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Alexei Starovoitov says: ==================== bpfilter v2->v3: - followed Luis's suggestion and significantly simplied first patch with shmem_kernel_file_setup+kernel_write. Added kdoc for new helper - fixed typos and race to access pipes with mutex - tested with bpfilter being 'builtin'. CONFIG_BPFILTER_UMH=y|m both work. Interesting to see a usermode executable being embedded inside vmlinux. - it doesn't hurt to enable bpfilter in .config. ip_setsockopt commands sent to usermode via pipes and -ENOPROTOOPT is returned from userspace, so kernel falls back to original iptables code v1->v2: this patch set is almost a full rewrite of the earlier umh modules approach The v1 of patches and follow up discussion was covered by LWN: https://lwn.net/Articles/749108/ I believe the v2 addresses all issues brought up by Andy and others. Mainly there are zero changes to kernel/module.c Instead of teaching module loading logic to recognize special umh module, let normal kernel modules execute part of its own .init.rodata as a new user space process (Andy's idea) Patch 1 introduces this new helper: int fork_usermode_blob(void *data, size_t len, struct umh_info *info); Input: data + len == executable file Output: struct umh_info { struct file *pipe_to_umh; struct file *pipe_from_umh; pid_t pid; }; Advantages vs v1: - the embedded user mode executable is stored as .init.rodata inside normal kernel module. These pages are freed when .ko finishes loading - the elf file is copied into tmpfs file. The user mode process is swappable. - the communication between user mode process and 'parent' kernel module is done via two unix pipes, hence protocol is not exposed to user space - impossible to launch umh on its own (that was the main issue of v1) and impossible to be man-in-the-middle due to pipes - bpfilter.ko consists of tiny kernel part that passes the data between kernel and umh via pipes and much bigger umh part that doing all the work - 'lsmod' shows bpfilter.ko as usual. 'rmmod bpfilter' removes kernel module and kills corresponding umh - signed bpfilter.ko covers the whole image including umh code Few issues: - the user can still attach to the process and debug it with 'gdb /proc/pid/exe pid', but 'gdb -p pid' doesn't work. (a bit worse comparing to v1) - tinyconfig will notice a small increase in .text +766 | TEXT | 7c8b94806bec umh: introduce fork_usermode_blob() helper ==================== Signed-off-by: David S. Miller <davem@davemloft.net>
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.