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SubjectRe: [PATCH v2 net-next 1/4] umh: introduce fork_usermode_blob() helper
On Thu, May 3, 2018 at 12:36 AM, Alexei Starovoitov <ast@kernel.org> wrote:
> Introduce helper:
> int fork_usermode_blob(void *data, size_t len, struct umh_info *info);
> struct umh_info {
> struct file *pipe_to_umh;
> struct file *pipe_from_umh;
> pid_t pid;
> };
>
> that GPLed kernel modules (signed or unsigned) can use it to execute part
> of its own data as swappable user mode process.
>
> The kernel will do:
> - mount "tmpfs"
> - allocate a unique file in tmpfs
> - populate that file with [data, data + len] bytes
> - user-mode-helper code will do_execve that file and, before the process
> starts, the kernel will create two unix pipes for bidirectional
> communication between kernel module and umh
> - close tmpfs file, effectively deleting it
> - the fork_usermode_blob will return zero on success and populate
> 'struct umh_info' with two unix pipes and the pid of the user process
>
> As the first step in the development of the bpfilter project
> the fork_usermode_blob() helper is introduced to allow user mode code
> to be invoked from a kernel module. The idea is that user mode code plus
> normal kernel module code are built as part of the kernel build
> and installed as traditional kernel module into distro specified location,
> such that from a distribution point of view, there is
> no difference between regular kernel modules and kernel modules + umh code.
> Such modules can be signed, modprobed, rmmod, etc. The use of this new helper
> by a kernel module doesn't make it any special from kernel and user space
> tooling point of view.
[...]
> +static struct vfsmount *umh_fs;
> +
> +static int init_tmpfs(void)
> +{
> + struct file_system_type *type;
> +
> + if (umh_fs)
> + return 0;
> + type = get_fs_type("tmpfs");
> + if (!type)
> + return -ENODEV;
> + umh_fs = kern_mount(type);
> + if (IS_ERR(umh_fs)) {
> + int err = PTR_ERR(umh_fs);
> +
> + put_filesystem(type);
> + umh_fs = NULL;
> + return err;
> + }
> + return 0;
> +}

Should init_tmpfs() be holding some sort of mutex if it's fiddling
with `umh_fs`? The current code only calls it in initcall context, but
if that ever changes and two processes try to initialize the tmpfs at
the same time, a few things could go wrong.
I guess Luis' suggestion (putting a call to init_tmpfs() in
do_basic_setup()) might be the easiest way to get rid of that problem.

> +static int alloc_tmpfs_file(size_t size, struct file **filp)
> +{
> + struct file *file;
> + int err;
> +
> + err = init_tmpfs();
> + if (err)
> + return err;
> + file = shmem_file_setup_with_mnt(umh_fs, "umh", size, VM_NORESERVE);
> + if (IS_ERR(file))
> + return PTR_ERR(file);
> + *filp = file;
> + return 0;
> +}

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