Re: [PATCH v2] target/arm: Allow user-mode code to write CPSR.E via MSR

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

On 5/18/20 7:28 AM, Peter Maydell wrote:
> Using the MSR instruction to write to CPSR.E is deprecated, but it is
> required to work from any mode including unprivileged code.  We were
> incorrectly forbidding usermode code from writing it because
> CPSR_USER did not include the CPSR_E bit.
> 
> We use CPSR_USER in only three places:
>  * as the mask of what to allow userspace MSR to write to CPSR
>  * when deciding what bits a linux-user signal-return should be
>    able to write from the sigcontext structure
>  * in target_user_copy_regs() when we set up the initial
>    registers for the linux-user process
> 
> In the first two cases not being able to update CPSR.E is a bug, and
> in the third case it doesn't matter because CPSR.E is always 0 there.
> So we can fix both bugs by adding CPSR_E to CPSR_USER.
> 
> Because the cpsr_write() in restore_sigcontext() is now changing
> a CPSR bit which is cached in hflags, we need to add an
> arm_rebuild_hflags() call there; the callsite in
> target_user_copy_regs() was already rebuilding hflags for other
> reasons.
> 
> (The recommended way to change CPSR.E is to use the 'SETEND'
> instruction, which we do correctly allow from usermode code.)
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> v2 changes:
>  * fixed wrong variable name in commit message
>  * added arm_rebuild_hflags() call in restore_sigcontext()
> ---
>  target/arm/cpu.h        | 2 +-
>  linux-user/arm/signal.c | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~