bc83f3d9bdf..4a69fe49a70d 100644
> > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > @@ -3404,6 +3404,21 @@
> > >  	panic_on_warn	panic() instead of WARN().  Useful to cause kdump
> > >  			on a WARN().
> > >  
> > > +	panic_on_taint=	[KNL] conditionally panic() in add_taint()
> > > +			Format: <str>
> > 			Changed it as 'Format: <string>' to be
> > consistent with the existing other options?
> 
> I can resubmit with the change, if it's a strong req and the surgery
> cannot be done at merge time.

Yeah, maybe maintainer can help adjust this, not sure who will pick it.
No, it's not a strong request, people might get a little bit confusion
about which format should be referred to when a new kernel option is added.

> 
> 
> > > +			Specifies, as a string, the TAINT flag set that will
> > > +			compose a bitmask for calling panic() when the kernel
> > > +			gets tainted.
> > > +			See Documentation/admin-guide/tainted-kernels.rst for
> > > +			details on the taint flags that users can pick to
> > > +			compose the bitmask to assign to panic_on_taint.
> > > +			When the string is prefixed with a '-' the bitmask
> > > +			set in panic_on_taint will be mutually exclusive
> > > +			with the sysctl knob kernel.tainted, and any attempt
> > > +			to write to that sysctl will fail with -EINVAL for
> > > +			any taint value that masks with the flags set for
> > > +			this option.
> > > +
> > >  	crash_kexec_post_notifiers
> > >  			Run kdump after running panic-notifiers and dumping
> > >  			kmsg. This only for the users who doubt kdump always
> > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > > index 9b7a8d74a9d6..66bc102cb59a 100644
> > > --- a/include/linux/kernel.h
> > > +++ b/include/linux/kernel.h
> > > @@ -528,6 +528,8 @@ extern int panic_on_oops;
> > >  extern int panic_on_unrecovered_nmi;
> > >  extern int panic_on_io_nmi;
> > >  extern int panic_on_warn;
> > > +extern unsigned long panic_on_taint;
> > > +extern bool panic_on_taint_exclusive;
> > >  extern int sysctl_panic_on_rcu_stall;
> > >  extern int sysctl_panic_on_stackoverflow;
> > >  
> > > diff --git a/kernel/panic.c b/kernel/panic.c
> > > index b69ee9e76cb2..65c62f8a1de8 100644
> > > --- a/kernel/panic.c
> > > +++ b/kernel/panic.c
> > > @@ -25,6 +25,7 @@
> > >  #include <linux/kexec.h>
> > >  #include <linux/sched.h>
> > >  #include <linux/sysrq.h>
> > > +#include <linux/ctype.h>
> > >  #include <linux/init.h>
> > >  #include <linux/nmi.h>
> > >  #include <linux/console.h>
> > > @@ -44,6 +45,8 @@ static int pause_on_oops_flag;
> > >  static DEFINE_SPINLOCK(pause_on_oops_lock);
> > >  bool crash_kexec_post_notifiers;
> > >  int panic_on_warn __read_mostly;
> > > +unsigned long panic_on_taint;
> > > +bool panic_on_taint_exclusive = false;
> > >  
> > >  int panic_timeout = CONFIG_PANIC_TIMEOUT;
> > >  EXPORT_SYMBOL_GPL(panic_timeout);
> > > @@ -434,6 +437,11 @@ void add_taint(unsigned flag, enum lockdep_ok lockdep_ok)
> > >  		pr_warn("Disabling lock debugging due to kernel taint\n");
> > >  
> > >  	set_bit(flag, &tainted_mask);
> > > +
> > > +	if (tainted_mask & panic_on_taint) {
> > > +		panic_on_taint = 0;
> > 
> > This panic_on_taint resetting is redundant? It will trigger crash, do we
> > need care if it's 0 or not?
> >
> 
> We might still get more than one CPU hitting a taint adding code path after 
> the one that tripped here called panic. To avoid multiple calls to panic, 
> in that particular scenario, we clear the panic_on_taint bitmask out. 
> Also, albeit non-frequent, we might be tracking TAINT_WARN, and still hit 
> a WARN_ON() in the panic / kdump path, thus incurring in a second 
> (and unwanted) call to panic here.  

Hmm, this cpu will set panic_cpu firstly, all other cpu need stop and
have no chance to execute panic. But yes, clearing panic_on_taint makes
code easier to understand.

> 
>  
> > > +		panic("panic_on_taint set ...");
> > > +	}
> > >  }
> > >  EXPORT_SYMBOL(add_taint);
> > >  
> > > @@ -686,3 +694,35 @@ static int __init oops_setup(char *s)
> > >  	return 0;
> > >  }
> > >  early_param("oops", oops_setup);
> > > +
> > > +static int __init panic_on_taint_setup(char *s)
> > > +{
> > > +	/* we just ignore panic_on_taint if passed without flags */
> > > +	if (!s)
> > > +		goto out;
> > > +
> > > +	for (; *s; s++) {
> > > +		int i;
> > > +
> > > +		if (*s == '-') {
> > > +			panic_on_taint_exclusive = true;
> > > +			continue;
> > > +		}
> > > +
> > > +		for (i = 0; i < TAINT_FLAGS_COUNT; i++) {
> > > +			if (toupper(*s) == taint_flags[i].c_true) {
> > > +				set_bit(i, &panic_on_taint);
> > > +				break;
> > > +			}
> > > +		}
> > 
> > Read admin-guide/tainted-kernels.rst, but still do not get what 'G' means.
> > If I specify 'panic_on_taint="G"' or 'panic_on_taint="-G"' in cmdline,
> > what is expected for this customer behaviour?
> > 
> 
> This will not panic the system as no taint flag gets actually set in 
> panic_on_taint bitmask for G.
> 
> G is the counterpart of P, and appears on print_tainted() whenever
> TAINT_PROPRIETARY_MODULE is not set. panic_on_taint doesn't set
> anything for G, as it doesn't represent any taint, but the lack
> of one particular taint, instead.
> 
> (apparently, TAINT_PROPRIETARY_MODULE is the only taint flag
> that follows that pattern of having an extra assigned letter 
> that means its absence, and perhaps it should be removed)

Yeah, agree. I will make a draft patch to remove it, see if there's
objection from people.

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