linux-kernel - Re: [PATCH] x86/head_32: Fix overflow warning with 32-bit binutils

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Date:	Fri, 19 Jun 2015 13:49:06 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/head_32: Fix overflow warning with 32-bit binutils

On Fri, Jun 19, 2015 at 12:46:28PM +0200, Ingo Molnar wrote:
> Grumble. I guess the workaround is OK then because I cannot think
> of any cleaner solution - but I'd suggest to put a comment there at
> minimum, to explain what it's about.
>
> ... and chances are that GAS might start warning about 2<<31 in the
> future as well.

Reportedly it won't because it'll wrap around to 0 and that is fine...

Anyway, here's v2:

---
From: Borislav Petkov <bp@...e.de>
Subject: [PATCH -v2] x86/head_32: Fix overflow warning with 32-bit binutils

When building the kernel with 32-bit binutils built with support only
for the i386 target, we get the following warning:

  arch/x86/kernel/head_32.S:66: Warning: shift count out of range (32 is not between 0 and 31)

The problem is that in that case, binutils' internal type representation
is 32-bit wide and the shift range overflows.

In order to fix this, manipulate the shift expression which creates the
4GiB constant to not overflow the shift count.

Reported-and-tested-by: Enrico Mioso <mrkiko.rs@...il.com>
Suggested-by: Michael Matz <matz@...e.de>
Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/kernel/head_32.S | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 53eeb226657c..7e429c99c728 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -62,9 +62,16 @@
 #define PAGE_TABLE_SIZE(pages) ((pages) / PTRS_PER_PGD)
 #endif
 
-/* Number of possible pages in the lowmem region */
-LOWMEM_PAGES = (((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT)
-	
+/*
+ * Number of possible pages in the lowmem region.
+ *
+ * We shift 2 by 31 instead of 1 by 32 to the left in order to avoid a
+ * gas warning about overflowing shift count when gas has been compiled
+ * with only a host target support using a 32-bit type for internal
+ * representation.
+ */
+LOWMEM_PAGES = (((2<<31) - __PAGE_OFFSET) >> PAGE_SHIFT)
+
 /* Enough space to fit pagetables for the low memory linear map */
 MAPPING_BEYOND_END = PAGE_TABLE_SIZE(LOWMEM_PAGES) << PAGE_SHIFT
 
-- 
2.3.5

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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