[PATCH 0/5 v16] KASan for Arm

[Fāng-ruì Sòng]

On Fri, Oct 30, 2020 at 2:09 AM Nathan Chancellor
<natechancellor at gmail.com> wrote:
>
> On Fri, Oct 30, 2020 at 09:51:27AM +0100, Arnd Bergmann wrote:
> > On Fri, Oct 30, 2020 at 9:45 AM Nathan Chancellor
> > <natechancellor at gmail.com> wrote:
> > > On Fri, Oct 30, 2020 at 09:10:56AM +0100, Ard Biesheuvel wrote:
> > > 4043            futex_hashsize = 1UL << futex_shift;
> > > (gdb)
> > > 4045            futex_detect_cmpxchg();
> > > (gdb)
> >
> > I can't explain it, but I'd point out that futex_detect_cmpxchg() has caused
> > problems in the past, with multiple patches for it proposed in the past
> > and none of them merged. One of the patches I had sent for it was:
> >
> > commit 8232a8ffc332fa6a50296a51c4d85200a747256c
> > Author: Arnd Bergmann <arnd at arndb.de>
> > Date:   Mon Mar 4 17:33:00 2019 +0100
> >
> >     [SUBMITTED 20190307] futex: mark futex_detect_cmpxchg() as 'noinline'
> >
> >     On 32-bit ARM, I got a link failure in futex_init() when building
> >     with clang in some random configurations:
> >
> >     kernel/futex.o:(.text.fixup+0x5c): relocation truncated to fit:
> > R_ARM_JUMP24 against `.init.text'
> >
> >     As far as I can tell, the problem is that a branch is over 16MB
> >     apart in those configurations, but only if it branches back to
> >     the init text.
> >
> >     Marking the futex_detect_cmpxchg() function as noinline and
> >     not __init avoids the problem for me.
> >
> >     Signed-off-by: Arnd Bergmann <arnd at arndb.de>
> >
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index e646661f6282..6fbbed45f51b 100644
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -4061,7 +4061,7 @@ SYSCALL_DEFINE6(futex_time32, u32 __user *,
> > uaddr, int, op, u32, val,
> >  }
> >  #endif /* CONFIG_COMPAT_32BIT_TIME */
> >
> > -static void __init futex_detect_cmpxchg(void)
> > +static noinline void futex_detect_cmpxchg(void)
> >  {
> >  #ifndef CONFIG_HAVE_FUTEX_CMPXCHG
> >         u32 curval;
> >
> > I forgot what the problem was, but you might try that patch, or find
> > the previous discussions in the archive.
> >
> >        Arnd
>
> As it turns out, that is the patch that fixes the ld.bfd issue that I
> mentioned earlier in this thread and it fixes this issue as well.
>
> To summarize, with the below diff, I can successfully boot
> multi_v7_defconfig + CONFIG_KASAN=y when LLVM=1 is used (and I see KASAN
> get initialized properly).
>
> Cheers,
> Nathan
>
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index 4d76eab2b22d..3c0a64cefe52 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -10,7 +10,7 @@
>  #
>  # Copyright (C) 1995-2001 by Russell King
>
> -LDFLAGS_vmlinux        := --no-undefined -X --pic-veneer
> +LDFLAGS_vmlinux        := --no-undefined -X --pic-veneer -z norelro
>  ifeq ($(CONFIG_CPU_ENDIAN_BE8),y)
>  LDFLAGS_vmlinux        += --be8
>  KBUILD_LDFLAGS_MODULE  += --be8
> diff --git a/kernel/futex.c b/kernel/futex.c
> index be68ac0d49ad..226bb20d175f 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -4004,7 +4004,7 @@ SYSCALL_DEFINE6(futex_time32, u32 __user *, uaddr, int, op, u32, val,
>  }
>  #endif /* CONFIG_COMPAT_32BIT_TIME */
>
> -static void __init futex_detect_cmpxchg(void)
> +static noinline void futex_detect_cmpxchg(void)
>  {
>  #ifndef CONFIG_HAVE_FUTEX_CMPXCHG
>         u32 curval;
>
> --
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About "ARM: Replace string mem* functions for KASan", it added .weak
memcpy to arch/arm/lib/memcpy.S
However, the convention is to change .weak + ENTRY(...) to WEAK(...)

In particular, clang 12 will reject `.weak memcpy; .global memcpy`
(they had different semantics in GNU as and LLVM integrated assembler
for a long time AND binutils maintainer agreed that .weak and .global
are strange and should not be used - though binutils will not make the
change to cause disruption)

Please see https://github.com/ClangBuiltLinux/linux/issues/1190