Re: [PATCH v2] linux-user/armeb: Fix __kernel_cmpxchg() for armeb

["Markus F.X.J. Oberhumer" <markus@oberhumer.com>]

The fix in arm_kernel_cmpxchg64_helper probably should use tswap64() instead
of tswap32().

~Markus

On 2023-07-28 06:54, Helge Deller wrote:
> Commit 7f4f0d9ea870 ("linux-user/arm: Implement __kernel_cmpxchg with host
> atomics") switched to use qatomic_cmpxchg() to swap a word with the memory
> content, but missed to endianess-swap the oldval and newval values when
> emulating an armeb CPU, which expects words to be stored in big endian in
> the guest memory.
> 
> The bug can be verified with qemu >= v7.2 on any little-endian host, when
> starting the armeb binary of the upx program, which just hangs without
> this patch.
> 
> Signed-off-by: Helge Deller <deller@gmx.de>
> Cc: Richard Henderson <richard.henderson@linaro.org>
> Cc: Peter Maydell <peter.maydell@linaro.org>
> Cc: qemu-stable@nongnu.org
> Reported-by: "Markus F.X.J. Oberhumer" <markus@oberhumer.com>
> Reported-by: John Reiser <jreiser@BitWagon.com>
> Closes: https://github.com/upx/upx/issues/687
> 
> --
> v2:
> - add tswap32() in arm_kernel_cmpxchg64_helper()
> 
> 
> diff --git a/linux-user/arm/cpu_loop.c b/linux-user/arm/cpu_loop.c
> index a992423257..0907cd8c15 100644
> --- a/linux-user/arm/cpu_loop.c
> +++ b/linux-user/arm/cpu_loop.c
> @@ -117,8 +117,9 @@ static void arm_kernel_cmpxchg32_helper(CPUARMState *env)
>  {
>      uint32_t oldval, newval, val, addr, cpsr, *host_addr;
> 
> -    oldval = env->regs[0];
> -    newval = env->regs[1];
> +    /* endianess-swap if emulating armeb */
> +    oldval = tswap32(env->regs[0]);
> +    newval = tswap32(env->regs[1]);
>      addr = env->regs[2];
> 
>      mmap_lock();
> @@ -174,6 +175,10 @@ static void arm_kernel_cmpxchg64_helper(CPUARMState *env)
>          return;
>      }
> 
> +    /* endianess-swap if emulating armeb */
> +    oldval = tswap32(oldval);
> +    newval = tswap32(newval);
> +
>  #ifdef CONFIG_ATOMIC64
>      val = qatomic_cmpxchg__nocheck(host_addr, oldval, newval);
>      cpsr = (val == oldval) * CPSR_C;
> 

-- 
Markus Oberhumer, <markus@oberhumer.com>, http://www.oberhumer.com/