accessing mmap of file that is overwritten causes bus error

[mhvk]

BPO	40720
Nosy	@ronaldoussoren, @graingert, @mhvk, @iritkatriel

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-05-21.21:32:53.071>
labels = ['3.10', 'library', '3.9', 'type-crash', '3.11']
title = 'accessing mmap of file that is overwritten causes bus error'
updated_at = <Date 2021-10-18.22:07:24.553>
user = 'https://github.com/mhvk'

bugs.python.org fields:

activity = <Date 2021-10-18.22:07:24.553>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-05-21.21:32:53.071>
creator = 'mhvk'
dependencies = []
files = []
hgrepos = []
issue_num = 40720
keywords = []
message_count = 5.0
messages = ['369543', '369635', '388337', '388339', '404230']
nosy_count = 4.0
nosy_names = ['ronaldoussoren', 'graingert', 'mhvk', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue40720'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[mhvk]

While debugging a strange failure with tests and np.memmap, I realized that the following direct use of mmap reliably leads to a bus error. Here, obviously mmap'ing a file, closing it, opening the file for writing but not writing anything, and then again accessing the mmap is not something one should do (but a test case did it anyway), but it would nevertheless be nice to avoid a crash!

import mmap


with open('test.dat', 'wb') as fh:
 fh.write(b'abcdefghijklmnopqrstuvwxyz')


with open('test.dat', 'rb') as fh:
 mm = mmap.mmap(fh.fileno(), 0, access=mmap.ACCESS_READ)


with open('test.dat', 'wb') as fh:
 pass # Note: if something is written, then I get no bus error.


mm[2]

[mhvk]

I should probably have added that the bus error happens on linux. On Windows, the opening of the file for writing leads to an error, as the file is still opened for reading inside the mmap.

[graingert]

I can confirm this happens on py3.5-3.10

import mmap
import pathlib
import tempfile


def main():
 with tempfile.TemporaryDirectory() as tmp:
 tmp_path = pathlib.Path(tmp)
 path = tmp_path / "eg"

 path.write_bytes(b"Hello, World!")

 with path.open("rb") as rf:
 mm = mmap.mmap(rf.fileno(), 0, mmap.MAP_SHARED, mmap.PROT_READ)
 path.write_bytes(b"")
 bytes(mm)

if __name__ == "__main__":
 main()