linux-kernel - Re: [PATCH 1/1] x86/fpu: Allow clearcpuid= to clear several bits

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Message-ID: <20200422143554.GI608746@tassilo.jf.intel.com>
Date:   Wed, 22 Apr 2020 07:35:54 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     John Haxby <john.haxby@...cle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>,
        x86@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/1] x86/fpu: Allow clearcpuid= to clear several bits


Thanks good catch.

>  	if (cmdline_find_option(boot_command_line, "clearcpuid", arg,
> -				sizeof(arg)) &&
> -	    get_option(&argptr, &bit) &&
> -	    bit >= 0 &&
> -	    bit < NCAPINTS * 32)
> -		setup_clear_cpu_cap(bit);
> +				sizeof(arg))) {
> +		/* cpuid bit numbers are mostly three digits */
> +		enum  { nints = sizeof(arg)/(3+1) + 1 };

Not sure what the digits have to do with the stack space of an int array.

We should have enough stack to afford some more than 8.

Would be good to have a warning if the arguments are longer.

Maybe it would be simpler to fix the early arg parser
to allow multiple instances again? That would also avoid the limit,
and keep everything compatible.

-Andi


> +		int i, bits[nints];
> +
> +		get_options(arg, nints, bits);
> +		for (i = 1; i <= bits[0]; i++) {
> +			if (bits[i] >= 0 && bits[i] < NCAPINTS * 32)
> +				setup_clear_cpu_cap(bits[i]);
> +		}
> +	}
>  }
>  
>  /*
> -- 
> 2.25.3
>