linux-kernel - Re: [PATCH] x86/efi-bgrt: remove the check of the version field

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Date:	Wed, 17 Aug 2016 13:44:13 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	Icenowy Zheng <icenowy@...c.xyz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Josh Triplett <josh@...htriplett.org>,
	Josh Boyer <jwboyer@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH] x86/efi-bgrt: remove the check of the version field

On 08/15/16 at 01:56pm, Matt Fleming wrote:
> On Tue, 09 Aug, at 01:25:46PM, Icenowy Zheng wrote:
> > Some broken firmwares have a wrongly filled version field in BGRT table.
> > (See http://wiki.osdev.org/Broken_UEFI_implementations )
> > 
> > As we know, these firmwares can also provide correct BGRT image, although
> > the table is wrong.
> > 
> > After removing the check of the version field, the kernel can now extract
> > the image correctly, and the information is also correct.
> > 
> > Tested on a Thinkpad E531 (68854UC).
> > 
> > Signed-off-by: Icenowy Zheng <icenowy@...c.xyz>
> > ---
> >  arch/x86/platform/efi/efi-bgrt.c | 5 -----
> >  1 file changed, 5 deletions(-)
> > 
> > diff --git a/arch/x86/platform/efi/efi-bgrt.c b/arch/x86/platform/efi/efi-bgrt.c
> > index 6a2f569..f492ea0 100644
> > --- a/arch/x86/platform/efi/efi-bgrt.c
> > +++ b/arch/x86/platform/efi/efi-bgrt.c
> > @@ -47,11 +47,6 @@ void __init efi_bgrt_init(void)
> >  		       bgrt_tab->header.length, sizeof(*bgrt_tab));
> >  		return;
> >  	}
> > -	if (bgrt_tab->version != 1) {
> > -		pr_notice("Ignoring BGRT: invalid version %u (expected 1)\n",
> > -		       bgrt_tab->version);
> > -		return;
> > -	}
> >  	if (bgrt_tab->status & 0xfe) {
> >  		pr_notice("Ignoring BGRT: reserved status bits are non-zero %u\n",
> >  		       bgrt_tab->status);
> 
> This would be less scary if we checked for known broken and known good
> version values instead of removing the check altogether, i.e. 0 and 1.

Could we add some quirk for these broken hardware instead of changing
the normal code?

> 
> The whole point of the version field is that it tells us about the
> layout of the BGRT table, so it's not exactly a useless check.

Agreed.

Thanks
Dave