linux-kernel - [tip:x86/urgent 3/3] arch/x86//kernel/cpu/mcheck/mce.c:601:52: note: in expansion of macro 'BIT'

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Date:   Thu, 16 Nov 2017 19:47:18 +0800
From:   kbuild test robot <fengguang.wu@...el.com>
To:     Tony Luck <tony.luck@...el.com>
Cc:     kbuild-all@...org, linux-kernel@...r.kernel.org,
        tipbuild@...or.com, Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>
Subject: [tip:x86/urgent 3/3] arch/x86//kernel/cpu/mcheck/mce.c:601:52: note:
 in expansion of macro 'BIT'

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/urgent
head:   f1a041552c403949ab3c0902c1030c3a3d186ec1
commit: f1a041552c403949ab3c0902c1030c3a3d186ec1 [3/3] x86/mm, mm/hwpoison: Don't unconditionally unmap kernel 1: 1 pages
config: i386-randconfig-x001-201746 (attached as .config)
compiler: gcc-6 (Debian 6.4.0-9) 6.4.0 20171026
reproduce:
        git checkout f1a041552c403949ab3c0902c1030c3a3d186ec1
        # save the attached .config to linux build tree
        make ARCH=i386 

All warnings (new ones prefixed by >>):

   In file included from include/linux/kernel.h:11:0,
                    from include/asm-generic/bug.h:16,
                    from arch/x86/include/asm/bug.h:82,
                    from include/linux/bug.h:5,
                    from include/linux/thread_info.h:12,
                    from arch/x86//kernel/cpu/mcheck/mce.c:13:
   arch/x86//kernel/cpu/mcheck/mce.c: In function 'mce_unmap_kpfn':
   include/linux/bitops.h:7:24: warning: left shift count >= width of type [-Wshift-count-overflow]
    #define BIT(nr)   (1UL << (nr))
                           ^
>> arch/x86//kernel/cpu/mcheck/mce.c:601:52: note: in expansion of macro 'BIT'
     decoy_addr = (pfn << PAGE_SHIFT) + (PAGE_OFFSET ^ BIT(63));
                                                       ^~~

vim +/BIT +601 arch/x86//kernel/cpu/mcheck/mce.c

   573	
   574	/*
   575	 * Unmap this page from the kernel 1:1 mappings to make sure we don't log
   576	 * more errors because of speculative access to the page.
   577	 *
   578	 * We would like to just call:
   579	 *	set_memory_np((unsigned long)pfn_to_kaddr(pfn), 1);
   580	 *
   581	 * but doing that would radically increase the odds of a speculative access
   582	 * to the poison page because we'd have the virtual address of the kernel
   583	 * 1:1 mapping sitting around in registers.
   584	 * Instead we get tricky.  We create a non-canonical address that looks
   585	 * just like the one we want, but has bit 63 flipped.  This relies on
   586	 * set_memory_np() not checking whether we passed a legal address.
   587	 */
   588	static void mce_unmap_kpfn(unsigned long pfn)
   589	{
   590		unsigned long decoy_addr;
   591	
   592		/*
   593		 * Build time check to see if we have a spare virtual bit. Don't
   594		 * want to leave this until run time because most developers don't
   595		 * have a system that can exercise this code path. This will only
   596		 * become a problem if/when we move beyond 5-level page tables.
   597		 *
   598		 * Hard code "9" here because cpp doesn't grok ilog2(PTRS_PER_PGD)
   599		 */
   600	#if PGDIR_SHIFT + 9 < 63
 > 601		decoy_addr = (pfn << PAGE_SHIFT) + (PAGE_OFFSET ^ BIT(63));
   602	#else
   603	#error "No unused virtual bit available"
   604	#endif
   605	
   606		if (set_memory_np(decoy_addr, 1))
   607			pr_warn("Could not invalidate pfn=0x%lx from 1:1 map\n", pfn);
   608	}
   609	

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