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308 lines
9.1 KiB
308 lines
9.1 KiB
// SPDX-License-Identifier: GPL-2.0 |
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#include <linux/err.h> |
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#include <linux/slab.h> |
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#include <linux/mm_types.h> |
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#include <linux/sched/task.h> |
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#include <asm/branch.h> |
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#include <asm/cacheflush.h> |
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#include <asm/fpu_emulator.h> |
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#include <asm/inst.h> |
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#include <asm/mipsregs.h> |
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#include <linux/uaccess.h> |
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/** |
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* struct emuframe - The 'emulation' frame structure |
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* @emul: The instruction to 'emulate'. |
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* @badinst: A break instruction to cause a return to the kernel. |
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* |
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* This structure defines the frames placed within the delay slot emulation |
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* page in response to a call to mips_dsemul(). Each thread may be allocated |
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* only one frame at any given time. The kernel stores within it the |
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* instruction to be 'emulated' followed by a break instruction, then |
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* executes the frame in user mode. The break causes a trap to the kernel |
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* which leads to do_dsemulret() being called unless the instruction in |
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* @emul causes a trap itself, is a branch, or a signal is delivered to |
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* the thread. In these cases the allocated frame will either be reused by |
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* a subsequent delay slot 'emulation', or be freed during signal delivery or |
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* upon thread exit. |
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* |
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* This approach is used because: |
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* |
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* - Actually emulating all instructions isn't feasible. We would need to |
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* be able to handle instructions from all revisions of the MIPS ISA, |
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* all ASEs & all vendor instruction set extensions. This would be a |
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* whole lot of work & continual maintenance burden as new instructions |
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* are introduced, and in the case of some vendor extensions may not |
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* even be possible. Thus we need to take the approach of actually |
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* executing the instruction. |
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* |
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* - We must execute the instruction within user context. If we were to |
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* execute the instruction in kernel mode then it would have access to |
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* kernel resources without very careful checks, leaving us with a |
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* high potential for security or stability issues to arise. |
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* |
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* - We used to place the frame on the users stack, but this requires |
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* that the stack be executable. This is bad for security so the |
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* per-process page is now used instead. |
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* |
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* - The instruction in @emul may be something entirely invalid for a |
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* delay slot. The user may (intentionally or otherwise) place a branch |
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* in a delay slot, or a kernel mode instruction, or something else |
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* which generates an exception. Thus we can't rely upon the break in |
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* @badinst always being hit. For this reason we track the index of the |
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* frame allocated to each thread, allowing us to clean it up at later |
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* points such as signal delivery or thread exit. |
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* |
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* - The user may generate a fake struct emuframe if they wish, invoking |
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* the BRK_MEMU break instruction themselves. We must therefore not |
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* trust that BRK_MEMU means there's actually a valid frame allocated |
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* to the thread, and must not allow the user to do anything they |
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* couldn't already. |
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*/ |
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struct emuframe { |
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mips_instruction emul; |
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mips_instruction badinst; |
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}; |
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static const int emupage_frame_count = PAGE_SIZE / sizeof(struct emuframe); |
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static inline __user struct emuframe *dsemul_page(void) |
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{ |
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return (__user struct emuframe *)STACK_TOP; |
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} |
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static int alloc_emuframe(void) |
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{ |
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mm_context_t *mm_ctx = ¤t->mm->context; |
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int idx; |
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retry: |
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spin_lock(&mm_ctx->bd_emupage_lock); |
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/* Ensure we have an allocation bitmap */ |
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if (!mm_ctx->bd_emupage_allocmap) { |
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mm_ctx->bd_emupage_allocmap = |
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kcalloc(BITS_TO_LONGS(emupage_frame_count), |
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sizeof(unsigned long), |
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GFP_ATOMIC); |
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if (!mm_ctx->bd_emupage_allocmap) { |
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idx = BD_EMUFRAME_NONE; |
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goto out_unlock; |
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} |
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} |
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/* Attempt to allocate a single bit/frame */ |
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idx = bitmap_find_free_region(mm_ctx->bd_emupage_allocmap, |
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emupage_frame_count, 0); |
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if (idx < 0) { |
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/* |
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* Failed to allocate a frame. We'll wait until one becomes |
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* available. We unlock the page so that other threads actually |
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* get the opportunity to free their frames, which means |
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* technically the result of bitmap_full may be incorrect. |
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* However the worst case is that we repeat all this and end up |
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* back here again. |
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*/ |
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spin_unlock(&mm_ctx->bd_emupage_lock); |
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if (!wait_event_killable(mm_ctx->bd_emupage_queue, |
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!bitmap_full(mm_ctx->bd_emupage_allocmap, |
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emupage_frame_count))) |
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goto retry; |
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/* Received a fatal signal - just give in */ |
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return BD_EMUFRAME_NONE; |
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} |
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/* Success! */ |
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pr_debug("allocate emuframe %d to %d\n", idx, current->pid); |
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out_unlock: |
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spin_unlock(&mm_ctx->bd_emupage_lock); |
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return idx; |
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} |
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static void free_emuframe(int idx, struct mm_struct *mm) |
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{ |
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mm_context_t *mm_ctx = &mm->context; |
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spin_lock(&mm_ctx->bd_emupage_lock); |
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pr_debug("free emuframe %d from %d\n", idx, current->pid); |
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bitmap_clear(mm_ctx->bd_emupage_allocmap, idx, 1); |
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/* If some thread is waiting for a frame, now's its chance */ |
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wake_up(&mm_ctx->bd_emupage_queue); |
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spin_unlock(&mm_ctx->bd_emupage_lock); |
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} |
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static bool within_emuframe(struct pt_regs *regs) |
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{ |
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unsigned long base = (unsigned long)dsemul_page(); |
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if (regs->cp0_epc < base) |
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return false; |
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if (regs->cp0_epc >= (base + PAGE_SIZE)) |
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return false; |
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return true; |
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} |
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bool dsemul_thread_cleanup(struct task_struct *tsk) |
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{ |
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int fr_idx; |
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/* Clear any allocated frame, retrieving its index */ |
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fr_idx = atomic_xchg(&tsk->thread.bd_emu_frame, BD_EMUFRAME_NONE); |
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/* If no frame was allocated, we're done */ |
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if (fr_idx == BD_EMUFRAME_NONE) |
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return false; |
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task_lock(tsk); |
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/* Free the frame that this thread had allocated */ |
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if (tsk->mm) |
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free_emuframe(fr_idx, tsk->mm); |
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task_unlock(tsk); |
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return true; |
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} |
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bool dsemul_thread_rollback(struct pt_regs *regs) |
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{ |
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struct emuframe __user *fr; |
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int fr_idx; |
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/* Do nothing if we're not executing from a frame */ |
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if (!within_emuframe(regs)) |
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return false; |
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/* Find the frame being executed */ |
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fr_idx = atomic_read(¤t->thread.bd_emu_frame); |
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if (fr_idx == BD_EMUFRAME_NONE) |
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return false; |
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fr = &dsemul_page()[fr_idx]; |
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/* |
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* If the PC is at the emul instruction, roll back to the branch. If |
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* PC is at the badinst (break) instruction, we've already emulated the |
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* instruction so progress to the continue PC. If it's anything else |
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* then something is amiss & the user has branched into some other area |
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* of the emupage - we'll free the allocated frame anyway. |
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*/ |
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if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->emul) |
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regs->cp0_epc = current->thread.bd_emu_branch_pc; |
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else if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->badinst) |
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regs->cp0_epc = current->thread.bd_emu_cont_pc; |
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atomic_set(¤t->thread.bd_emu_frame, BD_EMUFRAME_NONE); |
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free_emuframe(fr_idx, current->mm); |
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return true; |
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} |
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void dsemul_mm_cleanup(struct mm_struct *mm) |
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{ |
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mm_context_t *mm_ctx = &mm->context; |
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kfree(mm_ctx->bd_emupage_allocmap); |
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} |
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int mips_dsemul(struct pt_regs *regs, mips_instruction ir, |
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unsigned long branch_pc, unsigned long cont_pc) |
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{ |
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int isa16 = get_isa16_mode(regs->cp0_epc); |
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mips_instruction break_math; |
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unsigned long fr_uaddr; |
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struct emuframe fr; |
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int fr_idx, ret; |
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/* NOP is easy */ |
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if (ir == 0) |
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return -1; |
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/* microMIPS instructions */ |
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if (isa16) { |
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union mips_instruction insn = { .word = ir }; |
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/* NOP16 aka MOVE16 $0, $0 */ |
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if ((ir >> 16) == MM_NOP16) |
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return -1; |
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/* ADDIUPC */ |
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if (insn.mm_a_format.opcode == mm_addiupc_op) { |
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unsigned int rs; |
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s32 v; |
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rs = (((insn.mm_a_format.rs + 0xe) & 0xf) + 2); |
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v = regs->cp0_epc & ~3; |
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v += insn.mm_a_format.simmediate << 2; |
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regs->regs[rs] = (long)v; |
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return -1; |
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} |
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} |
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pr_debug("dsemul 0x%08lx cont at 0x%08lx\n", regs->cp0_epc, cont_pc); |
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/* Allocate a frame if we don't already have one */ |
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fr_idx = atomic_read(¤t->thread.bd_emu_frame); |
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if (fr_idx == BD_EMUFRAME_NONE) |
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fr_idx = alloc_emuframe(); |
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if (fr_idx == BD_EMUFRAME_NONE) |
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return SIGBUS; |
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/* Retrieve the appropriately encoded break instruction */ |
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break_math = BREAK_MATH(isa16); |
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/* Write the instructions to the frame */ |
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if (isa16) { |
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union mips_instruction _emul = { |
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.halfword = { ir >> 16, ir } |
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}; |
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union mips_instruction _badinst = { |
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.halfword = { break_math >> 16, break_math } |
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}; |
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fr.emul = _emul.word; |
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fr.badinst = _badinst.word; |
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} else { |
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fr.emul = ir; |
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fr.badinst = break_math; |
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} |
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/* Write the frame to user memory */ |
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fr_uaddr = (unsigned long)&dsemul_page()[fr_idx]; |
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ret = access_process_vm(current, fr_uaddr, &fr, sizeof(fr), |
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FOLL_FORCE | FOLL_WRITE); |
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if (unlikely(ret != sizeof(fr))) { |
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MIPS_FPU_EMU_INC_STATS(errors); |
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free_emuframe(fr_idx, current->mm); |
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return SIGBUS; |
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} |
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/* Record the PC of the branch, PC to continue from & frame index */ |
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current->thread.bd_emu_branch_pc = branch_pc; |
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current->thread.bd_emu_cont_pc = cont_pc; |
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atomic_set(¤t->thread.bd_emu_frame, fr_idx); |
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/* Change user register context to execute the frame */ |
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regs->cp0_epc = fr_uaddr | isa16; |
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return 0; |
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} |
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bool do_dsemulret(struct pt_regs *xcp) |
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{ |
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/* Cleanup the allocated frame, returning if there wasn't one */ |
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if (!dsemul_thread_cleanup(current)) { |
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MIPS_FPU_EMU_INC_STATS(errors); |
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return false; |
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} |
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/* Set EPC to return to post-branch instruction */ |
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xcp->cp0_epc = current->thread.bd_emu_cont_pc; |
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pr_debug("dsemulret to 0x%08lx\n", xcp->cp0_epc); |
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MIPS_FPU_EMU_INC_STATS(ds_emul); |
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return true; |
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}
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