Re: [PATCH] riscv: Fix spurious errors from __get/put_kernel_nofault

Charlie Jenkins
2024-03-13T02:53:59Z

On Mon, Mar 11, 2024 at 07:19:13PM -0700, Samuel Holland wrote:
> These macros did not initialize __kr_err, so they could fail even if
> the access did not fault.
>
> Cc: stable@vger.kernel.org
> Fixes: d464118cdc41 ("riscv: implement __get_kernel_nofault and __put_user_nofault")
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
> ---
> Found while testing the unaligned access speed series[1]. The observed
> behavior was that with RISCV_EFFICIENT_UNALIGNED_ACCESS=y, the
> copy_from_kernel_nofault() in prepend_copy() failed every time when
> filling out /proc/self/mounts, so all of the mount points were "xxx".
>
> I'm surprised this hasn't been seen before. For reference, I'm compiling
> with clang 18.
>
> [1]: https://lore.kernel.org/linux-riscv/20240308-disable_misaligned_probe_config-v9-0-a388770ba0ce@rivosinc.com/
>
> arch/riscv/include/asm/uaccess.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
> index ec0cab9fbddd..72ec1d9bd3f3 100644
> --- a/arch/riscv/include/asm/uaccess.h
> +++ b/arch/riscv/include/asm/uaccess.h
> @@ -319,7 +319,7 @@ unsigned long __must_check clear_user(void __user *to, unsigned long n)
>
> #define __get_kernel_nofault(dst, src, type, err_label) \
> do { \
> - long __kr_err; \
> + long __kr_err = 0; \
> \
> __get_user_nocheck(*((type *)(dst)), (type *)(src), __kr_err); \
> if (unlikely(__kr_err)) \
> @@ -328,7 +328,7 @@ do { \
>
> #define __put_kernel_nofault(dst, src, type, err_label) \
> do { \
> - long __kr_err; \
> + long __kr_err = 0; \
> \
> __put_user_nocheck(*((type *)(src)), (type *)(dst), __kr_err); \
> if (unlikely(__kr_err)) \
> --
> 2.43.1
>
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

I am not able to reproduce this using Clang 18 with
RISCV_EFFICIENT_UNALIGNED_ACCESS=y on 6.8. However I can see how this
could be an issue.

Going down the rabbit hold of macros here, I end up at
arch/riscv/include/asm/asm-extable.h where the register that hold 'err'
is written into the __ex_table section:

#define EX_DATA_REG(reg, gpr) \
"((.L__gpr_num_" #gpr ") << " __stringify(EX_DATA_REG_##reg##_SHIFT) ")"

#define _ASM_EXTABLE_UACCESS_ERR_ZERO(insn, fixup, err, zero) \
__DEFINE_ASM_GPR_NUMS \
__ASM_EXTABLE_RAW(#insn, #fixup, \
__stringify(EX_TYPE_UACCESS_ERR_ZERO), \
"(" \
EX_DATA_REG(ERR, err) " | " \
EX_DATA_REG(ZERO, zero) \
")")

I am wondering if setting this value to zero solves the problem by
hiding another issue. It seems like this shouldn't need to be
initialized to zero, however I am lost as to how this extable setup
works so perhaps this is the proper solution.

- Charlie