Re: [PATCH v2 3/9] x86: refcount: prevent gcc distortions

Nadav Amit
2018-06-04T22:20:54Z

Kees Cook <keescook@chromium.org> wrote:

> On Mon, Jun 4, 2018 at 4:21 AM, Nadav Amit <namit@vmware.com> wrote:
>> GCC considers the number of statements in inlined assembly blocks,
>> according to new-lines and semicolons, as an indication to the cost of
>> the block in time and space. This data is distorted by the kernel code,
>> which puts information in alternative sections. As a result, the
>> compiler may perform incorrect inlining and branch optimizations.
>>
>> The solution is to set an assembly macro and call it from the inlined
>> assembly block. As a result GCC considers the inline assembly block as
>> a single instruction.
>>
>> This patch allows to inline functions such as __get_seccomp_filter().
>> Interestingly, this allows more aggressive inlining while reducing the
>> kernel size.
>>
>> text data bss dec hex filename
>> 18140970 10225412 2957312 31323694 1ddf62e ./vmlinux before
>> 18140140 10225284 2957312 31322736 1ddf270 ./vmlinux after (-958)
>>
>> Static text symbols:
>> Before: 40302
>> After: 40286 (-16)
>>
>> Functions such as kref_get(), free_user(), fuse_file_get() now get
>> inlined.
>>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: x86@kernel.org
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Jan Beulich <JBeulich@suse.com>
>> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
>>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> ---
>> arch/x86/include/asm/refcount.h | 73 ++++++++++++++++++++-------------
>> arch/x86/kernel/macros.S | 1 +
>> 2 files changed, 45 insertions(+), 29 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
>> index 4cf11d88d3b3..53462f32b58e 100644
>> --- a/arch/x86/include/asm/refcount.h
>> +++ b/arch/x86/include/asm/refcount.h
>> @@ -4,6 +4,9 @@
>> * x86-specific implementation of refcount_t. Based on PAX_REFCOUNT from
>> * PaX/grsecurity.
>> */
>> +
>> +#ifndef __ASSEMBLY__
>
> Can you swap the order here, so that the asm macros are visible first
> in the file?
>
> #ifdef __ASSEMBLY__
> ...macros
> #else
> ....C
> #endif

Done. I also noticed that I forgot in one instance (REFCOUNT_CHECK_LT_ZERO)
to explicitly mark the parameter name (“counter=\counter”), so I’ll fix it
as well in the next version.