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Document inode_checkperm / inode_checkpathperm, mountpoint traverse vs open semantics, and the optional mountpt_operations.permission hook in file_permission.rst. Signed-off-by: Abhishek Mishra <mishra.abhishek2808@gmail.com>
62 lines
2.3 KiB
ReStructuredText
62 lines
2.3 KiB
ReStructuredText
========
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LITTLEFS
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========
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A little fail-safe filesystem designed for microcontrollers from
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https://github.com/littlefs-project/littlefs.
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In NuttX, littlefs can be interacted with through the virtual file system. This
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means that it can be used normally with ``read()``, ``write()``, etc. calls, as
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well as file operations (``fopen()``, ``fclose()``, etc).
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.. note::
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Since littlefs is power-fail safe and must commit writes before they are
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permanently stored (corruption prevention mechanism), it is required to
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periodically call ``fsync`` on the file to commit your writes. The exact
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semantics of when littlefs files are committed are discussed in `this issue
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<https://github.com/apache/nuttx/issues/15840>`_.
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.. note::
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If your littlefs setup is experiencing crashes when you boot, try
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troubleshooting by tweaking the ``BLOCK_SIZE_FACTOR`` options in Kconfig. A
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factor of 4 works well for SD cards.
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.. warning::
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The littlefs support on NuttX only works with mtd drivers, for storage
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devices such as flash chips, SD cards and eMMC. Performance on SD cards and
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eMMC devices is worse than flash.
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User identity and permissions
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=============================
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When both ``CONFIG_FS_PERMISSION`` and ``CONFIG_FS_LITTLEFS_ATTR_UPDATE`` are
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enabled, littlefs stores owner, group, and mode in per-file custom attributes.
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This requires :ref:`user-identity` (``CONFIG_SCHED_USER_IDENTITY``).
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**Metadata.** ``chmod``/``chown`` (via ``chstat``) update the stored mode,
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UID, and GID. ``stat`` and ``ls -l`` read them back.
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**Creation.** New files and directories are created with the effective UID and
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GID of the creating task.
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**Open checks.** ``open()`` enforces POSIX permission bits using the caller's
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effective identity:
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* Existing files: each path component must grant search (``X_OK``) permission;
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the final component must grant the access requested by ``oflags``.
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* ``O_CREAT`` on a non-existent file: the parent directory must grant write and
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search permission.
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**Directory reads.** ``opendir()`` requires read and search permission on the
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target directory.
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Files created before permission support was enabled, or without stored
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attributes, default to mode ``0777`` until ``chmod``/``chown`` sets explicit
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metadata.
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Path components above the littlefs mountpoint are enforced by the VFS; see
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:ref:`file-permission`.
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