Hi all,
I'be trying to drill down on what is the minimum consistency model that
still allows function prototype changes: Adding, removing or modifying
arguments of a function or changing its return type.
Because I believe this is the most important functionality that the null
model, where functions are simply replaced immediately using ftrace is
lacking.
Under my proposed classification, LEAVE_PATCHED_SET + SWITCH_THREAD
offers this.
But even that is fairly complex and would require large parts of kpatch,
kGraft and Masami's code.
The really trivial solution is:
static void lpc_ftrace_handler(unsigned long ip, unsigned long parent_ip,
struct ftrace_ops *ops, struct pt_regs *regs)
{
struct lpc_func *func = ops->private;
if (!within_patched_set(parent_ip))
regs->ip = func->new_addr;
}
Obviously, within_patched_set() would need an efficient data structure,
like an interval tree, to quickly decide whether the parent ip address
is within the patched set.
This is enough to make sure that function calls are consistent.
No stack checking, no stop_machine(), no forcing threads through
userspace.
A disadvantage is that it doesn't give an indication when patching is
over and the trampoline can be removed. For that, I currently don't see
a better method than tracking kernel exits and/or stack analysis.
What do you think?
Vojtech
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