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753b323687
For inline assembly, we place exception fixups out-of-line in the `.fixup` section such that these are out of the way of the fast path. This has a few drawbacks: * Since the fixup code is anonymous, backtraces will symbolize fixups as offsets from the nearest prior symbol, currently `__entry_tramp_text_end`. This is confusing, and painful to debug without access to the relevant vmlinux. * Since the exception handler adjusts the PC to execute the fixup, and the fixup uses a direct branch back into the function it fixes, backtraces of fixups miss the original function. This is confusing, and violates requirements for RELIABLE_STACKTRACE (and therefore LIVEPATCH). * Inline assembly and associated fixups are generated from templates, and we have many copies of logically identical fixups which only differ in which specific registers are written to and which address is branched to at the end of the fixup. This is potentially wasteful of I-cache resources, and makes it hard to add additional logic to fixups without significant bloat. * In the case of load_unaligned_zeropad(), the logic in the fixup requires a temporary register that we must allocate even in the fast-path where it will not be used. This patch address all four concerns for load_unaligned_zeropad() fixups by adding a dedicated exception handler which performs the fixup logic in exception context and subsequent returns back after the faulting instruction. For the moment, the fixup logic is identical to the old assembly fixup logic, but in future we could enhance this by taking the ESR and FAR into account to constrain the faults we try to fix up, or to specialize fixups for MTE tag check faults. Other than backtracing, there should be no functional change as a result of this patch. Signed-off-by: Mark Rutland <mark.rutland@arm.com> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: James Morse <james.morse@arm.com> Cc: Robin Murphy <robin.murphy@arm.com> Cc: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20211019160219.5202-13-mark.rutland@arm.com Signed-off-by: Will Deacon <will@kernel.org>
89 lines
1.9 KiB
C
89 lines
1.9 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Based on arch/arm/mm/extable.c
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*/
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#include <linux/bitfield.h>
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#include <linux/extable.h>
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#include <linux/uaccess.h>
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#include <asm/asm-extable.h>
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#include <asm/ptrace.h>
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typedef bool (*ex_handler_t)(const struct exception_table_entry *,
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struct pt_regs *);
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static inline unsigned long
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get_ex_fixup(const struct exception_table_entry *ex)
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{
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return ((unsigned long)&ex->fixup + ex->fixup);
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}
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static bool ex_handler_fixup(const struct exception_table_entry *ex,
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struct pt_regs *regs)
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{
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regs->pc = get_ex_fixup(ex);
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return true;
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}
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static bool ex_handler_uaccess_err_zero(const struct exception_table_entry *ex,
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struct pt_regs *regs)
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{
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int reg_err = FIELD_GET(EX_DATA_REG_ERR, ex->data);
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int reg_zero = FIELD_GET(EX_DATA_REG_ZERO, ex->data);
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pt_regs_write_reg(regs, reg_err, -EFAULT);
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pt_regs_write_reg(regs, reg_zero, 0);
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regs->pc = get_ex_fixup(ex);
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return true;
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}
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static bool
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ex_handler_load_unaligned_zeropad(const struct exception_table_entry *ex,
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struct pt_regs *regs)
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{
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int reg_data = FIELD_GET(EX_DATA_REG_DATA, ex->type);
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int reg_addr = FIELD_GET(EX_DATA_REG_ADDR, ex->type);
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unsigned long data, addr, offset;
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addr = pt_regs_read_reg(regs, reg_addr);
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offset = addr & 0x7UL;
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addr &= ~0x7UL;
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data = *(unsigned long*)addr;
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#ifndef __AARCH64EB__
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data >>= 8 * offset;
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#else
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data <<= 8 * offset;
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#endif
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pt_regs_write_reg(regs, reg_data, data);
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regs->pc = get_ex_fixup(ex);
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return true;
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}
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bool fixup_exception(struct pt_regs *regs)
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{
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const struct exception_table_entry *ex;
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ex = search_exception_tables(instruction_pointer(regs));
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if (!ex)
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return false;
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switch (ex->type) {
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case EX_TYPE_FIXUP:
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return ex_handler_fixup(ex, regs);
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case EX_TYPE_BPF:
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return ex_handler_bpf(ex, regs);
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case EX_TYPE_UACCESS_ERR_ZERO:
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return ex_handler_uaccess_err_zero(ex, regs);
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case EX_TYPE_LOAD_UNALIGNED_ZEROPAD:
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return ex_handler_load_unaligned_zeropad(ex, regs);
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}
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BUG();
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}
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