On 20/07/2020 18:00, Dave Martin wrote:
On Mon, Jul 20, 2020 at 04:30:35PM +0100, Kevin Brodsky wrote:
On 15/07/2020 18:08, Catalin Marinas wrote:
By default, even if PROT_MTE is set on a memory range, there is no tag
check fault reporting (SIGSEGV). Introduce a set of option to the
exiting prctl(PR_SET_TAGGED_ADDR_CTRL) to allow user control of the tag
check fault mode:
PR_MTE_TCF_NONE - no reporting (default)
PR_MTE_TCF_SYNC - synchronous tag check fault reporting
PR_MTE_TCF_ASYNC - asynchronous tag check fault reporting
These options translate into the corresponding SCTLR_EL1.TCF0 bitfield,
context-switched by the kernel. Note that uaccess done by the kernel is
not checked and cannot be configured by the user.
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
---
Notes:
v3:
- Use SCTLR_EL1_TCF0_NONE instead of 0 for consistency.
- Move mte_thread_switch() in this patch from an earlier one. In
addition, it is called after the dsb() in __switch_to() so that any
asynchronous tag check faults have been registered in the TFSR_EL1
registers (to be added with the in-kernel MTE support.
v2:
- Handle SCTLR_EL1_TCF0_NONE explicitly for consistency with PR_MTE_TCF_NONE.
- Fix SCTLR_EL1 register setting in flush_mte_state() (thanks to Peter
Collingbourne).
- Added ISB to update_sctlr_el1_tcf0() since, with the latest
architecture update/fix, the TCF0 field is used by the uaccess
routines.
[...]
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
[...]
+void mte_thread_switch(struct task_struct *next)
+{
+ if (!system_supports_mte())
+ return;
+
+ /* avoid expensive SCTLR_EL1 accesses if no change */
+ if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
I think this could be improved by checking whether `next` is a kernel
thread, in which case thread.sctlr_tcf0 is 0 but there is no point in
setting SCTLR_EL1.TCF0, since there should not be any access via TTBR0.
Out of interest, do we have a nice way of testing for a kernel thread
now?
Isn't it as simple as checking if PF_KTHREAD is set in tsk->flags? At least this is
what ssbs_thread_switch() does.
Kevin
I remember fpsimd_thread_switch() used to check for task->mm, but we
seem to have got rid of that at some point. set_mm() can defeat this,
and anyway the heavy lifting for FPSIMD is now deferred until returning
to userspace.
Cheers
---Dave
