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tun simply advances iov_iter when it needs to pad virtio header, which leaves the garbage in the buffer as is. This will become especially problematic when tun starts to allow enabling the hash reporting feature; even if the feature is enabled, the packet may lack a hash value and may contain a hole in the virtio header because the packet arrived before the feature gets enabled or does not contain the header fields to be hashed. If the hole is not filled with zero, it is impossible to tell if the packet lacks a hash value. In theory, a user of tun can fill the buffer with zero before calling read() to avoid such a problem, but leaving the garbage in the buffer is awkward anyway so replace advancing the iterator with writing zeros. A user might have initialized the buffer to some non-zero value, expecting tun to skip writing it. As this was never a documented feature, this seems unlikely. The overhead of filling the hole in the header is negligible when the header size is specified according to the specification as doing so will not make another cache line dirty under a reasonable assumption. Below is a proof of this statement: The first 10 bytes of the header is always written and tun also writes the packet itself immediately after the packet unless the packet is empty. This makes a hole between these writes whose size is: sz - 10 where sz is the specified header size. Therefore, we will never make another cache line dirty when: sz < L1_CACHE_BYTES + 10 where L1_CACHE_BYTES is the cache line size. Assuming L1_CACHE_BYTES >= 16, this inequation holds when: sz < 26. sz <= 20 according to the current specification so we even have a margin of 5 bytes in case that the header size grows in a future version of the specification. Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com> Link: https://patch.msgid.link/20250215-buffers-v2-1-1fbc6aaf8ad6@daynix.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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 reStructuredText markup notation.
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.