Re: [PATCH 1/2] uprobes: document the usage of mm->mmap_lock

[Oleg Nesterov <oleg@redhat.com>]

On 07/10, Masami Hiramatsu wrote:
>
> On Wed, 10 Jul 2024 16:00:45 +0200
> Oleg Nesterov <oleg@redhat.com> wrote:
>
> > The comment above uprobe_write_opcode() is wrong, unapply_uprobe() calls
> > it under mmap_read_lock() and this is correct.
> >
> > And it is completely unclear why register_for_each_vma() takes mmap_lock
> > for writing, add a comment to explain that mmap_write_lock() is needed to
> > avoid the following race:
> >
> > 	- A task T hits the bp installed by uprobe and calls
> > 	  find_active_uprobe()
> >
> > 	- uprobe_unregister() removes this uprobe/bp
> >
> > 	- T calls find_uprobe() which returns NULL
> >
> > 	- another uprobe_register() installs the bp at the same address
> >
> > 	- T calls is_trap_at_addr() which returns true
> >
> > 	- T returns to handle_swbp() and gets SIGTRAP.

...

> >  int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
> > @@ -1046,7 +1046,12 @@ register_for_each_vma(struct uprobe *uprobe, struct uprobe_consumer *new)
> >
> >  		if (err && is_register)
> >  			goto free;
> > -
> > +		/*
> > +		 * We take mmap_lock for writing to avoid the race with
> > +		 * find_active_uprobe(), install_breakpoint() must not
> > +		 * make is_trap_at_addr() true right after find_uprobe()
> > +		 * returns NULL.
>
> Sorry, I couldn't catch the latter part. What is the relationship of
> taking the mmap_lock and install_breakpoint() and is_trap_at_addr() here?

Please the the changelog above, it tries to explain this race with more
details...

> You meant that find_active_uprobe() is using find_uprobe() which searchs
> uprobe form rbtree?

Yes,

> But it seems uprobe is already inserted to the rbtree
> in alloc_uprobe() so find_uprobe() will not return NULL here, right?

uprobe_register() -> alloc_uprobe() can come after
find_active_uprobe() -> find_uprobe() returns NULL.

Now, if uprobe_register() -> register_for_each_vma() used mmap_read_lock(), it
could do install_breakpoint() before find_active_uprobe() calls is_trap_at_addr().

In this case find_active_uprobe() returns with uprobe == NULL and is_swbp == 1,
handle_swbp() treat this case as the "normal" int3 without uprobe and do

	if (!uprobe) {
		if (is_swbp > 0) {
			/* No matching uprobe; signal SIGTRAP. */
			force_sig(SIGTRAP);

Does this answer your question?

Oleg.