microblaze: preserve the MSR carry flags across signals

microblaze: preserve the MSR carry flags across signals

setup_sigcontext() and restore_sigcontext() copy r0-r31, pc, ear, esr and
fsr to and from the signal frame but never touch MSR.  The interrupted
MSR is therefore dropped from the signal context entirely: the handler's
ucontext does not expose it, a handler cannot adjust the resumed
arithmetic flags through uc_mcontext.regs.msr, and -- because
restore_sigcontext() leaves regs->msr as whatever the rt_sigreturn trap
left in it -- the interrupted context resumes with the carry produced by
the syscall entry path (_user_exception does "addi r14, r14, 4", which
writes carry), not with its own.

Other architectures round-trip the user-visible flags through the signal
frame (arm's cpsr, csky's carry, x86's eflags), so a handler can both read
and adjust the resumed flags; purely privileged status registers such as
riscv's sstatus are left out.  MicroBlaze should do the same for the
user-writable bits.

Concretely, MSR[C] (carry) is lost across signal delivery.  Code that
keeps a live carry across a point where a signal can be delivered -- for
example an lwx/swx compare-and-swap retry loop, between the swx and the
carry test -- resumes with the handler's carry and mis-evaluates the
result; the same failure class as the rt_sigreturn r3/r4 clobber, reached
through a different register.  Demonstrated under qemu-system-microblazeel
(machine petalogix-s3adsp1800): a handler that sets MSR_C in
uc_mcontext.regs.msr has no effect before this change (0 of 132 in-window
signals propagated) and takes effect after (130 of 132).

Save MSR in setup_sigcontext() so the handler's ucontext exposes it.  The
signal frame is user-writable, so restore_sigcontext() must not restore it
verbatim: MicroBlaze packs the user-writable carry (MSR_C, MSR_CC) and the
privileged control bits (MSR_UM, MSR_VM, MSR_IE, MSR_EE, ...) into the one
register, and a verbatim restore would let userspace alter privileged
return state -- rtbd derives the resumed mode from MSR_UMS/MSR_VMS.
Restore only MSR_C | MSR_CC from the frame and keep the rest from the
current regs->msr.  This mirrors x86's
restore_sigcontext(), which masks the restored EFLAGS to FIX_EFLAGS for
the same reason; arches whose status register is purely privileged (e.g.
riscv sstatus) simply do not restore it at all.

Comment on the exposed MSR reworded: it is not read-only,
restore_sigcontext() applies MSR_C|MSR_CC from the frame.  Author's
Signed-off-by kept; he agreed to comment and changelog cleanups.

Fixes: 2148daa9c45f ("microblaze_v8: Signal support")
Cc: stable@vger.kernel.org
Signed-off-by: Sam Price <thesamprice@gmail.com>
Signed-off-by: Ramin Moussavi <ramin.moussavi@yacoub.de>
Tested-by: Waldemar Brodkorb <wbx@openadk.org>
Link: https://patch.msgid.link/20260821151809.1233057-6-ramin.moussavi@yacoub.de
Signed-off-by: Michal Simek <michal.simek@amd.com>