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Date:   Sun, 30 Jul 2023 10:58:44 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     paulmck@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, 29 Jul 2023 09:16:56 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> 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\\.`.

^^^ /proc/cmdline - `/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.

Ah, I got it. Indeed, for kexec, we need to drop the options from
the bootconfig.

> 
> > 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.)

Not sure, I guess it is possible to use kexec() for kdump or warm reboot,
but it should be rare and we can expand this if someone need it.

> 
> 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.

Yeah, maybe it is the best way to do.
Anyway, I understand the reason why we need this interface.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you!

> 
> > 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>


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>