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# Copyright 2018 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Verify VDSO ELF does not contain any relocations and is directly mmappable.
"""
import argparse
import logging
import re
import subprocess
PAGE_SIZE = 4096
def PageRoundDown(addr):
"""Rounds down to the nearest page.
Args:
addr: An address.
Returns:
The address rounded down to thie nearest page.
"""
return addr & ~(PAGE_SIZE - 1)
def Fatal(*args, **kwargs):
"""Logs a critical message and exits with code 1.
Args:
*args: Args to pass to logging.critical.
**kwargs: Keyword args to pass to logging.critical.
"""
logging.critical(*args, **kwargs)
exit(1)
def CheckSegments(vdso_path):
"""Verifies layout of PT_LOAD segments.
PT_LOAD segments must be laid out such that the ELF is directly mmappable.
Specifically, check that:
* PT_LOAD file offsets are equivalent to the memory offset from the first
segment.
* No extra zeroed space (memsz) is required.
* PT_LOAD segments are in order (required for any ELF).
* No two PT_LOAD segments share part of the same page.
The readelf line format looks like:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0xffffffffff700000 0xffffffffff700000 0x000e68 0x000e68 R E 0x1000
Args:
vdso_path: Path to VDSO binary.
"""
output = subprocess.check_output(["readelf", "-lW", vdso_path])
lines = output.split("\n")
segments = []
for line in lines:
if not line.startswith(" LOAD"):
continue
components = line.split()
segments.append({
"offset": int(components[1], 16),
"addr": int(components[2], 16),
"filesz": int(components[4], 16),
"memsz": int(components[5], 16),
})
if not segments:
Fatal("No PT_LOAD segments in VDSO")
first = segments[0]
if first["offset"] != 0:
Fatal("First PT_LOAD segment has non-zero file offset: %s", first)
for i, segment in enumerate(segments):
memoff = segment["addr"] - first["addr"]
if memoff != segment["offset"]:
Fatal("PT_LOAD segment has different memory and file offsets: %s",
segments)
if segment["memsz"] != segment["filesz"]:
Fatal("PT_LOAD segment memsz != filesz: %s", segment)
if i > 0:
last_end = segments[i-1]["addr"] + segments[i-1]["memsz"]
if segment["addr"] < last_end:
Fatal("PT_LOAD segments out of order")
last_page = PageRoundDown(last_end)
start_page = PageRoundDown(segment["addr"])
if last_page >= start_page:
Fatal("PT_LOAD segments share a page: %s and %s", segment,
segments[i - 1])
# Matches the section name in readelf -SW output.
_SECTION_NAME_RE = re.compile(r"""^\s+\[\ ?\d+\]\s+
(?P<name>\.\S+)\s+
(?P<type>\S+)\s+
(?P<addr>[0-9a-f]+)\s+
(?P<off>[0-9a-f]+)\s+
(?P<size>[0-9a-f]+)""", re.VERBOSE)
def CheckData(vdso_path):
"""Verifies the VDSO contains no .data or .bss sections.
The readelf line format looks like:
There are 15 section headers, starting at offset 0x15f0:
Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .hash HASH ffffffffff700120 000120 000040 04 A 2 0 8
[ 2] .dynsym DYNSYM ffffffffff700160 000160 000108 18 A 3 1 8
...
[13] .strtab STRTAB 0000000000000000 001448 000123 00 0 0 1
[14] .shstrtab STRTAB 0000000000000000 00156b 000083 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
Args:
vdso_path: Path to VDSO binary.
"""
output = subprocess.check_output(["readelf", "-SW", vdso_path])
lines = output.split("\n")
found_text = False
for line in lines:
m = re.search(_SECTION_NAME_RE, line)
if not m:
continue
if not line.startswith(" ["):
continue
name = m.group("name")
size = int(m.group("size"), 16)
if name == ".text" and size != 0:
found_text = True
# Clang will typically omit these sections entirely; gcc will include them
# but with size 0.
if name.startswith(".data") and size != 0:
Fatal("VDSO contains non-empty .data section:\n%s" % output)
if name.startswith(".bss") and size != 0:
Fatal("VDSO contains non-empty .bss section:\n%s" % output)
if not found_text:
Fatal("VDSO contains no/empty .text section? Bad parsing?:\n%s" % output)
def CheckRelocs(vdso_path):
"""Verifies that the VDSO includes no relocations.
Args:
vdso_path: Path to VDSO binary.
"""
output = subprocess.check_output(["readelf", "-r", vdso_path])
if output.strip() != "There are no relocations in this file.":
Fatal("VDSO contains relocations: %s", output)
def main():
parser = argparse.ArgumentParser(description="Verify VDSO ELF.")
parser.add_argument("--vdso", required=True, help="Path to VDSO ELF")
parser.add_argument(
"--check-data",
action="store_true",
help="Check that the ELF contains no .data or .bss sections")
args = parser.parse_args()
CheckSegments(args.vdso)
CheckRelocs(args.vdso)
if args.check_data:
CheckData(args.vdso)
if __name__ == "__main__":
main()
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