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

Nathan Chancellor
2024-04-10T15:35:45Z

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? Your patch had

Cc: stable@vger.kernel.org

in it, should this one as well?

> > ---
> > 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
> >