118 lines
3.2 KiB
Python
118 lines
3.2 KiB
Python
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#!/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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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 elftools.elf.elffile import ELFFile
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from elftools.elf.sections import SymbolTableSection
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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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# ...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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# Limit argument always in bytes
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def chop_base_limit(base, limit):
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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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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) | (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 get_symbols(obj):
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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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global args
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parser = argparse.ArgumentParser(description = __doc__,
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formatter_class = argparse.RawDescriptionHelpFormatter)
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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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def main():
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parse_args()
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with open(args.kernel, "rb") as fp:
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kernel = ELFFile(fp)
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syms = get_symbols(kernel)
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num_entries = 3
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gdt_base = syms["_gdt"]
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with open(args.output_gdt, "wb") as 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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fp.write(create_gdt_pseudo_desc(gdt_base, num_entries * 8))
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# Selector 0x08: code descriptor
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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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fp.write(create_code_data_entry(0, 0xFFFFF, 0,
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FLAGS_GRAN, ACCESS_RW))
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if __name__ == "__main__":
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main()
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