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Subject[RFD] Efficient unit test and fuzz tools for kernel/libc porting

When I working on the ILP32 ABI for ARMv8 in last two years, I has encountered lots of syscall issues such as wrong number of arguments, different data type in binary interface. I realized that the correctness of argument passing between the C library and core kernel code is a common problem when bringing up new architecture or ABI to kernel and libc. Existing fuzz testing tools such as trinity[1], syzkaller[2] and triforce[3] only generate random or boundary values for syscall parameters and then inject them into kernel, but those tools won't validate if the results of those syscalls are correct or not. Thus they can not act as an unit test for ILP32. In this year, considering the abi of ILP32 is changes during discussion, I am thinking if I could use some sort of automatically tools to check whether the wrapper is correct or not. After learn and compare some fuzz tools, I feel that there is no such fuzz tools could help me. So, I wrote a new fuzz tools base on the trinity and it
found several wrapper issues in glibc. I will first explain the different with existing fuzz tools and paste my propsosal in the end.

Trinity is developed in a long time. It could randomize the parameter of syscall and run individual syscall standalone or parallel. When I do the long time parallel test(not for ILP32), it could report some bug, e.g. hang, panic. It is useful but it is indeed hard to debug because it usually fail after a long time running. I do not know what does it exactly do.

Compare with Trinity, syzkaller is quite different. Here is the comparision between syzkaller and our tools:
1. Syzkaller could recursively randomize base date type in syscall which means it is possible generate more meaningfull syscall test. But it only test the syscall through syscall() function. It assume that the c library is correct and stable. But it is wrong if we are porting new abi(such as ILP32) or architecture to glibc and kernel. We need to take c library into account. This is what my tools could do.

2. Syzkaller could generate the readable short testcases. Our tools could only test individual syscall and check the correctness of parameter and return value. I think it is enough for the unit test which tests syscall one by one.

3. Syzkaller could do the coverage. Our tools could not. I think it is useful for me. I plan to add the coverage later.

In my ILP32 works, my tools reported several off_t endian issues in glibc. Our tools work like this:

Dump the function Dump the function
prototype from prototype from c
vmlinux from the library from the
sys_call_table given list(posix
array in kernel. interfaces or user
| defined).
| |
| |
\|/ \|/
` `
Generate jprobe Modity Trinity to Generate struct
hook according to support run syscall fuzz generator
prototype which syscall from c from the prototype.
will recursively libray instead And add them of
print the syscall syscall() function to trinity. Trinity
value. | will recursively
\ | print the function
\ | parameter.
\ | /
Run the trinity each syscall once
and compare the function parameter
printed in kernel and userspace
If inconsistent, print specific
information, such endian issue,
32<->64bit conversion issue and
so on.

Tools of function dump and hook generator based on the abi-dumper[4]. Other functions base on trinity. Return value test is similar except generating the kretprobe hook instead jprobe hook.

There are some hacks and the original funtion of trinity may be broken. The main changes in trinity are as follows:
1. Call syscall through c library via call_glibc_syscalls() instead of direct syscall via syscall().
2. Add new file generate-struct.c including the missing data type mentioned in syscall. This file is generated by ./ with a little modification. It should be fully auto generated in future.
3. Add more date types in fill_arg()(generate-args.c) and include/syscall.h.
4. Modify the syscallentry struct in syscalls directory according to the newly added data types.
5. Add or Change some output message for script.
6. Add jprobe hooks in modules directory. Such hook will be inserted before trinity test and removed after test.

Step to generate generate-struct.c and jprobe hooks:
1. Compile the kernel/libc/binary which include the functions you want to generate.
2. Dump function and struct information through modified, named them as symbolinfo and typeinfo. You may need add "--all" and/or "--dump-static" depends on your binaries.
3. Generate the file through trinity/scripts/

We also submit a proposal to linuxcon europe last month. Hope we could get a chance to share our works face to face.

The patches could be found at [5] and [6]. It works but ugly. We hope we could get some early response, such as if community is interested in this tools, whether this tools could merge into existence fuzz tools.

Thanks in advance.

Best wishes



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