Re: [Qemu-devel] [PATCH] user-exec.c: Set is_write correctly in the ARM cpu_signal_handler()

["Andreas Färber" <afaerber@suse.de>]

Am 04.06.2013 15:31, schrieb Peter Maydell:
> In the ARM implementation of cpu_signal_handler(), set is_write
> correctly using the FSR value which the kernel passes us in the
> error_code field of uc_mcontext. Since the WnR bit of the FSR was
> only introduced in ARMv6, this means that v5 cores will continue
> to behave as before this patch, but they are not really supported
> as hosts for linux-user mode anyway since they do not have the
> modern behaviour for unaligned accesses.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> Without this linux-user won't work very well. In particular after
> fork() bash will segfault, with this in the QEMU_STRACE output
> immediately preceding:
>  sigreturn(18,4390912,1082130608,0,0,0) = -1 errno=255 (Unknown error 255)
> at least for PPC and MIPSEL guests.
> 
>  user-exec.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/user-exec.c b/user-exec.c
> index 71bd6c5..336ac70 100644
> --- a/user-exec.c
> +++ b/user-exec.c
> @@ -20,6 +20,7 @@
>  #include "cpu.h"
>  #include "disas/disas.h"
>  #include "tcg.h"
> +#include "qemu/bitops.h"
>  
>  #undef EAX
>  #undef ECX
> @@ -441,8 +442,11 @@ int cpu_signal_handler(int host_signum, void *pinfo,
>  #else
>      pc = uc->uc_mcontext.arm_pc;
>  #endif
> -    /* XXX: compute is_write */
> -    is_write = 0;
> +
> +    /* error_code is the FSR value, in which bit 11 is WnR (assuming a v6 or
> +     * later processor; on v5 we will always report this as a read).
> +     */
> +    is_write = extract32(uc->uc_mcontext.error_code, 11, 1);

You seem to be relying on v5 and earlier reading zero here - wondering
if that is true for all implementations (OMAP, PXA, etc.)? Safer and
closer to the comment might be an explicit check for v6+ if that were
possible.

Cheers,
Andreas

>      return handle_cpu_signal(pc, (unsigned long)info->si_addr,
>                               is_write,
>                               &uc->uc_sigmask, puc);

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