netfs: zero the tail of a short DIO/unbuffered readgrafted

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org

1 files changed, 187 insertions, 0 deletions

diff --git a/Documentation/doc-guide/parse-headers.rst b/Documentation/doc-guide/parse-headers.rst
new file mode 100644
index 000000000..a7bb01ff0
--- /dev/null
+++ b/Documentation/doc-guide/parse-headers.rst
@@ -0,0 +1,187 @@
+===========================
+Including uAPI header files
+===========================
+
+Sometimes, it is useful to include header files and C example codes in
+order to describe the userspace API and to generate cross-references
+between the code and the documentation. Adding cross-references for
+userspace API files has an additional advantage: Sphinx will generate warnings
+if a symbol is not found at the documentation. That helps to keep the
+uAPI documentation in sync with the Kernel changes.
+The :ref:`parse_headers.py <parse_headers>` provides a way to generate such
+cross-references. It has to be called via Makefile, while building the
+documentation. Please see ``Documentation/userspace-api/media/Makefile`` for an example
+about how to use it inside the Kernel tree.
+
+.. _parse_headers:
+
+tools/docs/parse_headers.py
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+NAME
+****
+
+parse_headers.py - parse a C file, in order to identify functions, structs,
+enums and defines and create cross-references to a Sphinx book.
+
+USAGE
+*****
+
+parse-headers.py [-h] [-d] [-t] ``FILE_IN`` ``FILE_OUT`` ``FILE_RULES``
+
+SYNOPSIS
+********
+
+Converts a C header or source file ``FILE_IN`` into a ReStructured Text
+included via ..parsed-literal block with cross-references for the
+documentation files that describe the API. It accepts an optional
+``FILE_RULES`` file to describe what elements will be either ignored or
+be pointed to a non-default reference type/name.
+
+The output is written at ``FILE_OUT``.
+
+It is capable of identifying ``define``, ``struct``, ``typedef``, ``enum``
+and enum ``symbol``, creating cross-references for all of them.
+
+It is also capable of distinguishing ``#define`` used for specifying
+Linux-specific macros used to define ``ioctl``.
+
+The optional ``FILE_RULES`` contains a set of rules like::
+
+    ignore ioctl VIDIOC_ENUM_FMT
+    replace ioctl VIDIOC_DQBUF vidioc_qbuf
+    replace define V4L2_EVENT_MD_FL_HAVE_FRAME_SEQ :c:type:`v4l2_event_motion_det`
+
+POSITIONAL ARGUMENTS
+********************
+
+  ``FILE_IN``
+      Input C file
+
+  ``FILE_OUT``
+      Output RST file
+
+  ``FILE_RULES``
+      Exceptions file (optional)
+
+OPTIONS
+*******
+
+  ``-h``, ``--help``
+      show a help message and exit
+  ``-d``, ``--debug``
+      Increase debug level. Can be used multiple times
+  ``-t``, ``--toc``
+      instead of a literal block, outputs a TOC table at the RST file
+
+
+DESCRIPTION
+***********
+
+Creates an enriched version of a Kernel header file with cross-links
+to each C data structure type, from ``FILE_IN``, formatting it with
+reStructuredText notation, either as-is or as a table of contents.
+
+It accepts an optional ``FILE_RULES`` which describes what elements will be
+either ignored or be pointed to a non-default reference, and optionally
+defines the C namespace to be used.
+
+It is meant to allow having more comprehensive documentation, where
+uAPI headers will create cross-reference links to the code.
+
+The output is written at the ``FILE_OUT``.
+
+The ``FILE_RULES`` may contain contain three types of statements:
+**ignore**, **replace** and **namespace**.
+
+By default, it create rules for all symbols and defines, but it also
+allows parsing an exception file. Such file contains a set of rules
+using the syntax below:
+
+1. Ignore rules:
+
+    ignore *type* *symbol*
+
+Removes the symbol from reference generation.
+
+2. Replace rules:
+
+    replace *type* *old_symbol* *new_reference*
+
+    Replaces *old_symbol* with a *new_reference*.
+    The *new_reference* can be:
+
+    - A simple symbol name;
+    - A full Sphinx reference.
+
+3. Namespace rules
+
+    namespace *namespace*
+
+    Sets C *namespace* to be used during cross-reference generation. Can
+    be overridden by replace rules.
+
+On ignore and replace rules, *type* can be:
+
+    - ioctl:
+        for defines of the form ``_IO*``, e.g., ioctl definitions
+
+    - define:
+        for other defines
+
+    - symbol:
+        for symbols defined within enums;
+
+    - typedef:
+        for typedefs;
+
+    - enum:
+        for the name of a non-anonymous enum;
+
+    - struct:
+        for structs.
+
+
+EXAMPLES
+********
+
+- Ignore a define ``_VIDEODEV2_H`` at ``FILE_IN``::
+
+    ignore define _VIDEODEV2_H
+
+- On an data structure like this enum::
+
+    enum foo { BAR1, BAR2, PRIVATE };
+
+  It won't generate cross-references for ``PRIVATE``::
+
+    ignore symbol PRIVATE
+
+  At the same struct, instead of creating one cross reference per symbol,
+  make them all point to the ``enum foo`` C type::
+
+    replace symbol BAR1 :c:type:\`foo\`
+    replace symbol BAR2 :c:type:\`foo\`
+
+
+- Use C namespace ``MC`` for all symbols at ``FILE_IN``::
+
+    namespace MC
+
+BUGS
+****
+
+
+Report bugs to Mauro Carvalho Chehab <mchehab@kernel.org>
+
+
+COPYRIGHT
+*********
+
+
+Copyright (c) 2016, 2025 by Mauro Carvalho Chehab <mchehab+huawei@kernel.org>.
+
+License GPLv2: GNU GPL version 2 <https://gnu.org/licenses/gpl.html>.
+
+This is free software: you are free to change and redistribute it.
+There is NO WARRANTY, to the extent permitted by law.