linux-kernel - Re: [PATCH v2] x86/earlyprintk: Add a force option for pciserial device

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Date:   Tue, 2 Oct 2018 23:31:10 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        H Peter Anvin <hpa@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Stuart R . Anderson" <stuart.r.anderson@...el.com>,
        alan@...ux.intel.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/earlyprintk: Add a force option for pciserial
 device

On Tue, Oct 02, 2018 at 08:10:46PM +0800, Feng Tang wrote:
> Hi Thomas,
> 
> On Tue, Oct 02, 2018 at 12:44:36PM +0200, Thomas Gleixner wrote:
> > On Tue, 2 Oct 2018, Feng Tang wrote:
> > > On Tue, Oct 02, 2018 at 11:17:57AM +0200, Thomas Gleixner wrote:
> > > > On Tue, 2 Oct 2018, Feng Tang wrote:
> > > > 
> > > > > Hi Boris,
> > > > > 
> > > > > On Mon, Oct 01, 2018 at 10:30:04PM +0200, Borislav Petkov wrote:
> > > > > > On Mon, Oct 01, 2018 at 10:18:10PM +0800, Feng Tang wrote:
> > > > > > > As I rechecked, the baud rate for pciserial is optional, so there may
> > > > > > > be no ",baudrate" following the "force". So this 2 strncmp is to
> > > > > > > cover conditions for with and without baudrate.
> > > > > > 
> > > > > > And what guarantees you have a space after the "force"?
> > > > > > 
> > > > > > 	!strncmp(s, "force ", 6)
> > > > > > 			  ^
> > > > > 
> > > > > You are right, it can't be guranteed. Can we still use strncmp(s, "force", 5)
> > > > > and rely on developer to set it right? any suggestions? thanks,
> > > > 
> > > > I don't know why you want strncmp() in the first place. "force" is null
> > > > terminated already.
> > > 
> > > Current code uses: 
> > > 	earlyprintk=pciserial,bus:device.function[,baudrate]
> > > with the patch, it will be:
> > > 	earlyprintk=pciserial,bus:device.function[,force][,baudrate]
> > > 
> > > So the force could be followed by ",baudrate".
> > 
> > Sure, but that has nothing to do with strncmp(). The earlyprintk argument
> > string is tokenized by commata. It really does not matter where the 'force'
> > string is, but it matters that you check whether it is 'force' and not
> > 'force$RANDOMCHARACTERS'. That's why strncmp() is the wrong thing to do,
> > because it only compares the first 5 characters and ignores anything
> > beyond, unless you make sure that the command line part is exactly 5
> > characters long.
> 
> I don't know if I get it right due to my poor English, so for both cases
> of "xxx,force,baudrate" and "xxx,force", it could be covered by
> 
> 	if ((!strcmp(s, "force") || !strncmp(s, "force," 6))
> 		force = 1;

One idea is to change the new format to
	earlyprintk=pciserial[,force],bus:device.function[,baudrate]
Then we can use what Boris has suggested:
	!strncmp(s, "force,", 6)

Any thoughts?

Thanks,
Feng