linux-kernel - Re: [PATCH] init: Handle bootloader head in kernel parameters

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Message-ID: <CAAhV-H69oz-Rmz4Q2Gad-x5AR0C2cxtK7Mgsc5iJHALP_NcEhw@mail.gmail.com>
Date: Fri, 11 Jul 2025 20:51:28 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Huacai Chen <chenhuacai@...ngson.cn>, Andrew Morton <akpm@...ux-foundation.org>, 
	linux-mm@...ck.org, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH] init: Handle bootloader head in kernel parameters

On Fri, Jul 11, 2025 at 8:41 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Fri, Jul 11, 2025 at 08:34:25PM +0800, Huacai Chen wrote:
> > Hi, Greg,
> >
> > On Fri, Jul 11, 2025 at 7:06 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > >
> > > On Fri, Jul 11, 2025 at 06:24:55PM +0800, Huacai Chen wrote:
> > > > BootLoader may pass a head such as "BOOT_IMAGE=/boot/vmlinuz-x.y.z" to
> > > > kernel parameters. But this head is not recognized by the kernel so will
> > > > be passed to user space. However, user space init program also doesn't
> > > > recognized it.
> > >
> > > Then why is it on the kernel command line if it is not recognized?
> > UEFI put it at the beginning of the command line, you can see it from
> > /proc/cmdline, both on x86 and LoongArch.
>
> Then fix UEFI :)
>
> My boot command line doesn't have that on x86, perhaps you need to fix
> your bootloader?
Not only UEFI, Grub also do this, for many years, not now. I don't
know why they do this, but I think at least it is not a bug. For
example, maybe it just tells user the path of kernel image via
/proc/cmdline.

[chenhuacai@...nelserver linux-official.git]$ uname -a
Linux kernelserver 6.12.0-84.el10.x86_64 #1 SMP PREEMPT_DYNAMIC Tue
May 13 13:39:02 UTC 2025 x86_64 GNU/Linux
[chenhuacai@...nelserver linux-official.git]$ cat /proc/cmdline
BOOT_IMAGE=(hd0,gpt2)/vmlinuz-6.12.0-84.el10.x86_64
root=UUID=c8fcb11a-0f2f-48e5-a067-4cec1d18a721 ro
crashkernel=2G-64G:256M,64G-:512M
resume=UUID=1c320fec-3274-4b5b-9adf-a06
42e7943c0 rhgb quiet

>
> > > > KEXEC may also pass a head such as "kexec" on some architectures.
> > >
> > > That's fine, kexec needs this.
> > >
> > > > So the the best way is handle it by the kernel itself, which can avoid
> > > > such boot warnings:
> > > >
> > > > Kernel command line: BOOT_IMAGE=(hd0,1)/vmlinuz-6.x root=/dev/sda3 ro console=tty
> > > > Unknown kernel command line parameters "BOOT_IMAGE=(hd0,1)/vmlinuz-6.x", will be passed to user space.
> > >
> > > Why is this a problem?  Don't put stuff that is not needed on the kernel
> > > command line :)
> > Both kernel and user space don't need it, and if it is passed to user
> > space then may cause some problems. For example, if there is
> > init=/bin/bash, then bash will crash with this parameter.
>
> Again, fix the bootloader to not do this, why is the kernel responsible
> for this?
>
> What has suddenly changed to now require this when we never have needed
> it before?
Because init=/bin/bash is not a usual use case, so in most cases it is
just a warning in dmesg. But once we see it, we need to fix it.

>
> > > > Cc: stable@...r.kernel.org
> > > > Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> > > > ---
> > > >  init/main.c | 7 +++++++
> > > >  1 file changed, 7 insertions(+)
> > > >
> > > > diff --git a/init/main.c b/init/main.c
> > > > index 225a58279acd..9e0a7e8913c0 100644
> > > > --- a/init/main.c
> > > > +++ b/init/main.c
> > > > @@ -545,6 +545,7 @@ static int __init unknown_bootoption(char *param, char *val,
> > > >                                    const char *unused, void *arg)
> > > >  {
> > > >       size_t len = strlen(param);
> > > > +     const char *bootloader[] = { "BOOT_IMAGE", "kexec", NULL };
> > >
> > > You need to document why these are ok to "swallow" and not warn for.
> > Because they are bootloader heads, not really a wrong parameter. We
> > only need a warning if there is a wrong parameter.
>
> Again, fix the bootloader.
But I don't think this is a bootloader bug.

>
> > > >
> > > >       /* Handle params aliased to sysctls */
> > > >       if (sysctl_is_alias(param))
> > > > @@ -552,6 +553,12 @@ static int __init unknown_bootoption(char *param, char *val,
> > > >
> > > >       repair_env_string(param, val);
> > > >
> > > > +     /* Handle bootloader head */
> > >
> > > Handle it how?
> > argv_init and envp_init arrays will be passed to userspace, so just
> > return early (before argv_init and envp_init handling) can avoid it
> > being passed.
>
> You need to document this way better.
>
> But again, please just fix your bootloader to not pass on lines to the
> kernel that it can not parse.
OK, I will update the document, but again, I don't think this is a
bootloader bug.

Huacai

>
> thanks,
>
> greg k-h