On Fri, Dec 6, 2019 at 6:10 AM Tycho Andersen <tycho@xxxxxxxx> wrote: > > On Thu, Dec 05, 2019 at 11:44:53PM +0000, Sargun Dhillon wrote: > > PTRACE_GETFD is a generic ptrace API that allows the tracer to > > get file descriptors from the traceee. > > > > The primary reason to use this syscall is to allow sandboxers to > > I might change this to "one motivation to use this ptrace command", > because I'm sure people will invent other crazy uses soon after it's > added :) > Another use-case that's come up has been transparent proxy for service meshes. Rather than doing intercept at L4 (iptables), or DNS, just rewriting the connect is nicer. A side benefit is that getpeername still works. > > take action on an FD on behalf of the tracee. For example, this > > can be combined with seccomp's user notification feature to extract > > a file descriptor and call privileged syscalls, like binding > > a socket to a privileged port. > > This can already be accomplished via injecting parasite code like CRIU > does; adding a ptrace() command like this makes it much nicer to be > sure, but it is redundant. > > Tycho How can you do this if the tracee doesn't have privilege? For example, if the tracee doesn't have CAP_SYS_BIND_SERVICE, how could you get it to bind to a port that's privileged without taking the file descriptor and doing it in a process that does have CAP_SYS_BIND_SERVICE? The other aspect is that doing the parasitic code thing is kind of slow, in that it requires quite a few operations.
