Re: [PATCH v2 1/2] arm64/signal: Restore TPIDR2 register rather than memory state

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Jun 22, 2023 at 02:39:45PM +0100, Mark Brown wrote:
> Currently when restoring the TPIDR2 signal context we set the new value
> from the signal frame in the thread data structure but not the register,
> following the pattern for the rest of the data we are restoring. This does
> not work in the case of TPIDR2, the register always has the value for the
> current task. This means that either we return to userspace and ignore the
> new value or we context switch and save the register value on top of the
> newly restored value.
> 
> Load the value from the signal context into the register instead.
> 
> Fixes: 39e54499280f ("arm64/signal: Include TPIDR2 in the signal context")
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Cc: stable@vger.kernel.org
> ---
>  arch/arm64/kernel/signal.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index 2cfc810d0a5b..10b407672c42 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -398,7 +398,7 @@ static int restore_tpidr2_context(struct user_ctxs *user)
>  
>  	__get_user_error(tpidr2_el0, &user->tpidr2->tpidr2, err);
>  	if (!err)
> -		current->thread.tpidr2_el0 = tpidr2_el0;
> +		write_sysreg_s(tpidr2_el0, SYS_TPIDR2_EL0);

I guess the other way around may also be true - the libc sets tpidr2_el0
to something else and doesn't want the kernel to restore its original
value from sigcontext.

For tpidr_el0 we don't bother with sigcontext, not sure what the use for
tpidr2_el0 in signals is. If we assume the context saved is only
informative (like esr), we can simply ignore restoring it from the
signal stack.

I guess we need to ask Szabolcs what his preference is. The current code
is wrong either way since current->thread.tpidr2_el0 would be overridden
at thread switch.

-- 
Catalin