Re: [PATCH V4 19/24] xen/arm: io: Abstract sign-extension

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 12 Jan 2021, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> 
> In order to avoid code duplication (both handle_read() and
> handle_ioserv() contain the same code for the sign-extension)
> put this code to a common helper to be used for both.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
> [On Arm only]
> Tested-by: Wei Chen <Wei.Chen@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> Please note, this is a split/cleanup/hardening of Julien's PoC:
> "Add support for Guest IO forwarding to a device emulator"
> 
> Changes V1 -> V2:
>    - new patch
> 
> Changes V2 -> V3:
>    - no changes
> 
> Changes V3 -> V4:
>    - no changes here, but in new patch:
>      "xen/arm: io: Harden sign extension check"
> ---
>  xen/arch/arm/io.c           | 18 ++----------------
>  xen/arch/arm/ioreq.c        | 17 +----------------
>  xen/include/asm-arm/traps.h | 24 ++++++++++++++++++++++++
>  3 files changed, 27 insertions(+), 32 deletions(-)
> 
> diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
> index 9814481..307c521 100644
> --- a/xen/arch/arm/io.c
> +++ b/xen/arch/arm/io.c
> @@ -24,6 +24,7 @@
>  #include <asm/cpuerrata.h>
>  #include <asm/current.h>
>  #include <asm/mmio.h>
> +#include <asm/traps.h>
>  #include <asm/hvm/ioreq.h>
>  
>  #include "decode.h"
> @@ -40,26 +41,11 @@ static enum io_state handle_read(const struct 
> mmio_handler *handler,
>       * setting r).
>       */
>      register_t r = 0;
> -    uint8_t size = (1 << dabt.size) * 8;
>  
>      if ( !handler->ops->read(v, info, &r, handler->priv) )
>          return IO_ABORT;
>  
> -    /*
> -     * Sign extend if required.
> -     * Note that we expect the read handler to have zeroed the bits
> -     * outside the requested access size.
> -     */
> -    if ( dabt.sign && (r & (1UL << (size - 1))) )
> -    {
> -        /*
> -         * We are relying on register_t using the same as
> -         * an unsigned long in order to keep the 32-bit assembly
> -         * code smaller.
> -         */
> -        BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long));
> -        r |= (~0UL) << size;
> -    }
> +    r = sign_extend(dabt, r);
>  
>      set_user_reg(regs, dabt.reg, r);
>  
> diff --git a/xen/arch/arm/ioreq.c b/xen/arch/arm/ioreq.c
> index 3c4a24d..40b9e59 100644
> --- a/xen/arch/arm/ioreq.c
> +++ b/xen/arch/arm/ioreq.c
> @@ -28,7 +28,6 @@ enum io_state handle_ioserv(struct cpu_user_regs *regs, 
> struct vcpu *v)
>      const union hsr hsr = { .bits = regs->hsr };
>      const struct hsr_dabt dabt = hsr.dabt;
>      /* Code is similar to handle_read */
> -    uint8_t size = (1 << dabt.size) * 8;
>      register_t r = v->io.req.data;
>  
>      /* We are done with the IO */
> @@ -37,21 +36,7 @@ enum io_state handle_ioserv(struct cpu_user_regs *regs, 
> struct vcpu *v)
>      if ( dabt.write )
>          return IO_HANDLED;
>  
> -    /*
> -     * Sign extend if required.
> -     * Note that we expect the read handler to have zeroed the bits
> -     * outside the requested access size.
> -     */
> -    if ( dabt.sign && (r & (1UL << (size - 1))) )
> -    {
> -        /*
> -         * We are relying on register_t using the same as
> -         * an unsigned long in order to keep the 32-bit assembly
> -         * code smaller.
> -         */
> -        BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long));
> -        r |= (~0UL) << size;
> -    }
> +    r = sign_extend(dabt, r);
>  
>      set_user_reg(regs, dabt.reg, r);
>  
> diff --git a/xen/include/asm-arm/traps.h b/xen/include/asm-arm/traps.h
> index 997c378..e301c44 100644
> --- a/xen/include/asm-arm/traps.h
> +++ b/xen/include/asm-arm/traps.h
> @@ -83,6 +83,30 @@ static inline bool VABORT_GEN_BY_GUEST(const struct 
> cpu_user_regs *regs)
>          (unsigned long)abort_guest_exit_end == regs->pc;
>  }
>  
> +/* Check whether the sign extension is required and perform it */
> +static inline register_t sign_extend(const struct hsr_dabt dabt, register_t 
> r)
> +{
> +    uint8_t size = (1 << dabt.size) * 8;
> +
> +    /*
> +     * Sign extend if required.
> +     * Note that we expect the read handler to have zeroed the bits
> +     * outside the requested access size.
> +     */
> +    if ( dabt.sign && (r & (1UL << (size - 1))) )
> +    {
> +        /*
> +         * We are relying on register_t using the same as
> +         * an unsigned long in order to keep the 32-bit assembly
> +         * code smaller.
> +         */
> +        BUILD_BUG_ON(sizeof(register_t) != sizeof(unsigned long));
> +        r |= (~0UL) << size;
> +    }
> +
> +    return r;
> +}
> +
>  #endif /* __ASM_ARM_TRAPS__ */
>  /*
>   * Local variables:
> -- 
> 2.7.4
>