[PATCH 00/14] Make the user mode driver code a better citizen

[Tetsuo Handa]

On 2020/06/29 4:44, Alexei Starovoitov wrote:
> But all the defensive programming kinda goes against general kernel style.
> I wouldn't do it. Especially pr_info() ?!
> Though I don't feel strongly about it.

Honestly speaking, caller should check for errors and print appropriate
messages. info->wd.mnt->mnt_root != info->wd.dentry indicates that something
went wrong (maybe memory corruption). But other conditions are not fatal.
That is, I consider even pr_info() here should be unnecessary.

> 
> I would like to generalize elf_header_check() a bit and call it
> before doing blob_to_mnt() to make sure that all blobs are elf files only.
> Supporting '#!/bin/bash' or other things as blobs seems wrong to me.

Why? There is no point with forbidding "#!", for users can use a wrapper
ELF binary which contains instructions including glibc's execv()/system()
functions even if "#!" cannot be used.

What is more important is what protection/isolation properties processes started
via fork_usermode_driver() should hold, for ELF binary can contain arbitrary
instructions, these processes run as daemons (reading request from stdin and
writing response to stdout) but hidden from "/usr/bin/pstree -p 1" (because
they are forked from kthreadd kernel thread).