[PATCH v3] init: Add strictinit to disable init= fallbacks

From: Andy Lutomirski
Date: Fri Sep 26 2014 - 15:14:12 EST

If a user puts init=/whatever on the command line and /whatever
can't be run, then the kernel will try a few default options before
giving up. If init=/whatever came from a bootloader prompt, then
this probably makes sense. On the other hand, if it comes from a
script (e.g. a tool like virtme or perhaps a future kselftest
script), then the fallbacks are likely to exist, but they'll do the
wrong thing. For example, they might unexpectedly invoke systemd.

This adds a new option called strictinit. If init= and strictinit
are both set, and the init= binary is not executable, then the
kernel will panic immediately. If strictinit is set but init= is
not set, then strictinit will have no effect, because the only real
alternative would be to panic regardless of the contents of the root

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Documentation/kernel-parameters.txt | 8 ++++++++
init/main.c | 16 ++++++++++++++--
2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 10d51c2f10d7..1576273edce6 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3236,6 +3236,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
stifb= [HW]
Format: bpp:<bpp1>[:<bpp2>[:<bpp3>...]]

+ strictinit [KNL,BOOT]
+ Normally, if the kernel can't find the init binary
+ specified by rdinit= and/or init=, then it will
+ try several fallbacks. If strictinit is set
+ and the value specified by init= does not work,
+ then the kernel will panic instead.
+ This option makes no sense if init= is not specified.
diff --git a/init/main.c b/init/main.c
index bb1aed928f21..2ae0f2776155 100644
--- a/init/main.c
+++ b/init/main.c
@@ -131,6 +131,7 @@ static char *initcall_command_line;

static char *execute_command;
static char *ramdisk_execute_command;
+static bool strictinit;

* Used to generate warnings if static_key manipulation functions are used
@@ -347,6 +348,13 @@ static int __init rdinit_setup(char *str)
__setup("rdinit=", rdinit_setup);

+static int __init strictinit_setup(char *str)
+ strictinit = true;
+ return 1;
+__setup("strictinit", strictinit_setup);
#ifndef CONFIG_SMP
static const unsigned int setup_max_cpus = NR_CPUS;
@@ -960,8 +968,12 @@ static int __ref kernel_init(void *unused)
ret = run_init_process(execute_command);
if (!ret)
return 0;
- pr_err("Failed to execute %s (error %d). Attempting defaults...\n",
- execute_command, ret);
+ if (strictinit)
+ panic("Requested init %s failed (error %d) and strictinit was set.",
+ execute_command, ret);
+ else
+ pr_err("Failed to execute %s (error %d). Attempting defaults...\n",
+ execute_command, ret);
if (!try_to_run_init_process("/sbin/init") ||
!try_to_run_init_process("/etc/init") ||

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