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![Peter Collingbourne](/assets/img/avatar_default.png)
It is not necessary to write to GCR_EL1 on every kernel entry and exit when HW tag-based KASAN is disabled because the kernel will not execute any IRG instructions in that mode. Since accessing GCR_EL1 can be expensive on some microarchitectures, avoid doing so by moving the access to task switch when HW tag-based KASAN is disabled. Signed-off-by: Peter Collingbourne <pcc@google.com> Acked-by: Andrey Konovalov <andreyknvl@gmail.com> Link: https://linux-review.googlesource.com/id/I78e90d60612a94c24344526f476ac4ff216e10d2 Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Link: https://lore.kernel.org/r/20210924010655.2886918-1-pcc@google.com Signed-off-by: Will Deacon <will@kernel.org>
482 lines
12 KiB
C
482 lines
12 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* Copyright (C) 2020 ARM Ltd.
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*/
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#include <linux/bitops.h>
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#include <linux/cpu.h>
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#include <linux/kernel.h>
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#include <linux/mm.h>
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#include <linux/prctl.h>
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#include <linux/sched.h>
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#include <linux/sched/mm.h>
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#include <linux/string.h>
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#include <linux/swap.h>
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#include <linux/swapops.h>
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#include <linux/thread_info.h>
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#include <linux/types.h>
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#include <linux/uio.h>
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#include <asm/barrier.h>
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#include <asm/cpufeature.h>
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#include <asm/mte.h>
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#include <asm/ptrace.h>
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#include <asm/sysreg.h>
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static DEFINE_PER_CPU_READ_MOSTLY(u64, mte_tcf_preferred);
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#ifdef CONFIG_KASAN_HW_TAGS
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/* Whether the MTE asynchronous mode is enabled. */
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DEFINE_STATIC_KEY_FALSE(mte_async_mode);
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EXPORT_SYMBOL_GPL(mte_async_mode);
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#endif
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static void mte_sync_page_tags(struct page *page, pte_t old_pte,
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bool check_swap, bool pte_is_tagged)
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{
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if (check_swap && is_swap_pte(old_pte)) {
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swp_entry_t entry = pte_to_swp_entry(old_pte);
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if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
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return;
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}
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if (!pte_is_tagged)
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return;
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page_kasan_tag_reset(page);
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/*
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* We need smp_wmb() in between setting the flags and clearing the
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* tags because if another thread reads page->flags and builds a
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* tagged address out of it, there is an actual dependency to the
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* memory access, but on the current thread we do not guarantee that
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* the new page->flags are visible before the tags were updated.
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*/
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smp_wmb();
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mte_clear_page_tags(page_address(page));
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}
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void mte_sync_tags(pte_t old_pte, pte_t pte)
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{
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struct page *page = pte_page(pte);
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long i, nr_pages = compound_nr(page);
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bool check_swap = nr_pages == 1;
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bool pte_is_tagged = pte_tagged(pte);
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/* Early out if there's nothing to do */
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if (!check_swap && !pte_is_tagged)
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return;
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/* if PG_mte_tagged is set, tags have already been initialised */
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for (i = 0; i < nr_pages; i++, page++) {
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if (!test_and_set_bit(PG_mte_tagged, &page->flags))
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mte_sync_page_tags(page, old_pte, check_swap,
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pte_is_tagged);
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}
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}
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int memcmp_pages(struct page *page1, struct page *page2)
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{
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char *addr1, *addr2;
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int ret;
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addr1 = page_address(page1);
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addr2 = page_address(page2);
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ret = memcmp(addr1, addr2, PAGE_SIZE);
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if (!system_supports_mte() || ret)
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return ret;
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/*
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* If the page content is identical but at least one of the pages is
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* tagged, return non-zero to avoid KSM merging. If only one of the
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* pages is tagged, set_pte_at() may zero or change the tags of the
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* other page via mte_sync_tags().
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*/
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if (test_bit(PG_mte_tagged, &page1->flags) ||
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test_bit(PG_mte_tagged, &page2->flags))
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return addr1 != addr2;
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return ret;
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}
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static inline void __mte_enable_kernel(const char *mode, unsigned long tcf)
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{
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/* Enable MTE Sync Mode for EL1. */
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sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, tcf);
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isb();
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pr_info_once("MTE: enabled in %s mode at EL1\n", mode);
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}
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#ifdef CONFIG_KASAN_HW_TAGS
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void mte_enable_kernel_sync(void)
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{
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/*
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* Make sure we enter this function when no PE has set
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* async mode previously.
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*/
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WARN_ONCE(system_uses_mte_async_mode(),
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"MTE async mode enabled system wide!");
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__mte_enable_kernel("synchronous", SCTLR_ELx_TCF_SYNC);
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}
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void mte_enable_kernel_async(void)
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{
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__mte_enable_kernel("asynchronous", SCTLR_ELx_TCF_ASYNC);
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/*
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* MTE async mode is set system wide by the first PE that
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* executes this function.
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*
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* Note: If in future KASAN acquires a runtime switching
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* mode in between sync and async, this strategy needs
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* to be reviewed.
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*/
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if (!system_uses_mte_async_mode())
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static_branch_enable(&mte_async_mode);
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}
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#endif
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#ifdef CONFIG_KASAN_HW_TAGS
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void mte_check_tfsr_el1(void)
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{
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u64 tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
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if (unlikely(tfsr_el1 & SYS_TFSR_EL1_TF1)) {
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/*
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* Note: isb() is not required after this direct write
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* because there is no indirect read subsequent to it
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* (per ARM DDI 0487F.c table D13-1).
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*/
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write_sysreg_s(0, SYS_TFSR_EL1);
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kasan_report_async();
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}
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}
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#endif
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static void mte_update_sctlr_user(struct task_struct *task)
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{
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/*
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* This must be called with preemption disabled and can only be called
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* on the current or next task since the CPU must match where the thread
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* is going to run. The caller is responsible for calling
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* update_sctlr_el1() later in the same preemption disabled block.
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*/
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unsigned long sctlr = task->thread.sctlr_user;
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unsigned long mte_ctrl = task->thread.mte_ctrl;
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unsigned long pref, resolved_mte_tcf;
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pref = __this_cpu_read(mte_tcf_preferred);
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resolved_mte_tcf = (mte_ctrl & pref) ? pref : mte_ctrl;
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sctlr &= ~SCTLR_EL1_TCF0_MASK;
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if (resolved_mte_tcf & MTE_CTRL_TCF_ASYNC)
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sctlr |= SCTLR_EL1_TCF0_ASYNC;
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else if (resolved_mte_tcf & MTE_CTRL_TCF_SYNC)
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sctlr |= SCTLR_EL1_TCF0_SYNC;
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task->thread.sctlr_user = sctlr;
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}
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static void mte_update_gcr_excl(struct task_struct *task)
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{
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/*
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* SYS_GCR_EL1 will be set to current->thread.mte_ctrl value by
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* mte_set_user_gcr() in kernel_exit, but only if KASAN is enabled.
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*/
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if (kasan_hw_tags_enabled())
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return;
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write_sysreg_s(
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((task->thread.mte_ctrl >> MTE_CTRL_GCR_USER_EXCL_SHIFT) &
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SYS_GCR_EL1_EXCL_MASK) | SYS_GCR_EL1_RRND,
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SYS_GCR_EL1);
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}
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void __init kasan_hw_tags_enable(struct alt_instr *alt, __le32 *origptr,
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__le32 *updptr, int nr_inst)
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{
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BUG_ON(nr_inst != 1); /* Branch -> NOP */
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if (kasan_hw_tags_enabled())
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*updptr = cpu_to_le32(aarch64_insn_gen_nop());
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}
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void mte_thread_init_user(void)
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{
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if (!system_supports_mte())
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return;
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/* clear any pending asynchronous tag fault */
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dsb(ish);
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write_sysreg_s(0, SYS_TFSRE0_EL1);
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clear_thread_flag(TIF_MTE_ASYNC_FAULT);
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/* disable tag checking and reset tag generation mask */
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set_mte_ctrl(current, 0);
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}
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void mte_thread_switch(struct task_struct *next)
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{
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if (!system_supports_mte())
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return;
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mte_update_sctlr_user(next);
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mte_update_gcr_excl(next);
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/*
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* Check if an async tag exception occurred at EL1.
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*
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* Note: On the context switch path we rely on the dsb() present
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* in __switch_to() to guarantee that the indirect writes to TFSR_EL1
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* are synchronized before this point.
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*/
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isb();
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mte_check_tfsr_el1();
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}
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void mte_suspend_enter(void)
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{
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if (!system_supports_mte())
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return;
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/*
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* The barriers are required to guarantee that the indirect writes
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* to TFSR_EL1 are synchronized before we report the state.
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*/
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dsb(nsh);
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isb();
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/* Report SYS_TFSR_EL1 before suspend entry */
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mte_check_tfsr_el1();
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}
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long set_mte_ctrl(struct task_struct *task, unsigned long arg)
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{
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u64 mte_ctrl = (~((arg & PR_MTE_TAG_MASK) >> PR_MTE_TAG_SHIFT) &
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SYS_GCR_EL1_EXCL_MASK) << MTE_CTRL_GCR_USER_EXCL_SHIFT;
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if (!system_supports_mte())
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return 0;
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if (arg & PR_MTE_TCF_ASYNC)
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mte_ctrl |= MTE_CTRL_TCF_ASYNC;
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if (arg & PR_MTE_TCF_SYNC)
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mte_ctrl |= MTE_CTRL_TCF_SYNC;
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task->thread.mte_ctrl = mte_ctrl;
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if (task == current) {
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preempt_disable();
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mte_update_sctlr_user(task);
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mte_update_gcr_excl(task);
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update_sctlr_el1(task->thread.sctlr_user);
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preempt_enable();
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}
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return 0;
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}
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long get_mte_ctrl(struct task_struct *task)
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{
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unsigned long ret;
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u64 mte_ctrl = task->thread.mte_ctrl;
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u64 incl = (~mte_ctrl >> MTE_CTRL_GCR_USER_EXCL_SHIFT) &
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SYS_GCR_EL1_EXCL_MASK;
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if (!system_supports_mte())
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return 0;
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ret = incl << PR_MTE_TAG_SHIFT;
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if (mte_ctrl & MTE_CTRL_TCF_ASYNC)
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ret |= PR_MTE_TCF_ASYNC;
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if (mte_ctrl & MTE_CTRL_TCF_SYNC)
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ret |= PR_MTE_TCF_SYNC;
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return ret;
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}
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/*
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* Access MTE tags in another process' address space as given in mm. Update
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* the number of tags copied. Return 0 if any tags copied, error otherwise.
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* Inspired by __access_remote_vm().
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*/
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static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
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struct iovec *kiov, unsigned int gup_flags)
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{
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struct vm_area_struct *vma;
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void __user *buf = kiov->iov_base;
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size_t len = kiov->iov_len;
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int ret;
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int write = gup_flags & FOLL_WRITE;
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if (!access_ok(buf, len))
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return -EFAULT;
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if (mmap_read_lock_killable(mm))
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return -EIO;
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while (len) {
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unsigned long tags, offset;
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void *maddr;
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struct page *page = NULL;
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ret = get_user_pages_remote(mm, addr, 1, gup_flags, &page,
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&vma, NULL);
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if (ret <= 0)
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break;
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/*
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* Only copy tags if the page has been mapped as PROT_MTE
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* (PG_mte_tagged set). Otherwise the tags are not valid and
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* not accessible to user. Moreover, an mprotect(PROT_MTE)
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* would cause the existing tags to be cleared if the page
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* was never mapped with PROT_MTE.
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*/
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if (!(vma->vm_flags & VM_MTE)) {
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ret = -EOPNOTSUPP;
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put_page(page);
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break;
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}
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WARN_ON_ONCE(!test_bit(PG_mte_tagged, &page->flags));
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/* limit access to the end of the page */
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offset = offset_in_page(addr);
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tags = min(len, (PAGE_SIZE - offset) / MTE_GRANULE_SIZE);
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maddr = page_address(page);
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if (write) {
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tags = mte_copy_tags_from_user(maddr + offset, buf, tags);
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set_page_dirty_lock(page);
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} else {
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tags = mte_copy_tags_to_user(buf, maddr + offset, tags);
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}
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put_page(page);
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/* error accessing the tracer's buffer */
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if (!tags)
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break;
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len -= tags;
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buf += tags;
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addr += tags * MTE_GRANULE_SIZE;
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}
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mmap_read_unlock(mm);
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/* return an error if no tags copied */
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kiov->iov_len = buf - kiov->iov_base;
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if (!kiov->iov_len) {
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/* check for error accessing the tracee's address space */
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if (ret <= 0)
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return -EIO;
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else
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return -EFAULT;
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}
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return 0;
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}
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/*
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* Copy MTE tags in another process' address space at 'addr' to/from tracer's
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* iovec buffer. Return 0 on success. Inspired by ptrace_access_vm().
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*/
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static int access_remote_tags(struct task_struct *tsk, unsigned long addr,
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struct iovec *kiov, unsigned int gup_flags)
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{
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struct mm_struct *mm;
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int ret;
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mm = get_task_mm(tsk);
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if (!mm)
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return -EPERM;
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if (!tsk->ptrace || (current != tsk->parent) ||
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((get_dumpable(mm) != SUID_DUMP_USER) &&
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!ptracer_capable(tsk, mm->user_ns))) {
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mmput(mm);
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return -EPERM;
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}
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ret = __access_remote_tags(mm, addr, kiov, gup_flags);
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mmput(mm);
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return ret;
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}
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int mte_ptrace_copy_tags(struct task_struct *child, long request,
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unsigned long addr, unsigned long data)
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{
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int ret;
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struct iovec kiov;
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struct iovec __user *uiov = (void __user *)data;
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unsigned int gup_flags = FOLL_FORCE;
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if (!system_supports_mte())
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return -EIO;
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if (get_user(kiov.iov_base, &uiov->iov_base) ||
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get_user(kiov.iov_len, &uiov->iov_len))
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return -EFAULT;
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if (request == PTRACE_POKEMTETAGS)
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gup_flags |= FOLL_WRITE;
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/* align addr to the MTE tag granule */
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addr &= MTE_GRANULE_MASK;
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ret = access_remote_tags(child, addr, &kiov, gup_flags);
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if (!ret)
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ret = put_user(kiov.iov_len, &uiov->iov_len);
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return ret;
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}
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static ssize_t mte_tcf_preferred_show(struct device *dev,
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struct device_attribute *attr, char *buf)
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{
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switch (per_cpu(mte_tcf_preferred, dev->id)) {
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case MTE_CTRL_TCF_ASYNC:
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return sysfs_emit(buf, "async\n");
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case MTE_CTRL_TCF_SYNC:
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return sysfs_emit(buf, "sync\n");
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default:
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return sysfs_emit(buf, "???\n");
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}
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}
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static ssize_t mte_tcf_preferred_store(struct device *dev,
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struct device_attribute *attr,
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const char *buf, size_t count)
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{
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u64 tcf;
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if (sysfs_streq(buf, "async"))
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tcf = MTE_CTRL_TCF_ASYNC;
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else if (sysfs_streq(buf, "sync"))
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tcf = MTE_CTRL_TCF_SYNC;
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else
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return -EINVAL;
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device_lock(dev);
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per_cpu(mte_tcf_preferred, dev->id) = tcf;
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device_unlock(dev);
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return count;
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}
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static DEVICE_ATTR_RW(mte_tcf_preferred);
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static int register_mte_tcf_preferred_sysctl(void)
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{
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unsigned int cpu;
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if (!system_supports_mte())
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return 0;
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for_each_possible_cpu(cpu) {
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per_cpu(mte_tcf_preferred, cpu) = MTE_CTRL_TCF_ASYNC;
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device_create_file(get_cpu_device(cpu),
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&dev_attr_mte_tcf_preferred);
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}
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return 0;
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}
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subsys_initcall(register_mte_tcf_preferred_sysctl);
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