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FindingMemo

Framework for dynamically instrumenting the Linux kernel.

Steps

  1. Apply the patch in linux/add-hooks-metadata-section.patch
  2. Define hooking functions in impl.c
  3. Build the kernel module and start instrumenting with memo

Function hook definition

Hooks are defined in the hooks.c module. Below is shown an example of a hook for load_msg function defined in ipc/msgutil.c:

struct msg_msg *load_msg(const void __user *src, size_t len)

The macro FM_HOOK_FUNC_DEFINEx specifies the return type and arguments, where x is the number of arguments used by the function.

struct msg_msg *msg;

FM_HOOK_FUNC_DEFINE2(load_msg, struct msg_msg *, const void __user *, src,
                size_t, len)
{
        atomic_set(&curr_hook->mutex, false);
        msg = FM_HOOK_FUNC_PTR(load_msg)(src, len);
        atomic_set(&curr_hook->mutex, true);
        pr_info("fmemo: load_msg(): msg addr: %px\n", msg);
        return msg;
}

sysfs attribute definition

FM_HOOK_ATTR_DEFINE macro creates a read-only sysfs attribute file related the given hooked function, load_msg in the example that follows. The file is created in /sys/module/finder/show/, with a name that starts with fm_ and is followed by the name of the function.

FM_HOOK_ATTR_DEFINE(load_msg)
{
        return snprintf(buf, PAGE_SIZE, "%px\n", msg);
}

test@test:~$ ls -l /sys/module/finder/show
total 0
-r--r--r-- 1 root root 4096 Oct 15 16:14 fm_load_msg

Instrumentation flow

memo is the FindingMemo client. Its options are the follows:

test@test:~$ sudo ./memo -h
Usage: memo [OPTION]...
Configuration client to the FindingMemo hooking framework.

Arguments:
  -a, --add             Add hook symbol
  -s, --stop            Stop kernel instrumentation
  -i, --init            Initiate kernel instrumentation
  -h, --help            Display this help and exit
  1. Add hook functions
test@test:~$ sudo ./memo -a load_msg
Hook added for load_msg.
  1. Start kernel's dynamic instrumentation
test@test:~$ sudo ./memo -i
Linux hooking initiated.
  1. Stop kernel's dynamic instrumentation
test@test:~$ sudo ./memo -s
Linux hooking stopped.

Example of use

A test for the load_msg function is included in the test directory.

test@test:~$ ./msg-queue-test
+ Message queue creation
+ Send message
msg send: a message at Sat Aug 24 11:57:20 4448289

+ Get message
Message received: a message at Sat Aug 24 11:57:20 4448289

Adding the hook and sysfs attribute defined above for load_msg results in the msg address being shown in the kernel log and created sysfs file:

[86931.553933] fmemo: load_msg(): msg addr: ffff8880066e9480

test@test:~$ cat /sys/module/finder/show/fm_load_msg
ffff8880066e9480

Notes

  • Enable the Kernel Function Tracer: CONFIG_FUNCTION_TRACER