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

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Date: Wed, 10 Apr 2024 08:35:26 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: zhuqiuer <zhuqiuer1@...wei.com>, justinstitt@...gle.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux@...linux.org.uk, llvm@...ts.linux.dev, morbo@...gle.com,
	ndesaulniers@...gle.com
Subject: Re: [PATCH v2] ARM: Add a memory clobber to the fmrx instruction

On Wed, Apr 10, 2024 at 02:31:11PM +0200, Ard Biesheuvel wrote:
> On Wed, 10 Apr 2024 at 04:41, zhuqiuer <zhuqiuer1@...wei.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@...wei.com>
> 
> This also fixes the issue I observed so
> 
> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

This can probably go in Russell's patch tracker? Your patch had

Cc: stable@...r.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
> >