Re: [PATCH] x86: Make variable_test_bit reference all of *addr

[Josh Stone]

On 10/06/2011 04:58 PM, Josh Stone wrote:
> [...]/arch/x86/include/asm/bitops.h: In function ‘can_boost.part.1’:
> [...]/arch/x86/include/asm/bitops.h:319:2: warning: use of memory input without lvalue in asm operand 1 is deprecated [enabled by default]

I probably should have noted that Jakub also blamed gcc's behavior, for
transforming const memory into a literal constant and then complaining
about lvalues.  He fixed that upstream, and applied to 4.6.1-10.fc16:
  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50571

I didn't figure out any automated way to detect the problem in general
(apart from the presence of that warning), but here's how I'm checking
kprobes' in particular.

Using gcc-4.6.1-9.fc15.i686:
> $ objdump -rd arch/x86/kernel/kprobes.o | grep -A1 -w bt
>      551:	0f a3 05 00 00 00 00 	bt     %eax,0x0
> 			554: R_386_32	.rodata.cst4
> $ objdump -s -j .rodata -j .rodata.cst4 arch/x86/kernel/kprobes.o
> 
> arch/x86/kernel/kprobes.o:     file format elf32-i386
> 
> Contents of section .rodata:
>  0000 02000000                             ....            
> Contents of section .rodata.cst4:
>  0000 4c030000                             L...            

Using gcc-4.6.1-9.fc15.i686, with my variable_test_bit patch:
> $ objdump -rd arch/x86/kernel/kprobes.o | grep -A1 -w bt
>      551:	0f a3 05 20 00 00 00 	bt     %eax,0x20
> 			554: R_386_32	.rodata
> $ objdump -s -j .rodata arch/x86/kernel/kprobes.o
> 
> arch/x86/kernel/kprobes.o:     file format elf32-i386
> 
> Contents of section .rodata:
>  0000 02000000 00000000 00000000 00000000  ................
>  0010 00000000 00000000 00000000 00000000  ................
>  0020 4c030000 0f000200 ffff0000 ffcff0c0  L...............
>  0030 0000ffff 3bbbfff8 03ff2ebb 26bb2e77  ....;.......&..w

Using gcc-4.6.1-10.fc16.i686, with Jakub's fix, without my patch:
> $ objdump -rd arch/x86/kernel/kprobes.o | grep -A1 -w bt
>      551:	0f a3 05 20 00 00 00 	bt     %eax,0x20
> 			554: R_386_32	.rodata
> $ objdump -s -j .rodata arch/x86/kernel/kprobes.o
> 
> arch/x86/kernel/kprobes.o:     file format elf32-i386
> 
> Contents of section .rodata:
>  0000 02000000 00000000 00000000 00000000  ................
>  0010 00000000 00000000 00000000 00000000  ................
>  0020 4c030000 0f000200 ffff0000 ffcff0c0  L...............
>  0030 0000ffff 3bbbfff8 03ff2ebb 26bb2e77  ....;.......&..w

There's some zero-padding on the previous .rodata contents, but then
starting at 0x20 it now has the full 32-bytes of twobyte_is_boostable[].

So Jakub's gcc change fixes this issue independently of my patch, but I
got the impression from him that the way the kernel is expressing this
is still in the realm of "gcc might break your expectations here".  If
that's not the case, then my patch here is only needed if you want to
cope with prior broken versions.  Jakub, do you have an idea of the
range of gcc versions broken in this way?

On 10/06/2011 07:02 PM, Andi Kleen wrote:
> "hpanvin@xxxxxxxxx" <hpa@xxxxxxxxx> writes:
>> This is concerning... the kernel relies heavily on asm volatile being
>> a universal memory consumer.  If that is suddenly broken, we are f***
>> in many, many, MANY places in the kernel all of a sudden!
> 
> I don't think that's true. We generally add "memory" clobbers for this
> purpose. asm volatile just means "don't move" 
> 
> Just this one doesn't have it for unknown reasons (someone overoptimizing?)

Which overoptimizing part are you referring to?  The only part of
variable_test_bit that's not volatile is "m" (*(unsigned long *)addr),
and throwing volatile in that cast does nothing for the problem (at
least on gcc-4.6.1-9.fc15).

We can make twobyte_is_boostable[] volatile instead, which does the
trick, but that seems a kludge to me.


Josh
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