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Date:   Sat, 29 Jul 2023 09:16:56 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     akpm@linux-foundation.org, adobriyan@gmail.com, arnd@kernel.org,
        ndesaulniers@google.com, sfr@canb.auug.org.au,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com
Subject: Re: [PATCH RFC bootconfig] 1/2] fs/proc: Add /proc/cmdline_load for
 boot loader arguments
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On Sat, Jul 29, 2023 at 11:29:29PM +0900, Masami Hiramatsu wrote:
> Hi Paul,
> 
> On Thu, 27 Jul 2023 20:37:00 -0700
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
> 
> > In kernels built with CONFIG_BOOT_CONFIG_FORCE=y, /proc/cmdline will
> > show all kernel boot parameters, both those supplied by the boot loader
> > and those embedded in the kernel image.  This works well for those who
> > just want to see all of the kernel boot parameters, but is not helpful to
> > those who need to see only those parameters supplied by the boot loader.
> > This is especially important when these parameters are presented to the
> > boot loader by automation that might gather them from diverse sources.
> > 
> > Therefore, provide a /proc/cmdline_load file that shows only those kernel
> > boot parameters supplied by the boot loader.
> 
> If I understand correctly, /proc/cmdline_load is something like
> /proc/cmdline_load - `/proc/bootconfig | grep ^kernel\\.`.

Yes, very much something like that.

For one use case, suppose you have a kernel that gets some boot parameters
from the boot loader and some from bootconfig.  If you want to kexec()
into a new kernel, you must tell kexec() what the kernel boot parameters
are.  However, you must *not* tell kexec() about any of the current
kernel's parameters that came from bootconfig, because those should
instead be supplied by the new kernel being kexec()ed into.

So you must pass in only those parameters that came from the boot loader,
hence my proposed /proc/cmdline_load.

> BTW, what about CONFIG_CMDLINE? We already have that Kconfig and it is also
> merged with the command line specified by boot loader. Should we also
> expose that? (when CONFIG_CMDLINE_OVERRIDE=y, we don't need it because
> cmdline is always overridden by the CONFIG_CMDLINE) Unfortunatelly, this
> option is implemented in each arch init, so we have to change all of them...

The use case is embedded systems, right?  I have no idea whether they
have a use case requiring this.  Do those sorts of embedded systems
use kexec()?  (I don't know of any that do, but then again, I haven't
been looking.)

This arch init is in setup_arch(), correct?  If so, one option is to
make start_kernel() or something that it invokes make a copy of the
command line just before invoking setup_arch().  Full disclosure: I
have not yet looked at all the ins and outs of CONFIG_CMDLINE, so this
suggestion should be viewed with appropriate skepticism.

> Thank you,
> 
> > 
> > Why put this in /proc?  Because it is quite similar to /proc/cmdline, so
> > it makes sense to put it in the same place that /proc/cmdline is located.
> > 
> > [ sfr: Apply kernel test robot feedback. ]
> > 
> > Co-developed-by: Stephen Rothwell <sfr@canb.auug.org.au>
> > Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
> > Co-developed-by: Arnd Bergmann <arnd@kernel.org>
> > Signed-off-by: Arnd Bergmann <arnd@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Alexey Dobriyan <adobriyan@gmail.com>
> > Cc: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: <linux-fsdevel@vger.kernel.org>
> > ---
> >  fs/proc/cmdline.c    | 13 +++++++++++++
> >  include/linux/init.h |  3 ++-
> >  init/main.c          |  2 +-
> >  3 files changed, 16 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/proc/cmdline.c b/fs/proc/cmdline.c
> > index a6f76121955f..1d0ef9d2949d 100644
> > --- a/fs/proc/cmdline.c
> > +++ b/fs/proc/cmdline.c
> > @@ -3,6 +3,7 @@
> >  #include <linux/init.h>
> >  #include <linux/proc_fs.h>
> >  #include <linux/seq_file.h>
> > +#include <asm/setup.h>
> >  #include "internal.h"
> >  
> >  static int cmdline_proc_show(struct seq_file *m, void *v)
> > @@ -12,6 +13,13 @@ static int cmdline_proc_show(struct seq_file *m, void *v)
> >  	return 0;
> >  }
> >  
> > +static int cmdline_load_proc_show(struct seq_file *m, void *v)
> > +{
> > +	seq_puts(m, boot_command_line);
> > +	seq_putc(m, '\n');
> > +	return 0;
> > +}
> > +
> >  static int __init proc_cmdline_init(void)
> >  {
> >  	struct proc_dir_entry *pde;
> > @@ -19,6 +27,11 @@ static int __init proc_cmdline_init(void)
> >  	pde = proc_create_single("cmdline", 0, NULL, cmdline_proc_show);
> >  	pde_make_permanent(pde);
> >  	pde->size = saved_command_line_len + 1;
> > +	if (IS_ENABLED(CONFIG_BOOT_CONFIG_FORCE)) {
> > +		pde = proc_create_single("cmdline_load", 0, NULL, cmdline_load_proc_show);
> > +		pde_make_permanent(pde);
> > +		pde->size = strnlen(boot_command_line, COMMAND_LINE_SIZE) + 1;
> > +	}
> >  	return 0;
> >  }
> >  fs_initcall(proc_cmdline_init);
> > diff --git a/include/linux/init.h b/include/linux/init.h
> > index 266c3e1640d4..29e75bbe7984 100644
> > --- a/include/linux/init.h
> > +++ b/include/linux/init.h
> > @@ -112,6 +112,7 @@
> >  #define __REFCONST       .section       ".ref.rodata", "a"
> >  
> >  #ifndef __ASSEMBLY__
> > +
> >  /*
> >   * Used for initialization calls..
> >   */
> > @@ -143,7 +144,7 @@ struct file_system_type;
> >  
> >  /* Defined in init/main.c */
> >  extern int do_one_initcall(initcall_t fn);
> > -extern char __initdata boot_command_line[];
> > +extern char boot_command_line[];
> 
> FYI, boot_command_line[] is mixture of built-in cmdline string with
> bootloader cmdline string.

So if we also need to separate out the CONFIG_CMDLINE arguments, then
/proc/cmdline_load will need to come from some string saved off before
the CONFIG_CMDLINE processing, correct?  I would expect that to be a
separate patch series, but if it is needed, I would be happy to look
into setting it up, as long as I am in the area.

My tests indicate that boot_command_line[] doesn't contain any bootconfig
(and opposed to CONFIG_CMDLINE) arguments, but I could easily have missed
some other corner-case configuration.

And thank you for looking this over!

							Thanx, Paul

> >  extern char *saved_command_line;
> >  extern unsigned int saved_command_line_len;
> >  extern unsigned int reset_devices;
> > diff --git a/init/main.c b/init/main.c
> > index ad920fac325c..2121685c479a 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -135,7 +135,7 @@ EXPORT_SYMBOL(system_state);
> >  void (*__initdata late_time_init)(void);
> >  
> >  /* Untouched command line saved by arch-specific code. */
> > -char __initdata boot_command_line[COMMAND_LINE_SIZE];
> > +char boot_command_line[COMMAND_LINE_SIZE] __ro_after_init;
> >  /* Untouched saved command line (eg. for /proc) */
> >  char *saved_command_line __ro_after_init;
> >  unsigned int saved_command_line_len __ro_after_init;
> > -- 
> > 2.40.1
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@kernel.org>