Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls for prog_bpf_syscall

[Al Viro]

On Mon, Mar 14, 2022 at 10:07:31AM -0700, Hao Luo wrote:
> Hello Al,

> > In which contexts can those be called?
> >
> 
> In a sleepable context. The plan is to introduce a certain tracepoints
> as sleepable, a program that attaches to sleepable tracepoints is
> allowed to call these functions. In particular, the first sleepable
> tracepoint introduced in this patchset is one at the end of
> cgroup_mkdir(). Do you have any advices?

Yes - don't do it, unless you really want a lot of user-triggerable
deadlocks.

Pathname resolution is not locking-agnostic.  In particular, you can't
do it if you are under any ->i_rwsem, whether it's shared or exclusive.
That includes cgroup_mkdir() callchains.  And if the pathname passed
to these functions will have you walk through the parent directory,
you would get screwed (e.g. if the next component happens to be
inexistent, triggering a lookup, which takes ->i_rwsem shared).