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


From: Peter Maydell
Subject: Re: [PATCH v2] target/arm: Allow user-mode code to write CPSR.E via MSR
Date: Thu, 21 May 2020 18:04:33 +0100

On Mon, 18 May 2020 at 15:28, Peter Maydell <address@hidden> 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 <address@hidden>



Applied to target-arm.next, thanks.

-- PMM



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