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


Hi, Dmitry

On 2016/7/6 16:00, Dmitry Vyukov wrote:
On Wed, Jul 6, 2016 at 9:39 AM, Zhangjian (Bamvor)
<bamvor.zhangjian@huawei.com> wrote:
HI,

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 ./struct_extract.py 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 abi-dumper.pl,
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/struct_extract.py.

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.


Hi Bamvor,

Nice work!

Coverage should be easy to do with CONFIG_KCOV, but do you need
fuzzing/coverage? It seems that testing a predefined set of special
values for each arg should be enough for your use case. Namely special
values that can detect endianess/truncation/sign extension/etc issues.
Yes. We are trying to cover endianess/truncation/sign extension at this
moment.
For coverage, there are some code path in syscall wrapper in both glibc
and kernel. E.g. overflow check in glibc. I am thinking if coverage
could help on this.

I think there is also a number of glibc functions that don't directly
map to syscalls. Most notably wrappers around various ioctl's (e.g.
ptsname). Do you test them?
No. Currently, our tools only focus on the syscall function in glibc. In
these syscall level, we could compare the parameter and return value
directly. As you said, there are only several type of issues. It is easy
to handle by tools.

I do not know how to test these complex cases. E.g. the ptsname may call
ioctl, *stat* syscall. Compare the original parameter is meaningless. But
it seems a good type of testcase to show how the user use the syscalls.
Do you have some ideas?

Regards

Bamvor

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