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mknod: set cli specified file mode even when it's 0o666#8343

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fromTheJJ:fix-mknod-mode

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TheJJ
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@TheJJTheJJ commentedJul 16, 2025•
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libc's mknod function applies the process' umask in the usual way: the permissions of the created node aremode & ~umask.

the umask of uu-mknod is currently only set to 0 (= no mask) when the supplied mode is not 0o666:

let set_umask_0 = config.mode&MODE_RW_UGO !=MODE_RW_UGO;

becauseMODE_RW_UGO == 0o666 is also the default--mode argument value.

but when--mode 0666 is given explicitly, the permissions should be set exactly to the requested bits, and the umask should not be applied.

this patch fixes the issue by tracking if a custom mode was supplied by argument.

interestingly, GNU coreutils issue a call tofchmodat2 aftermknod was called, even though they also set the umask to 0 if a custom mode was specified via--mode. this patch doesn't add that, but it may be necessary for other cases in the future.

fixes#8342

this does now result in:

strace mknod --mode 0666 /tmp/rofl/null c 1 3umask(000)                              = 022mknodat(AT_FDCWD, "/tmp/rofl/null", S_IFCHR|0666, makedev(0x1, 0x3)) = 0umask(022)                              = 000-> crw-rw-rw- 1 root root 1, 3 Jul 16 23:54 /tmp/rofl/null

@TheJJTheJJ changed the titlefix(mknod): set specified file modemknod: set cli specified file mode even when it's 0o666Jul 16, 2025
@setharnold
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setharnold commentedJul 16, 2025•
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Forgive me if I've missed a conversation or discussion elsewhere. This feels more complex than the implementation that I expected:

  • setumask(0) at program start (and discard the old umask, what else will the process do?)
  • use default0o666 if the user doesn't include a--mode

What motivates the more complex implementation?

Thanks

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GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

@TheJJ
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TheJJ commentedJul 17, 2025•
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Good point, I don't know :) but I tried to change the currently implemented behavior as little as possible.

The umask restoring is probably done so the functions are usable inside single executables?

My updated implementation now does

  • 0o666+umask by default
  • --mode without umask (and restores the umask).

just like the GNU coreutils manpage says:

-m, --mode=MODE set file permission bits to MODE, not a=rw - umask

@TheJJTheJJforce-pushed thefix-mknod-mode branch 2 times, most recently from042b5d7 toa3c0e53CompareJuly 17, 2025 09:55
libc's mknod function applies the process' umask in the usual way:the permissions of the created node are `mode & ~umask`.the umask of uu-mknod is currently only set to 0 (= no mask)when the supplied mode is not 0o666:```rustlet set_umask_0 = config.mode & MODE_RW_UGO != MODE_RW_UGO;```because `MODE_RW_UGO == 0o666` is also the default `--mode` argument value.but when `--mode 0666` is given explicitly, the permissions should be setexactly to the requested bits, and the umask should not be applied.this patch fixes the issue by tracking if a custom mode was supplied by argument.interestingly, GNU coreutils issue a call to `fchmodat2` after `mknod` was called,even though they also set the umask to 0 if a custom mode was specified via `--mode`.this patch doesn't add that, but it may be necessary for other cases in the future.fixesuutils#8342
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Could you please add a test to make sure we don't regress?

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GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)

@TheJJ
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done, it requiresroot though.

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GNU testsuite comparison:

Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)

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mknod doesn't respect --mode
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