linux-kernel - Re: [PATCH 2/2] arch/x86/xen: Silence compiler warnings

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Date:	Sat, 12 Jul 2014 01:45:48 +0200
From:	Daniel Kiper <daniel.kiper@...cle.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	xen-devel@...ts.xenproject.org, andrew.cooper3@...rix.com,
	david.vrabel@...rix.com, hpa@...or.com, ian.campbell@...rix.com,
	jbeulich@...e.com, jeremy@...p.org, konrad.wilk@...cle.com,
	matt.fleming@...el.com, mingo@...hat.com,
	stefano.stabellini@...citrix.com, tglx@...utronix.de
Subject: Re: [PATCH 2/2] arch/x86/xen: Silence compiler warnings

On Fri, Jul 11, 2014 at 04:32:27PM -0400, Boris Ostrovsky wrote:
> On 07/11/2014 04:10 PM, Daniel Kiper wrote:
> >On Fri, Jul 11, 2014 at 04:03:46PM -0400, Boris Ostrovsky wrote:
> >>On 07/11/2014 03:54 PM, Daniel Kiper wrote:
> >>>Compiler complains in the following way when x86 32-bit kernel
> >>>with Xen support is build:
> >>>
> >>>   CC      arch/x86/xen/enlighten.o
> >>>arch/x86/xen/enlighten.c: In function ‘xen_start_kernel’:
> >>>arch/x86/xen/enlighten.c:1726:3: warning: right shift count >= width of type [enabled by default]
> >>>
> >>>Such line contains following EFI initialization code:
> >>>
> >>>boot_params.efi_info.efi_systab_hi = (__u32)(__pa(efi_systab_xen) >> 32);
> >>>
> >>>There is no issue if x86 64-bit kernel is build. However, 32-bit case
> >>>generate warning (even if that code will not be executed because Xen
> >>>does not work on 32-bit EFI platforms) due to __pa() returning unsigned long
> >>>type which has 32-bits width. So move whole EFI initialization stuff
> >>>to separate function and build its body conditionally to avoid above
> >>>mentioned warning on x86 32-bit architecture.
> >>>
> >>>Signed-off-by: Daniel Kiper <daniel.kiper@...cle.com>
> >>>---
> >>>  arch/x86/xen/enlighten.c |   35 ++++++++++++++++++++++-------------
> >>>  1 file changed, 22 insertions(+), 13 deletions(-)
> >>>
> >>>diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> >>>index bc89647..6abec74 100644
> >>>--- a/arch/x86/xen/enlighten.c
> >>>+++ b/arch/x86/xen/enlighten.c
> >>>@@ -1516,12 +1516,32 @@ static void __init xen_pvh_early_guest_init(void)
> >>>  #endif
> >>>  }
> >>>+static void __init xen_efi_init(void)
> >>>+{
> >>>+#ifdef CONFIG_XEN_EFI
> >>>+	efi_system_table_t *efi_systab_xen;
> >>>+
> >>>+	efi_systab_xen = xen_efi_probe();
> >>>+
> >>>+	if (efi_systab_xen == NULL)
> >>>+		return;
> >>>+
> >>>+	strncpy((char *)&boot_params.efi_info.efi_loader_signature, "Xen",
> >>>+			sizeof(boot_params.efi_info.efi_loader_signature));
> >>>+	boot_params.efi_info.efi_systab = (__u32)__pa(efi_systab_xen);
> >>>+	boot_params.efi_info.efi_systab_hi = (__u32)(__pa(efi_systab_xen) >> 32);
> >>>+
> >>>+	set_bit(EFI_BOOT, &efi.flags);
> >>>+	set_bit(EFI_PARAVIRT, &efi.flags);
> >>>+	set_bit(EFI_64BIT, &efi.flags);
> >>>+#endif
> >>>+}
> >>>+
> >>>  /* First C function to be called on Xen boot */
> >>>  asmlinkage __visible void __init xen_start_kernel(void)
> >>>  {
> >>>  	struct physdev_set_iopl set_iopl;
> >>>  	int rc;
> >>>-	efi_system_table_t *efi_systab_xen;
> >>>  	if (!xen_start_info)
> >>>  		return;
> >>>@@ -1717,18 +1737,7 @@ asmlinkage __visible void __init xen_start_kernel(void)
> >>>  	xen_setup_runstate_info(0);
> >>>-	efi_systab_xen = xen_efi_probe();
> >>>-
> >>>-	if (efi_systab_xen) {
> >>>-		strncpy((char *)&boot_params.efi_info.efi_loader_signature, "Xen",
> >>>-				sizeof(boot_params.efi_info.efi_loader_signature));
> >>>-		boot_params.efi_info.efi_systab = (__u32)__pa(efi_systab_xen);
> >>>-		boot_params.efi_info.efi_systab_hi = (__u32)(__pa(efi_systab_xen) >> 32);
> >>>-
> >>>-		set_bit(EFI_BOOT, &efi.flags);
> >>>-		set_bit(EFI_PARAVIRT, &efi.flags);
> >>>-		set_bit(EFI_64BIT, &efi.flags);
> >>>-	}
> >>>+	xen_efi_init();
> >>I'd put ifdef CONFIG_XEN_EFI around the call instead of having it
> >>inside the routine.
> >Well, I thought about that a bit and I prefer function like Konrad.
> >Could you agree with him which solution do you (as maintainers) prefer?
> >
>
> I am not arguing against having a separate routine. All I am saying
> is that calling xen_efi_init() when CONFIG_XEN_EFI is not defined
> doesn't look logical. It will also add an unnecessary call (although

Ahh... I misunderstood you. However, your proposal, as below:

#ifdef CONFIG_XEN_EFI
  xen_efi_init();
#endif

does not solve the problem because this vulnerable shift will be still
visible for compiler during x86 32-bit kernel build.

> compiler may optimize it out).

Please loot at arch/x86/xen/enlighten.c:xen_check_mwait() and
arch/x86/xen/enlighten.c:xen_boot_params_init_edd() (probably
there are more stuff like that around). As I can see this is fairly
common solution and probably compiler cope with it quite well.

Have a nice weekend,

Daniel
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/