The log file has been on external storage since it existed, and the
reason is stated at length in two places: /data/data/<pkg>/files needs
run-as against a debuggable build, /sdcard/Android/data/<pkg>/files is a
plain adb pull from any build, and a log nobody can retrieve is not a
diagnostic. The file was then opened 0600, which cancels that decision
out. On the tablet:
adb pull -> remote open failed: Permission denied
adb shell cat -> Permission denied
run-as -> package not debuggable
Every route off the device closed at once, on a file whose whole purpose
is to leave the device.
The mode is now per platform, because "who may read this" has two
different answers and the directory above the file is what makes them
differ. On the desktop, 0600 as before: $XDG_STATE_HOME/darkroom is in a
home directory on a machine that may have other accounts, and nothing
about that directory stops another local user reading a world-readable
file. On Android, 0644: /sdcard/Android/data is drwxrws--x
media_rw:ext_data_rw, so no other app can enter this app's subdirectory
and anyone who can traverse it is holding the unlocked tablet, which
already gets them the photographs the log merely names. What the read
bits buy is adb pull, which runs as shell — able to traverse a --x
directory, but then obliged to open the file as other.
The mode is also applied twice, and the second one is the fix rather
than belt and braces. OpenOptions::mode is a request: the kernel ANDs it
with the process umask, and an Android application process inherits
0o077 from the zygote, so asking for 0644 there creates 0600 and reports
nothing. It is ignored outright on a file that already exists, which
every launch after the first has. fchmod is subject to neither, and is
what the second call makes.
The comment claiming the mode was "ignored by the FAT-derived filesystem
Android presents as external storage" is gone with it. The device says
otherwise: the file it produced was 0600 exactly.
Two tests. One pins the literal mode per platform — only the desktop arm
can run under cargo test, and the comment says so rather than implying
the Android number is covered. The other reopens a log left behind with
the wrong mode, which is the one assertion on the host that fails if the
fchmod is deleted, since OpenOptions::mode cannot touch a file that is
already there.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
dtourolleandClaude Opus 5
c62edd3317