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432c6bacbd
In some cases the kernel needs to execute an instruction from the delay
slot of an emulated branch instruction. These cases include:
- Emulated floating point branch instructions (bc1[ft]l?) for systems
which don't include an FPU, or upon which the kernel is run with the
"nofpu" parameter.
- MIPSr6 systems running binaries targeting older revisions of the
architecture, which may include branch instructions whose encodings
are no longer valid in MIPSr6.
Executing instructions from such delay slots is done by writing the
instruction to memory followed by a trap, as part of an "emuframe", and
executing it. This avoids the requirement of an emulator for the entire
MIPS instruction set. Prior to this patch such emuframes are written to
the user stack and executed from there.
This patch moves FP branch delay emuframes off of the user stack and
into a per-mm page. Allocating a page per-mm leaves userland with access
to only what it had access to previously, and compared to other
solutions is relatively simple.
When a thread requires a delay slot emulation, it is allocated a frame.
A thread may only have one frame allocated at any one time, since it may
only ever be executing one instruction at any one time. In order to
ensure that we can free up allocated frame later, its index is recorded
in struct thread_struct. In the typical case, after executing the delay
slot instruction we'll execute a break instruction with the BRK_MEMU
code. This traps back to the kernel & leads to a call to do_dsemulret
which frees the allocated frame & moves the user PC back to the
instruction that would have executed following the emulated branch.
In some cases the delay slot instruction may be invalid, such as a
branch, or may trigger an exception. In these cases the BRK_MEMU break
instruction will not be hit. In order to ensure that frames are freed
this patch introduces dsemul_thread_cleanup() and calls it to free any
allocated frame upon thread exit. If the instruction generated an
exception & leads to a signal being delivered to the thread, or indeed
if a signal simply happens to be delivered to the thread whilst it is
executing from the struct emuframe, then we need to take care to exit
the frame appropriately. This is done by either rolling back the user PC
to the branch or advancing it to the continuation PC prior to signal
delivery, using dsemul_thread_rollback(). If this were not done then a
sigreturn would return to the struct emuframe, and if that frame had
meanwhile been used in response to an emulated branch instruction within
the signal handler then we would execute the wrong user code.
Whilst a user could theoretically place something like a compact branch
to self in a delay slot and cause their thread to become stuck in an
infinite loop with the frame never being deallocated, this would:
- Only affect the users single process.
- Be architecturally invalid since there would be a branch in the
delay slot, which is forbidden.
- Be extremely unlikely to happen by mistake, and provide a program
with no more ability to harm the system than a simple infinite loop
would.
If a thread requires a delay slot emulation & no frame is available to
it (ie. the process has enough other threads that all frames are
currently in use) then the thread joins a waitqueue. It will sleep until
a frame is freed by another thread in the process.
Since we now know whether a thread has an allocated frame due to our
tracking of its index, the cookie field of struct emuframe is removed as
we can be more certain whether we have a valid frame. Since a thread may
only ever have a single frame at any given time, the epc field of struct
emuframe is also removed & the PC to continue from is instead stored in
struct thread_struct. Together these changes simplify & shrink struct
emuframe somewhat, allowing twice as many frames to fit into the page
allocated for them.
The primary benefit of this patch is that we are now free to mark the
user stack non-executable where that is possible.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Leonid Yegoshin <leonid.yegoshin@imgtec.com>
Cc: Maciej Rozycki <maciej.rozycki@imgtec.com>
Cc: Faraz Shahbazker <faraz.shahbazker@imgtec.com>
Cc: Raghu Gandham <raghu.gandham@imgtec.com>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13764/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
85 lines
2.6 KiB
C
85 lines
2.6 KiB
C
/*
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* This program is free software; you can distribute it and/or modify it
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* under the terms of the GNU General Public License (Version 2) as
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* published by the Free Software Foundation.
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*
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* This program is distributed in the hope it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with this program; if not, write to the Free Software Foundation, Inc.,
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* 59 Temple Place - Suite 330, Boston MA 02111-1307, USA.
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*
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* Further private data for which no space exists in mips_fpu_struct.
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* This should be subsumed into the mips_fpu_struct structure as
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* defined in processor.h as soon as the absurd wired absolute assembler
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* offsets become dynamic at compile time.
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*
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* Kevin D. Kissell, kevink@mips.com and Carsten Langgaard, carstenl@mips.com
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* Copyright (C) 2000 MIPS Technologies, Inc. All rights reserved.
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*/
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#ifndef _ASM_FPU_EMULATOR_H
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#define _ASM_FPU_EMULATOR_H
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#include <linux/sched.h>
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#include <asm/dsemul.h>
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#include <asm/thread_info.h>
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#include <asm/inst.h>
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#include <asm/local.h>
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#include <asm/processor.h>
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#ifdef CONFIG_DEBUG_FS
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struct mips_fpu_emulator_stats {
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unsigned long emulated;
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unsigned long loads;
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unsigned long stores;
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unsigned long cp1ops;
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unsigned long cp1xops;
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unsigned long errors;
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unsigned long ieee754_inexact;
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unsigned long ieee754_underflow;
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unsigned long ieee754_overflow;
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unsigned long ieee754_zerodiv;
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unsigned long ieee754_invalidop;
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unsigned long ds_emul;
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};
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DECLARE_PER_CPU(struct mips_fpu_emulator_stats, fpuemustats);
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#define MIPS_FPU_EMU_INC_STATS(M) \
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do { \
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preempt_disable(); \
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__this_cpu_inc(fpuemustats.M); \
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preempt_enable(); \
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} while (0)
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#else
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#define MIPS_FPU_EMU_INC_STATS(M) do { } while (0)
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#endif /* CONFIG_DEBUG_FS */
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extern int fpu_emulator_cop1Handler(struct pt_regs *xcp,
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struct mips_fpu_struct *ctx, int has_fpu,
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void *__user *fault_addr);
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int process_fpemu_return(int sig, void __user *fault_addr,
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unsigned long fcr31);
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int isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
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unsigned long *contpc);
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int mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
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unsigned long *contpc);
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#define SIGNALLING_NAN 0x7ff800007ff80000LL
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static inline void fpu_emulator_init_fpu(void)
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{
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struct task_struct *t = current;
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int i;
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for (i = 0; i < 32; i++)
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set_fpr64(&t->thread.fpu.fpr[i], 0, SIGNALLING_NAN);
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}
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#endif /* _ASM_FPU_EMULATOR_H */
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