Re: [Qemu-devel] [PATCH] linux-user: ppc64: don't use volatile register during safe_syscall

[Richard Henderson <richard.henderson@linaro.org>]

On 07/25/2018 11:48 PM, Shivaprasad G Bhat wrote:
> r11 is a volatile register on PPC as per calling conventions.
> The safe_syscall code uses it to check if the signal_pending
> is set during the safe_syscall. When a syscall is interrupted
> on return from signal handling, the r11 might be corrupted
> before we retry the syscall leading to a crash. The registers
> r0-r13 are not to be used here as they have
> volatile/designated/reserved usages. Change the code to use
> r14 which is non-volatile and is appropriate for local use in
> safe_syscall.
> 
> Signed-off-by: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
> ---
> Steps to reproduce:
> On PPC host, issue `qemu-ppc64le /usr/bin/cc -E -`
> Attempt Ctrl-C, the issue is reproduced.
> 
> Reference:
> https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#REG
> 
>  linux-user/host/ppc64/safe-syscall.inc.S |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/linux-user/host/ppc64/safe-syscall.inc.S b/linux-user/host/ppc64/safe-syscall.inc.S
> index d30050a67c..b0cbbe6a69 100644
> --- a/linux-user/host/ppc64/safe-syscall.inc.S
> +++ b/linux-user/host/ppc64/safe-syscall.inc.S
> @@ -49,7 +49,7 @@ safe_syscall_base:
>  	 *               and returns the result in r3
>  	 * Shuffle everything around appropriately.
>  	 */
> -	mr	11, 3	/* signal_pending */
> +	mr	14, 3	/* signal_pending */

I do see that I was incorrect in assuming that r11 would be unmodified.  But
you can't simply write to a call-saved register -- you must preserve its value
for the caller.

Saving the value requires that you find some space on, or create, a stack
frame.  Note that there are two different conventions for _CALL_AIX and
_CALL_ELF==2.


r~