[PATCH v2 0/1] Allow setting hostname before userspace starts

[PATCH v2 0/1] Allow setting hostname before userspace starts

[Dan Moulding]

Some userspace processes may rely on gethostname to always return the
correct machine name. However, the only way that the hostname may be
set is by some other userspace process calling sethostname
first. During boot, if a process that depends on gethostname runs
before sethostname has been called, then the process that called
gethostname is going to get an incorrect result.

A real-world case where this comes up is with mdadm, which if
gethostname returns the wrong name, can cause local md-raid arrays to
appear to be foreign arrays. This can alter how mdadm assembles the
array, or can even cause array assembly to fail. I imagine there are
probbaly other real-world cases where undesirable behavior results when
the hostname is not set early enough.

I'm proposing adding the option to set the hostname from a kernel
parameter, so that the correct host name can be guaranteed to be set
before any userspace process can call gethostname.

I can imagine an even better way to do this would be to have the
hostname written to some non-volatile storage (like a firmware NVRAM
variable or such), which the kernel could read out during early
boot. But, alas, such designs require hardware support, standards, and
cooperation. This proposal is an alternative that can provide a simple
and immediate solution.

v2:
* Use strlcpy instead of strncpy to eliminate W=1 compiler warning
  (assuaging it from its string truncation fears). Thanks to kernel
  test robot for finding this.
* Move "hostname" after "hlt" in kernel-parameters.txt (I promise I
  know my ABCs).

Dan Moulding (1):
  init: Add "hostname" kernel parameter

 Documentation/admin-guide/kernel-parameters.txt | 13 +++++++++++++
 init/version.c                                  | 17 +++++++++++++++++
 2 files changed, 30 insertions(+)

-- 
2.35.1

[PATCH v2 1/1] init: Add "hostname" kernel parameter

[Dan Moulding]

The gethostname system call returns the hostname for the current
machine. However, the kernel has no mechanism to initially set the
current machine's name in such a way as to guarantee that the first
userspace process to call gethostname will receive a meaningful
result. It relies on some unspecified userspace process to first call
sethostname before gethostname can produce a meaningful name.

Traditionally the machine's hostname is set from userspace by the init
system. The init system, in turn, often relies on a configuration file
(say, /etc/hostname) to provide the value that it will supply in the
call to sethostname. Consequently, the file system containing
/etc/hostname usually must be available before the hostname will be
set. There may, however, be earlier userspace processes that could
call gethostname before the file system containing /etc/hostname is
mounted. Such a process will get some other, likely meaningless, name
from gethostname (such as "(none)", "localhost", or "darkstar").

A real-world example where this can happen, and lead to undesirable
results, is with mdadm. When assembling arrays, mdadm distinguishes
between "local" arrays and "foreign" arrays. A local array is one that
properly belongs to the current machine, and a foreign array is one
that is (possibly temporarily) attached to the current machine, but
properly belongs to some other machine. To determine if an array is
local or foreign, mdadm may compare the "homehost" recorded on the
array with the current hostname. If mdadm is run before the root file
system is mounted, perhaps because the root file system itself resides
on an md-raid array, then /etc/hostname isn't yet available and the
init system will not yet have called sethostname, causing mdadm to
incorrectly conclude that all of the local arrays are foreign.

Solving this problem *could* be delegated to the init system. It could
be left up to the init system (including any init system that starts
within an initramfs, if one is in use) to ensure that sethostname is
called before any other userspace process could possibly call
gethostname. However, it may not always be obvious which processes
could call gethostname (for example, udev itself might not call
gethostname, but it could via udev rules invoke processes that
do). Additionally, the init system has to ensure that the hostname
configuration value is stored in some place where it will be readily
accessible during early boot. Unfortunately, every init system will
attempt to (or has already attempted to) solve this problem in a
different, possibly incorrect, way. This makes getting consistently
working configurations harder for users.

I believe it is better for the kernel to provide the means by which
the hostname may be set early, rather than making this a problem for
the init system to solve. The option to set the hostname during early
startup, via a kernel parameter, provides a simple, reliable way to
solve this problem. It also could make system configuration easier for
some embedded systems.

Signed-off-by: Dan Moulding <dmoulding@me.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 13 +++++++++++++
 init/version.c                                  | 17 +++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3f1cc5e317ed..c9a76b7ca460 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1617,6 +1617,19 @@
 
 	hlt		[BUGS=ARM,SH]
 
+	hostname=	[KNL] Set the hostname (aka UTS nodename).
+			Format: <string>
+			This allows setting the system's hostname during early
+			startup. This sets the name returned by gethostname.
+			Using this parameter to set the hostname makes it
+			possible to ensure the hostname is correctly set before
+			any userspace processes run, avoiding the possibility
+			that a process may call gethostname before the hostname
+			has been explicitly set, resulting in the calling
+			process getting an incorrect result. The string must
+			not exceed the maximum allowed hostname length (usually
+			64 characters) and will be truncated otherwise.
+
 	hpet=		[X86-32,HPET] option to control HPET usage
 			Format: { enable (default) | disable | force |
 				verbose }
diff --git a/init/version.c b/init/version.c
index 1a356f5493e8..b7f9559d417c 100644
--- a/init/version.c
+++ b/init/version.c
@@ -11,6 +11,8 @@
 #include <linux/build-salt.h>
 #include <linux/elfnote-lto.h>
 #include <linux/export.h>
+#include <linux/init.h>
+#include <linux/printk.h>
 #include <linux/uts.h>
 #include <linux/utsname.h>
 #include <generated/utsrelease.h>
@@ -35,6 +37,21 @@ struct uts_namespace init_uts_ns = {
 };
 EXPORT_SYMBOL_GPL(init_uts_ns);
 
+static int __init early_hostname(char *arg)
+{
+	size_t bufsize = sizeof(init_uts_ns.name.nodename);
+	size_t maxlen  = bufsize - 1;
+	size_t arglen;
+
+	arglen = strlcpy(init_uts_ns.name.nodename, arg, bufsize);
+	if (arglen > maxlen) {
+		pr_warn("hostname parameter exceeds %zd characters and will be truncated",
+			maxlen);
+	}
+	return 0;
+}
+early_param("hostname", early_hostname);
+
 /* FIXED STRINGS! Don't touch! */
 const char linux_banner[] =
 	"Linux version " UTS_RELEASE " (" LINUX_COMPILE_BY "@"
-- 
2.35.1

Re: [PATCH v2 0/1] Allow setting hostname before userspace starts

[Pavel Machek]

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Hi1

> Some userspace processes may rely on gethostname to always return the
> correct machine name. However, the only way that the hostname may be
> set is by some other userspace process calling sethostname
> first. During boot, if a process that depends on gethostname runs
> before sethostname has been called, then the process that called
> gethostname is going to get an incorrect result.

Don't do that, then? :-).

init gets passed command line parameters kernel did not use. So init
can do that easily. Plus, there's initrd.

Best regards,
								Pavel
-- 
People of Russia, stop Putin before his war on Ukraine escalates.

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Re: [PATCH v2 0/1] Allow setting hostname before userspace starts

[Dan Moulding]

On May 6, 2022, at 1:23 AM, Pavel Machek <pavel@ucw.cz> wrote:

> init gets passed command line parameters kernel did not use. So init
> can do that easily.

I agree this is a problem that *can* be delegated to init to solve.
But is it *better* to do that? Why have the kernel pass the the hostname
to the init system, just so that the init system has to pass it back to the
kernel later?

> Plus, there's initrd.

This to me is a reason why it's better for the kernel to do this. If left up
to the init system, it must be sure it gets done correctly both when an
initramfs is used, and when one is not.

P.S. Apologies to anyone who received this twice, my first reply was
bounced by the list servers.

Cheers,

  -- Dan