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

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

[Borislav Petkov]

From: Borislav Petkov <bp@suse.de>

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@gmail.com>
Suggested-by: Michael Matz <matz@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/kernel/head_32.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 53eeb226657c..c601d1de5ae5 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -63,8 +63,8 @@
 #endif
 
 /* Number of possible pages in the lowmem region */
-LOWMEM_PAGES = (((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT)
-	
+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

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

[Ingo Molnar]

* Borislav Petkov <bp@alien8.de> wrote:

> From: Borislav Petkov <bp@suse.de>
> 
> 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@gmail.com>
> Suggested-by: Michael Matz <matz@suse.de>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> ---
>  arch/x86/kernel/head_32.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> index 53eeb226657c..c601d1de5ae5 100644
> --- a/arch/x86/kernel/head_32.S
> +++ b/arch/x86/kernel/head_32.S
> @@ -63,8 +63,8 @@
>  #endif
>  
>  /* Number of possible pages in the lowmem region */
> -LOWMEM_PAGES = (((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT)
> -	
> +LOWMEM_PAGES = (((2<<31) - __PAGE_OFFSET) >> PAGE_SHIFT)
> +

Hm, so if internally GAS uses 64-bit types, couldn't we 'make sure' it's a 64-bit 
type, by using something like 31LL? Assuming GAS understands that?

Or if the internal representation is 32 bits, then 2<<31 is just a fancy way of 
saying '0', right?

So this could be written as:

  LOWMEM_PAGES = (-__PAGE_OFFSET >> PAGE_SHIFT)

or:

  LOWMEM_PAGES = ((0 - __PAGE_OFFSET) >> PAGE_SHIFT)

right?

Or did it get it all wrong?

Thanks,

	Ingo

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

[Borislav Petkov]

On Fri, Jun 19, 2015 at 10:10:44AM +0200, Ingo Molnar wrote:
> 
> * Borislav Petkov <bp@alien8.de> wrote:
> 
> > From: Borislav Petkov <bp@suse.de>
> > 
> > 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@gmail.com>
> > Suggested-by: Michael Matz <matz@suse.de>
> > Signed-off-by: Borislav Petkov <bp@suse.de>
> > ---
> >  arch/x86/kernel/head_32.S | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> > index 53eeb226657c..c601d1de5ae5 100644
> > --- a/arch/x86/kernel/head_32.S
> > +++ b/arch/x86/kernel/head_32.S
> > @@ -63,8 +63,8 @@
> >  #endif
> >  
> >  /* Number of possible pages in the lowmem region */
> > -LOWMEM_PAGES = (((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT)
> > -	
> > +LOWMEM_PAGES = (((2<<31) - __PAGE_OFFSET) >> PAGE_SHIFT)
> > +
> 
> Hm, so if internally GAS uses 64-bit types, couldn't we 'make sure' it's a 64-bit 
> type, by using something like 31LL? Assuming GAS understands that?

I don't think it does.

This fix is only when gas is compiled with the i386 target which uses
solely 32-bit sized types for internal representation. The warning won't
fire on gas built with multiple targets support even if you build with
-m32.

> Or if the internal representation is 32 bits, then 2<<31 is just a fancy way of 
> saying '0', right?
> 
> So this could be written as:
> 
>   LOWMEM_PAGES = (-__PAGE_OFFSET >> PAGE_SHIFT)
> 
> or:
> 
>   LOWMEM_PAGES = ((0 - __PAGE_OFFSET) >> PAGE_SHIFT)
> 
> right?

Yes, you can do that only when the internal representation is a 32-bit
type. If it is not and you still do an -m32 build, you then want the
4Gib value there and not a 0:

arch/x86/kernel/head_32.S: Assembler messages:
arch/x86/kernel/head_32.S:227: Error: value of 18014395285110787 too large for field of 4 bytes at 146
make[2]: *** [arch/x86/kernel/head_32.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [arch/x86/kernel] Error 2
make: *** [arch/x86] Error 2
make: *** Waiting for unfinished jobs....

-- 
Regards/Gruss,
    Boris.

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

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

[Ingo Molnar]

* Borislav Petkov <bp@alien8.de> wrote:

> On Fri, Jun 19, 2015 at 10:10:44AM +0200, Ingo Molnar wrote:
> > 
> > * Borislav Petkov <bp@alien8.de> wrote:
> > 
> > > From: Borislav Petkov <bp@suse.de>
> > > 
> > > 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@gmail.com>
> > > Suggested-by: Michael Matz <matz@suse.de>
> > > Signed-off-by: Borislav Petkov <bp@suse.de>
> > > ---
> > >  arch/x86/kernel/head_32.S | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> > > index 53eeb226657c..c601d1de5ae5 100644
> > > --- a/arch/x86/kernel/head_32.S
> > > +++ b/arch/x86/kernel/head_32.S
> > > @@ -63,8 +63,8 @@
> > >  #endif
> > >  
> > >  /* Number of possible pages in the lowmem region */
> > > -LOWMEM_PAGES = (((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT)
> > > -	
> > > +LOWMEM_PAGES = (((2<<31) - __PAGE_OFFSET) >> PAGE_SHIFT)
> > > +
> > 
> > Hm, so if internally GAS uses 64-bit types, couldn't we 'make sure' it's a 64-bit 
> > type, by using something like 31LL? Assuming GAS understands that?
> 
> I don't think it does.
> 
> This fix is only when gas is compiled with the i386 target which uses solely 
> 32-bit sized types for internal representation. The warning won't fire on gas 
> built with multiple targets support even if you build with -m32.

Ugh, nasty.

> > Or if the internal representation is 32 bits, then 2<<31 is just a fancy way of 
> > saying '0', right?
> > 
> > So this could be written as:
> > 
> >   LOWMEM_PAGES = (-__PAGE_OFFSET >> PAGE_SHIFT)
> > 
> > or:
> > 
> >   LOWMEM_PAGES = ((0 - __PAGE_OFFSET) >> PAGE_SHIFT)
> > 
> > right?
> 
> Yes, you can do that only when the internal representation is a 32-bit
> type. If it is not and you still do an -m32 build, you then want the
> 4Gib value there and not a 0:
> 
> arch/x86/kernel/head_32.S: Assembler messages:
> arch/x86/kernel/head_32.S:227: Error: value of 18014395285110787 too large for field of 4 bytes at 146

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.

Thanks,

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

[Borislav Petkov]

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@suse.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@gmail.com>
Suggested-by: Michael Matz <matz@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.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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