3e6be4ff30
Introduce in-kernel headers which are made available as an archive through proc (/proc/kheaders.tar.xz file). This archive makes it possible to run eBPF and other tracing programs that need to extend the kernel for tracing purposes without any dependency on the file system having headers. A github PR is sent for the corresponding BCC patch at: https://github.com/iovisor/bcc/pull/2312 On Android and embedded systems, it is common to switch kernels but not have kernel headers available on the file system. Further once a different kernel is booted, any headers stored on the file system will no longer be useful. This is an issue even well known to distros. By storing the headers as a compressed archive within the kernel, we can avoid these issues that have been a hindrance for a long time. The best way to use this feature is by building it in. Several users have a need for this, when they switch debug kernels, they do not want to update the filesystem or worry about it where to store the headers on it. However, the feature is also buildable as a module in case the user desires it not being part of the kernel image. This makes it possible to load and unload the headers from memory on demand. A tracing program can load the module, do its operations, and then unload the module to save kernel memory. The total memory needed is 3.3MB. By having the archive available at a fixed location independent of filesystem dependencies and conventions, all debugging tools can directly refer to the fixed location for the archive, without concerning with where the headers on a typical filesystem which significantly simplifies tooling that needs kernel headers. The code to read the headers is based on /proc/config.gz code and uses the same technique to embed the headers. Other approaches were discussed such as having an in-memory mountable filesystem, but that has drawbacks such as requiring an in-kernel xz decompressor which we don't have today, and requiring usage of 42 MB of kernel memory to host the decompressed headers at anytime. Also this approach is simpler than such approaches. (Resolved minor conflicts in Makefile) (cherry picked from commit 43d8ce9d65a54846d378545770991e65838981e0) Bug: 78013494 Change-Id: Id40724018c0c68d5ea159822c269e23897d43826 Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
74 lines
1.8 KiB
C
74 lines
1.8 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Provide kernel headers useful to build tracing programs
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* such as for running eBPF tracing tools.
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*
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* (Borrowed code from kernel/configs.c)
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*/
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/proc_fs.h>
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#include <linux/init.h>
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#include <linux/uaccess.h>
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/*
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* Define kernel_headers_data and kernel_headers_data_end, within which the
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* compressed kernel headers are stored. The file is first compressed with xz.
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*/
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asm (
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" .pushsection .rodata, \"a\" \n"
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" .global kernel_headers_data \n"
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"kernel_headers_data: \n"
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" .incbin \"kernel/kheaders_data.tar.xz\" \n"
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" .global kernel_headers_data_end \n"
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"kernel_headers_data_end: \n"
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" .popsection \n"
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);
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extern char kernel_headers_data;
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extern char kernel_headers_data_end;
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static ssize_t
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ikheaders_read_current(struct file *file, char __user *buf,
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size_t len, loff_t *offset)
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{
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return simple_read_from_buffer(buf, len, offset,
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&kernel_headers_data,
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&kernel_headers_data_end -
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&kernel_headers_data);
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}
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static const struct file_operations ikheaders_file_ops = {
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.read = ikheaders_read_current,
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.llseek = default_llseek,
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};
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static int __init ikheaders_init(void)
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{
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struct proc_dir_entry *entry;
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/* create the current headers file */
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entry = proc_create("kheaders.tar.xz", S_IRUGO, NULL,
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&ikheaders_file_ops);
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if (!entry)
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return -ENOMEM;
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proc_set_size(entry,
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&kernel_headers_data_end -
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&kernel_headers_data);
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return 0;
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}
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static void __exit ikheaders_cleanup(void)
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{
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remove_proc_entry("kheaders.tar.xz", NULL);
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}
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module_init(ikheaders_init);
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module_exit(ikheaders_cleanup);
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MODULE_LICENSE("GPL v2");
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MODULE_AUTHOR("Joel Fernandes");
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MODULE_DESCRIPTION("Echo the kernel header artifacts used to build the kernel");
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