lua-lsm: expose kernel file load ids

[chenzongyao200127]

Summary

Expose kernel file/load ids to Lua policies and remove the redundant size
argument from post file data hooks.

The id arguments are now stable strings from the kernel helpers, so Lua policy
can compare readable values instead of enum numbers. The post hooks continue to
pass binary-safe Lua strings for the data buffer; policy can use the Lua length
operator when it needs the byte length.

Lua examples

return {
    name = "file-load-policy",
    author = "Zongyao Chen",
    license = "GPL",

    kernel_load_data = function(id, contents)
        if id == "kernel-module" and not contents then
            return false, errno.EPERM
        end
        return true
    end,

    kernel_post_load_data = function(data, id, description)
        if id ~= "kernel-module" then
            return true
        end

        if #data < 4 or data:sub(1, 4) ~= "\127ELF" then
            return false, errno.EPERM
        end

        return true
    end,

    kernel_read_file = function(file, id, contents)
        if id == "firmware" and not contents then
            return true
        end
        return true
    end,

    kernel_post_read_file = function(file, data, id)
        if id == "kernel-module" and #data >= 4 then
            return data:sub(1, 4) == "\127ELF"
        end
        return true
    end,
}

Testing

• git diff --check origin/lua-lsm..HEAD
• ./scripts/checkpatch.pl --git origin/lua-lsm..HEAD

[chenzongyao200127]

Recreated this PR from the dedicated head branch : #18

[chenzongyao200127]

Recreated this PR from the dedicated head branch lua-lsm-file-load-ids: #18