246 lines
7.9 KiB
Python
Executable File
246 lines
7.9 KiB
Python
Executable File
#!/usr/bin/env python3
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#
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# Copyright (c) 2017 Intel Corporation
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#
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# SPDX-License-Identifier: Apache-2.0
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"""Generate a Global Descriptor Table (GDT) for x86 CPUs.
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For additional detail on GDT and x86 memory management, please
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consult the IA Architecture SW Developer Manual, vol. 3.
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This script accepts as input the zephyr_prebuilt.elf binary,
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which is a link of the Zephyr kernel without various build-time
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generated data structures (such as the GDT) inserted into it.
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This kernel image has been properly padded such that inserting
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these data structures will not disturb the memory addresses of
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other symbols.
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The input kernel ELF binary is used to obtain the following
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information:
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- Memory addresses of the Main and Double Fault TSS structures
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so GDT descriptors can be created for them
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- Memory addresses of where the GDT lives in memory, so that this
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address can be populated in the GDT pseudo descriptor
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- whether userspace or HW stack protection are enabled in Kconfig
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The output is a GDT whose contents depend on the kernel
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configuration. With no memory protection features enabled,
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we generate flat 32-bit code and data segments. If hardware-
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based stack overflow protection or userspace is enabled,
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we additionally create descriptors for the main and double-
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fault IA tasks, needed for userspace privilege elevation and
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double-fault handling. If userspace is enabled, we also create
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flat code/data segments for ring 3 execution.
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"""
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import argparse
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import sys
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import struct
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import os
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from packaging import version
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import elftools
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from elftools.elf.elffile import ELFFile
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from elftools.elf.sections import SymbolTableSection
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if version.parse(elftools.__version__) < version.parse('0.24'):
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sys.exit("pyelftools is out of date, need version 0.24 or later")
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def debug(text):
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"""Display debug message if --verbose"""
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if args.verbose:
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sys.stdout.write(os.path.basename(sys.argv[0]) + ": " + text + "\n")
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def error(text):
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"""Exit program with an error message"""
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sys.exit(os.path.basename(sys.argv[0]) + ": " + text)
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GDT_PD_FMT = "<HIH"
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FLAGS_GRAN = 1 << 7 # page granularity
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ACCESS_EX = 1 << 3 # executable
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ACCESS_DC = 1 << 2 # direction/conforming
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ACCESS_RW = 1 << 1 # read or write permission
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# 6 byte pseudo descriptor, but we're going to actually use this as the
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# zero descriptor and return 8 bytes
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def create_gdt_pseudo_desc(addr, size):
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"""Create pseudo GDT descriptor"""
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debug("create pseudo descriptor: %x %x" % (addr, size))
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# ...and take back one byte for the Intel god whose Ark this is...
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size = size - 1
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return struct.pack(GDT_PD_FMT, size, addr, 0)
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def chop_base_limit(base, limit):
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"""Limit argument always in bytes"""
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base_lo = base & 0xFFFF
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base_mid = (base >> 16) & 0xFF
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base_hi = (base >> 24) & 0xFF
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limit_lo = limit & 0xFFFF
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limit_hi = (limit >> 16) & 0xF
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return (base_lo, base_mid, base_hi, limit_lo, limit_hi)
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GDT_ENT_FMT = "<HHBBBB"
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def create_code_data_entry(base, limit, dpl, flags, access):
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"""Create GDT entry for code or data"""
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debug("create code or data entry: %x %x %x %x %x" %
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(base, limit, dpl, flags, access))
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base_lo, base_mid, base_hi, limit_lo, limit_hi = chop_base_limit(base,
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limit)
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# This is a valid descriptor
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present = 1
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# 32-bit protected mode
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size = 1
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# 1 = code or data, 0 = system type
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desc_type = 1
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# Just set accessed to 1 already so the CPU doesn't need it update it,
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# prevents freakouts if the GDT is in ROM, we don't care about this
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# bit in the OS
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accessed = 1
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access = access | (present << 7) | (dpl << 5) | (desc_type << 4) | accessed
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flags = flags | (size << 6) | limit_hi
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return struct.pack(GDT_ENT_FMT, limit_lo, base_lo, base_mid,
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access, flags, base_hi)
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def create_tss_entry(base, limit, dpl):
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"""Create GDT TSS entry"""
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debug("create TSS entry: %x %x %x" % (base, limit, dpl))
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present = 1
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base_lo, base_mid, base_hi, limit_lo, limit_hi, = chop_base_limit(base,
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limit)
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type_code = 0x9 # non-busy 32-bit TSS descriptor
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gran = 0
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flags = (gran << 7) | limit_hi
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type_byte = (present << 7) | (dpl << 5) | type_code
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return struct.pack(GDT_ENT_FMT, limit_lo, base_lo, base_mid,
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type_byte, flags, base_hi)
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def get_symbols(obj):
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"""Extract all symbols from ELF file object"""
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for section in obj.iter_sections():
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if isinstance(section, SymbolTableSection):
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return {sym.name: sym.entry.st_value
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for sym in section.iter_symbols()}
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raise LookupError("Could not find symbol table")
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def parse_args():
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"""Parse command line arguments"""
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global args
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parser = argparse.ArgumentParser(
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description=__doc__,
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formatter_class=argparse.RawDescriptionHelpFormatter, allow_abbrev=False)
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parser.add_argument("-k", "--kernel", required=True,
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help="Zephyr kernel image")
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parser.add_argument("-v", "--verbose", action="store_true",
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help="Print extra debugging information")
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parser.add_argument("-o", "--output-gdt", required=True,
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help="output GDT binary")
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args = parser.parse_args()
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if "VERBOSE" in os.environ:
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args.verbose = 1
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def main():
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"""Main Program"""
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parse_args()
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with open(args.kernel, "rb") as elf_fp:
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kernel = ELFFile(elf_fp)
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syms = get_symbols(kernel)
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# NOTE: use-cases are extremely limited; we always have a basic flat
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# code/data segments. If we are doing stack protection, we are going to
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# have two TSS to manage the main task and the special task for double
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# fault exception handling
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if "CONFIG_USERSPACE" in syms:
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num_entries = 7
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elif "CONFIG_X86_STACK_PROTECTION" in syms:
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num_entries = 5
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else:
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num_entries = 3
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use_tls = False
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if ("CONFIG_THREAD_LOCAL_STORAGE" in syms) and ("CONFIG_X86_64" not in syms):
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use_tls = True
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# x86_64 does not use descriptor for thread local storage
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num_entries += 1
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gdt_base = syms["_gdt"]
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with open(args.output_gdt, "wb") as output_fp:
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# The pseudo descriptor is stuffed into the NULL descriptor
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# since the CPU never looks at it
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output_fp.write(create_gdt_pseudo_desc(gdt_base, num_entries * 8))
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# Selector 0x08: code descriptor
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output_fp.write(create_code_data_entry(0, 0xFFFFF, 0,
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FLAGS_GRAN, ACCESS_EX | ACCESS_RW))
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# Selector 0x10: data descriptor
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output_fp.write(create_code_data_entry(0, 0xFFFFF, 0,
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FLAGS_GRAN, ACCESS_RW))
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if num_entries >= 5:
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main_tss = syms["_main_tss"]
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df_tss = syms["_df_tss"]
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# Selector 0x18: main TSS
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output_fp.write(create_tss_entry(main_tss, 0x67, 0))
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# Selector 0x20: double-fault TSS
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output_fp.write(create_tss_entry(df_tss, 0x67, 0))
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if num_entries >= 7:
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# Selector 0x28: code descriptor, dpl = 3
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output_fp.write(create_code_data_entry(0, 0xFFFFF, 3,
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FLAGS_GRAN, ACCESS_EX | ACCESS_RW))
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# Selector 0x30: data descriptor, dpl = 3
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output_fp.write(create_code_data_entry(0, 0xFFFFF, 3,
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FLAGS_GRAN, ACCESS_RW))
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if use_tls:
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# Selector 0x18, 0x28 or 0x38 (depending on entries above):
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# data descriptor, dpl = 3
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#
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# for use with thread local storage while this will be
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# modified at runtime.
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output_fp.write(create_code_data_entry(0, 0xFFFFF, 3,
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FLAGS_GRAN, ACCESS_RW))
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if __name__ == "__main__":
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main()
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