Actually Opening The File

Entry function: open_namei, in fs/namei.c.

• If not opening in create mode (no O_CREAT flag given):
  lookup the file via the dentry cache (path_lookup -> path_walk -> link_path_walk).
• Otherwise (it is an open in create mode):
  1. Lookup the parent directory (path_lookup again). If it does not exist, or is not a directory - fail the operation.
  2. Lookup the file in the parent directory. If it does not exist, create it (see vfs_create later on).
  3. Handle special cases (flags mismatch, file is a directory, file is a link...).
• In both cases (open with create or file already exists):
  1. Perform sanity checks.
  2. Check permissions.
  3. Check various mode limitations (e.g. file is read-only and trying to open for write, trying to open a device file on a 'no_dev' file-system mount, etc.).
  4. Handle truncation.

Originally written by guy keren