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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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135 lines
3.7 KiB
Markdown
135 lines
3.7 KiB
Markdown
# GDB step debugging
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To GDB step debug the Linux kernel, first run:
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./run -d
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If you want to break immediately at a symbol, e.g. `start_kernel` of the boot sequence, run on another shell:
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./rungdb start_kernel
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Now QEMU will stop there, and you can use the normal GDB commands:
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l
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n
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c
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To skip the boot, run just:
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./rungdb
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and when you want to break, do `Ctrl + C` from GDB.
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To have some fun, you can first run inside QEMU:
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/count.sh
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which counts to infinity to stdout, and then in GDB:
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Ctrl + C
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break sys_write
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continue
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continue
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continue
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And you now control the counting from GDB.
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See also:
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- <http://stackoverflow.com/questions/11408041/how-to-debug-the-linux-kernel-with-gdb-and-qemu/33203642#33203642>
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- <http://stackoverflow.com/questions/4943857/linux-kernel-live-debugging-how-its-done-and-what-tools-are-used/42316607#42316607>
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`O=0` is an impossible dream, `O=2` being the default: <https://stackoverflow.com/questions/29151235/how-to-de-optimize-the-linux-kernel-to-and-compile-it-with-o0> So get ready for some weird jumps, and `<value optimized out>` fun. Why, Linux, why.
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## Kernel module debugging
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Loadable kernel modules are a bit trickier since the kernel can place them at different memory locations depending on load order.
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So we cannot set the breakpoints before `insmod`.
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However, the Linux kernel GDB scripts offer the `lx-symbols` command, which takes care of that beautifully for us:
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./run -d
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./rungdb
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In QEMU:
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insmod /fops.ko
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In GDB, hit `Ctrl + C`, and note how it says:
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scanning for modules in ../kernel_module-1.0/
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loading @0xffffffffa0000000: ../kernel_module-1.0//fops.ko
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That's `lx-symbols` working! Now simply:
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b fop_write
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c
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In QEMU:
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printf a >/sys/kernel/debug/lkmc_fops/f
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and GDB now breaks at our `fop_write` function!
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Just don't forget to remove your breakpoints after `rmmod`, or they will point to stale memory locations.
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TODO: why does `break work_func` for `insmod kthread.ko` not break the first time I `insmod`, but breaks the second time?
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See also: <http://stackoverflow.com/questions/28607538/how-to-debug-linux-kernel-modules-with-qemu/44095831#44095831>
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### Bypassing lx-symbols
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Useless, but a good way to show how hardcore you are. From inside QEMU:
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insmod /fops.ko
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cat /proc/modules
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This will give a line of form:
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fops 2327 0 - Live 0xfffffffa00000000
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And then tell GDB where the module was loaded with:
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Ctrl + C
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add-symbol-file ../kernel_module-1.0/fops.ko 0xfffffffa00000000
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## Debug kernel early boot
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TODO: why can't we break at early startup stuff such as:
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./rungdb extract_kernel
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./rungdb main
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See also: <https://stackoverflow.com/questions/2589845/what-are-the-first-operations-that-the-linux-kernel-executes-on-boot>
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## call
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GDB can call functions as explained at: <https://stackoverflow.com/questions/1354731/how-to-evaluate-functions-in-gdb>
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However this is failing for us:
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- some symbols are not visible to `call` even though `b` sees them
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- for those that are, `call` fails with an E14 error
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E.g.: if we break on `sys_write` on `/count.sh`:
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>>> call printk(0, "asdf")
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Could not fetch register "orig_rax"; remote failure reply 'E14'
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>>> b printk
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Breakpoint 2 at 0xffffffff81091bca: file kernel/printk/printk.c, line 1824.
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>>> call fdget_pos(fd)
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No symbol "fdget_pos" in current context.
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>>> b fdget_pos
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Breakpoint 3 at 0xffffffff811615e3: fdget_pos. (9 locations)
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>>>
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even though `fdget_pos` is the first thing `sys_write` does:
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581 SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
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582 size_t, count)
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583 {
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584 struct fd f = fdget_pos(fd);
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See also: <https://github.com/cirosantilli/linux-kernel-module-cheat/issues/19>
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