linux-user/aarch64: Do not clear PROT_MTE on mprotect

The documentation for PROT_MTE says that it cannot be cleared
by mprotect.  Further, the implementation of the VM_ARCH_CLEAR bit,
contains PROT_BTI confiming that bit should be cleared.

Introduce PAGE_TARGET_STICKY to allow target/arch/cpu.h to control
which bits may be reset during page_set_flags.  This is sort of the
opposite of VM_ARCH_CLEAR, but works better with qemu's PAGE_* bits
that are separate from PROT_* bits.

Reported-by: Vitaly Buka <vitalybuka@google.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220711031420.17820-1-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index 1e36a839ee..6afcc882f2 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -3392,9 +3392,12 @@ static inline MemTxAttrs *typecheck_memtxattrs(MemTxAttrs *x)
 
 /*
  * AArch64 usage of the PAGE_TARGET_* bits for linux-user.
+ * Note that with the Linux kernel, PROT_MTE may not be cleared by mprotect
+ * mprotect but PROT_BTI may be cleared.  C.f. the kernel's VM_ARCH_CLEAR.
  */
-#define PAGE_BTI  PAGE_TARGET_1
-#define PAGE_MTE  PAGE_TARGET_2
+#define PAGE_BTI            PAGE_TARGET_1
+#define PAGE_MTE            PAGE_TARGET_2
+#define PAGE_TARGET_STICKY  PAGE_MTE
 
 #ifdef TARGET_TAGGED_ADDRESSES
 /**