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The documentation for seq_file suggests that it is necessary to be able to move the iterator to a given offset, however that is not the case. If the iterator is stored in the private data and is stable from one read() syscall to the next, it is only necessary to support first/next interactions. Implementing this in a client is a little clumsy. - if ->start() is given a pos of zero, it should go to start of sequence. - if ->start() is given the name pos that was given to the most recent next() or start(), it should restore the iterator to state just before that last call - if ->start is given another number, it should set the iterator one beyond the start just before the last ->start or ->next call. Also, the documentation says that the implementation can interpret the pos however it likes (other than zero meaning start), but seq_file increments the pos sometimes which does impose on the implementation. This patch simplifies the interface for first/next iteration and simplifies the code, while maintaining complete backward compatability. Now: - if ->start() is given a pos of zero, it should return an iterator placed at the start of the sequence - if ->start() is given a non-zero pos, it should return the iterator in the same state it was after the last ->start or ->next. This is particularly useful for interators which walk the multiple chains in a hash table, e.g. using rhashtable_walk*. See fs/gfs2/glock.c and drivers/staging/lustre/lustre/llite/vvp_dev.c A large part of achieving this is to *always* call ->next after ->show has successfully stored all of an entry in the buffer. Never just increment the index instead. Also: - always pass &m->index to ->start() and ->next(), never a temp variable - don't clear ->from when ->count is zero, as ->from is dead when ->count is zero. Some ->next functions do not increment *pos when they return NULL. To maintain compatability with this, we still need to increment m->index in one place, if ->next didn't increment it. Note that such ->next functions are buggy and should be fixed. A simple demonstration is dd if=/proc/swaps bs=1000 skip=1 Choose any block size larger than the size of /proc/swaps. This will always show the whole last line of /proc/swaps. This patch doesn't work around buggy next() functions for this case. [neilb@suse.com: ensure ->from is valid] Link: http://lkml.kernel.org/r/87601ryb8a.fsf@notabene.neil.brown.name Signed-off-by: NeilBrown <neilb@suse.com> Acked-by: Jonathan Corbet <corbet@lwn.net> [docs] Tested-by: Jann Horn <jannh@google.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Kees Cook <keescook@chromium.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.