Re: [Qemu-devel] [PATCH] cpu-all.h: Don't accidentally sign extend in g2h()

["Andreas Färber" <afaerber@suse.de>]

Am 09.03.2012 15:33, schrieb Peter Maydell:
> Cast the argument of the g2h() macro to a target_ulong so that
> it isn't accidentally sign-extended if it is a signed 32 bit
> type and long is a 64 bit type. In particular, this fixes a
> bug where it would return the wrong value for 32 bit guests
> on 64 bit hosts when passed in one of the arg* values from
> do_syscall() [which are all abi_long and thus signed types].
> This could result in spurious failure of mlock(), among others.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> This should be committed before Alex's patch to make mmap allocate
> downwards (http://patchwork.ozlabs.org/patch/144476/) because that
> hugely increases the chances that g2h will get passed a pointer
> that has the high bit set.
> 
>  cpu-all.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/cpu-all.h b/cpu-all.h
> index 80e6d42..a174532 100644
> --- a/cpu-all.h
> +++ b/cpu-all.h
> @@ -197,7 +197,7 @@ extern unsigned long reserved_va;
>  #endif
>  
>  /* All direct uses of g2h and h2g need to go away for usermode softmmu.  */
> -#define g2h(x) ((void *)((unsigned long)(x) + GUEST_BASE))
> +#define g2h(x) ((void *)((unsigned long)(target_ulong)(x) + GUEST_BASE))

So *only* for a 32-bit guest does this cast from signed int to unsigned
int and then to unsigned long, avoiding the sign extension on 64-bit
host. For 64-bit guests it remains as broken as before. Commit message
could be clearer.

Reviewed-by: Andreas Färber <afaerber@suse.de>

Note that unsigned long would be wrong for Win64 (where we don't
currently have any user emulation using this macro).
uintptr_t would be cleaner.

Andreas

>  
>  #if HOST_LONG_BITS <= TARGET_VIRT_ADDR_SPACE_BITS
>  #define h2g_valid(x) 1


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