gh-89341: Support creation of a link to the file by fd #136302
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| "test needs fd support in os.link()" | ||
| ) | ||
| @unittest.skipUnless(root_in_posix, | ||
| "requires the CAP_DAC_READ_SEARCH capability") |
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On my Fedora 42, I can use AT_EMPTY_PATH as a regular user. I don't need to be root.
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This is difficult to check. What getpcaps $$ and cat /proc/$$/status | grep CapEff return to you.
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vstinner@mona$ getpcaps $$
103445: cap_wake_alarm=i
vstinner@mona$ cat /proc/$$/status | grep CapEff
CapEff: 0000000000000000
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It does not look like your shell has such capability. Maybe Fedora has patches that ignores it?
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My test which works as a normal user on Fedora 42: import tempfile
import os
import ctypes
TESTNAME = b"linkat_test"
libc = ctypes.cdll.LoadLibrary("libc.so.6")
linkat = libc.linkat
linkat.argtypes = (
ctypes.c_int,
ctypes.c_char_p,
ctypes.c_int,
ctypes.c_char_p,
ctypes.c_int,
)
linkat.restype = ctypes.c_int
AT_FDCWD = -100
AT_EMPTY_PATH = 0x1000
def link_file(fd):
res = linkat(fd, b"",
AT_FDCWD, TESTNAME,
AT_EMPTY_PATH)
if res:
errno = ctypes.get_errno()
print("linkat() failed: res", res, "errno", errno)
raise OSError(errno)
try:
os.unlink(TESTNAME)
except FileNotFoundError:
pass
fd = os.open(".", os.O_WRONLY | os.O_TMPFILE)
os.write(fd, b"hello world\n")
link_file(fd)
os.close(fd)
with open(TESTNAME) as fp:
print(fp.read(), end="")
os.unlink(TESTNAME) |
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It fails on Ubuntu. |
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I don't know why Fedora behaves differently. Kernel code: /*
* To use null names we require CAP_DAC_READ_SEARCH or
* that the open-time creds of the dfd matches current.
* This ensures that not everyone will be able to create
* a hardlink using the passed file descriptor.
*/
if (flags & AT_EMPTY_PATH)
how |= LOOKUP_LINKAT_EMPTY; |
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I think that torvalds/linux@42bd2af is the cause. There was no condition "or that the open-time creds of the dfd matches current" initially. This change makes the feature more usable. I have pretty old kernel (was not able to boot with newer kernels after upgrade), so I cannot test this. I am not particularly interested in this feature. You can continue work (update the test, add the documentation) if you wish. |
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