Re: [PATCH v2] ARM: Add a memory clobber to the fmrx instruction

Ard Biesheuvel
2024-04-11T07:02:48Z

On Wed, 10 Apr 2024 at 17:35, Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Wed, Apr 10, 2024 at 02:31:11PM +0200, Ard Biesheuvel wrote:
> > On Wed, 10 Apr 2024 at 04:41, zhuqiuer <zhuqiuer1@huawei.com> wrote:
> > >
> > > The instruction fmrx is used throughout the kernel,
> > > where it is sometimes expected to be skipped
> > > by incrementing the program counter, such as in vfpmodule.c:vfp_init().
> > > Therefore, the instruction should not be reordered when it is not intended.
> > > Adding a barrier() instruction before and after this call cannot prevent
> > > reordering by the compiler, as the fmrx instruction is constrained
> > > by '=r', meaning it works on the general register but not on memory.
> > > To ensure the order of the instruction after compiling,
> > > adding a memory clobber is necessary.
> > >
> > > Below is the code snippet disassembled from the method:
> > > vfpmodule.c:vfp_init(), compiled by LLVM.
> > >
> > > Before the patching:
> > > xxxxx: xxxxx bl c010c688 <register_undef_hook>
> > > xxxxx: xxxxx mov r0, r4
> > > xxxxx: xxxxx bl c010c6e4 <unregister_undef_hook>
> > > ...
> > > xxxxx: xxxxx bl c0791c8c <printk>
> > > xxxxx: xxxxx movw r5, #23132 ; 0x5a5c
> > > xxxxx: xxxxx vmrs r4, fpsid <- this is the fmrx instruction
> > >
> > > After the patching:
> > > xxxxx: xxxxx bl c010c688 <register_undef_hook>
> > > xxxxx: xxxxx mov r0, r4
> > > xxxxx: xxxxx vmrs r5, fpsid <- this is the fmrx instruction
> > > xxxxx: xxxxx bl c010c6e4 <unregister_undef_hook>
> > >
> > > Signed-off-by: zhuqiuer <zhuqiuer1@huawei.com>
> >
> > This also fixes the issue I observed so
> >
> > Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
>
> This can probably go in Russell's patch tracker?

Yes.

> Your patch had
>
> Cc: stable@vger.kernel.org
>
> in it, should this one as well?
>

Yes.

> > > ---
> > > arch/arm/vfp/vfpinstr.h | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/arm/vfp/vfpinstr.h b/arch/arm/vfp/vfpinstr.h
> > > index 3c7938fd40aa..ae2c9b9b7701 100644
> > > --- a/arch/arm/vfp/vfpinstr.h
> > > +++ b/arch/arm/vfp/vfpinstr.h
> > > @@ -68,14 +68,14 @@
> > > u32 __v; \
> > > asm(".fpu vfpv2\n" \
> > > "vmrs %0, " #_vfp_ \
> > > - : "=r" (__v) : : "cc"); \
> > > + : "=r" (__v) : : "memory", "cc"); \
> > > __v; \
> > > })
> > >
> > > #define fmxr(_vfp_,_var_) \
> > > asm(".fpu vfpv2\n" \
> > > "vmsr " #_vfp_ ", %0" \
> > > - : : "r" (_var_) : "cc")
> > > + : : "r" (_var_) : "memory", "cc")
> > >
> > > #else
> > >
> > > --
> > > 2.12.3
> > >