[PATCH v2] init: Add strictinit to disable init= fallbacks

[PATCH v2] init: Add strictinit to disable init= fallbacks

[Andy Lutomirski]

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
fs.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---

Now cc-ing Rusty (who probably cares more about startup options than
anyone else) and Shuah (who might want to use this at some point).

Changes from v1:
 - Add missing "if" to the docs (Randy Dunlap)

 Documentation/kernel-parameters.txt |  8 ++++++++
 init/main.c                         | 10 ++++++++++
 2 files changed, 18 insertions(+)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 10d51c2f10d7..3dc921714002 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 has no effect if init= is not specified.
+
 	sunrpc.min_resvport=
 	sunrpc.max_resvport=
 			[NFS,SUNRPC]
diff --git a/init/main.c b/init/main.c
index bb1aed928f21..4fd80dcef9d0 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;
 #ifdef CONFIG_X86_LOCAL_APIC
@@ -962,6 +970,8 @@ static int __ref kernel_init(void *unused)
 			return 0;
 		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
 			execute_command, ret);
+		if (strictinit)
+			panic("Requested init failed and strictinit was set.");
 	}
 	if (!try_to_run_init_process("/sbin/init") ||
 	    !try_to_run_init_process("/etc/init") ||
-- 
1.9.3

Re: [PATCH v2] init: Add strictinit to disable init= fallbacks

[Shuah Khan]

On 09/26/2014 12:04 PM, Andy Lutomirski wrote:
> 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
> fs.
> 
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
> 
> Now cc-ing Rusty (who probably cares more about startup options than
> anyone else) and Shuah (who might want to use this at some point).
> 
> Changes from v1:
>  - Add missing "if" to the docs (Randy Dunlap)
> 
>  Documentation/kernel-parameters.txt |  8 ++++++++
>  init/main.c                         | 10 ++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 10d51c2f10d7..3dc921714002 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 has no effect if init= is not specified.
> +
>  	sunrpc.min_resvport=
>  	sunrpc.max_resvport=
>  			[NFS,SUNRPC]
> diff --git a/init/main.c b/init/main.c
> index bb1aed928f21..4fd80dcef9d0 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;
>  #ifdef CONFIG_X86_LOCAL_APIC
> @@ -962,6 +970,8 @@ static int __ref kernel_init(void *unused)
>  			return 0;
>  		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
>  			execute_command, ret);
> +		if (strictinit)
> +			panic("Requested init failed and strictinit was set.");

It would make sense to not print Attempting defaults for strictinit
case. I would recommend making it an else case and changing the
strictinit message to or something along the lines:

panic("Failed to execute %s (error %d). Not attempting defaults in
strictinit mode...\n", execute_command, ret);


-- Shuah

-- 
Shuah Khan
Sr. Linux Kernel Developer
Samsung Research America (Silicon Valley)
shuahkh@osg.samsung.com | (970) 217-8978