djmil/standalone-kernel-module
djmil/standalone-kernel-module
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PLT section

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Andriy Petrov 2024-04-17 12:34:47 +02:00
parent 298edf8587
commit 08ca3702af
2 changed files with 19 additions and 4 deletions
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21
README.md
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@ -3,11 +3,27 @@ The process of [kernel modules compilation](https://www.kernel.org/doc/html/v5.6
> But what to do if, for the variety of reasons, for the kernel you are interest in, headers are unavailable?
This article focuses on Android, but most of information and techniques discussed here can be easily applied to generic Linux kernel as well.
The code in this repo focuses on Android, but most of the information and techniques discussed here can be easily applied to the generic Linux kernel as well.
# ELF symbols stealing
The main idea, is to use Android NDK to compile generic Linux kernel module. And embed it with ELF symbols collected from some existing kernel module (see `/vendor/lib/modules/*.ko`). In such a way, that the loader would be able to recognise and resolve all necessary dependencies and definitions.
The main idea, is to use Android NDK to compile generic Linux kernel module. And embed it with ELF symbols collected from some existing kernel module (see `/vendor/lib/modules/*.ko`). In such a way, that the LKM loader would be able to recognise and resolve all necessary dependencies and definitions.
## PLT section
This part is pretty straight forward, we simply have to define a few ELF symbols, that Android LKM is expecting to find within *normal* module. We are going to really on [compiler keyword](https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Variable-Attributes.html) `__attribute__` for this.
```c
// insmod error: module PLT section(s) missing
__attribute__((section(".plt")))
char plt = 0;
__attribute__((section(".init.plt")))
char initplt = 0;
__attribute__((section(".text.ftrace_trampoline")))
char textftrace_trampoline = 0; // TODO: probably an overkill
```
## .modinfo
@ -73,7 +89,6 @@ readelf -a $KMODULE -W
0000000000000310 000006ee00000101 R_AARCH64_ABS64 0000000000000000 cleanup_module + 0
```
## \_\_versions
In its essence is a byte-array for declaring external dependencies required by the module:

2
bmod.c
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@ -11,7 +11,7 @@ __attribute__((section(".init.plt")))
char initplt = 0;
__attribute__((section(".text.ftrace_trampoline")))
char textftrace_trampoline = 0; // TODO: probably irrelevant
char textftrace_trampoline = 0; // TODO: probably an overkill
/************************/