linux-kernel - Re: [PATCH] sched/isolation: don't do unbounded chomp on bootarg string

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Date:   Mon, 19 Apr 2021 16:38:55 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH] sched/isolation: don't do unbounded chomp on bootarg
 string

On Sun, Apr 18, 2021 at 05:54:26PM -0400, Paul Gortmaker wrote:
> After commit 3662daf02350 ("sched/isolation: Allow "isolcpus=" to skip
> unknown sub-parameters") the isolcpus= string is walked to skip over what
> might be any future flag comma separated additions.
> 
> However, there is a logic error, and so as can clearly be seen below, it
> will ignore its own arg len and search to the end of the bootarg string.
> 
>  $ dmesg|grep isol
>  Command line: BOOT_IMAGE=/boot/bzImage isolcpus=xyz pleasedontparseme=1 root=/dev/sda1 ro
>  isolcpus: Skipped unknown flag xyz
>  isolcpus: Invalid flag pleasedontparseme=1 root=/dev/sda1 ro
> 
> This happens because the flag "skip" code does an unconditional
> increment, which skips over the '\0' check the loop body looks for. If
> the isolcpus= happens to be the last bootarg, then you'd never notice?
> 
> So we only increment if the skipped flag is followed by a comma, as per
> what the existing "continue" flag matching code does.
> 
> Note that isolcpus= was declared deprecated as of v4.15 (b0d40d2b22fe),
> so we might want to revisit that if we are trying to future-proof it
> as recently as a year ago for as yet unseen new flags.

Thanks for report the issue.

Is cpuset going to totally replace "isolcpus="?  It seems most hk_flags will be
handled by nohz_full=, and HK_FLAG_DOMAIN can be done by cpuset.  However it
seems still the only place to set the new flag HK_FLAG_MANAGED_IRQ.  If one day
we'll finally obsolete isolcpus= we may need to think about where to put it?

When I looked at it, I also noticed I see no caller to set HK_FLAG_SCHED at
all.  Is it really used anywhere?

Regarding this patch...

> 
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Fixes: 3662daf02350 ("sched/isolation: Allow "isolcpus=" to skip unknown sub-parameters")
> Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
> 
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 5a6ea03f9882..9652dba7e938 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -188,7 +188,8 @@ static int __init housekeeping_isolcpus_setup(char *str)
>  		}
>  
>  		pr_info("isolcpus: Skipped unknown flag %.*s\n", len, par);
> -		str++;
> +		if (str[1] == ',')	/* above continue; match on "flag," */

.. wondering why it is not "str[0] == ','" instead?

Thanks,

> +			str++;
>  	}
>  
>  	/* Default behaviour for isolcpus without flags */
> -- 
> 2.25.1
> 

-- 
Peter Xu