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

[Feng Tang]

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;

Thanks,
Feng