linux-kernel - Re: [GIT PULL] tracing: Changes for 5.6

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Date:   Tue, 4 Feb 2020 07:28:56 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [GIT PULL] tracing: Changes for 5.6

On Tue, 4 Feb 2020 11:54:09 +0000
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Feb 4, 2020 at 10:32 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> >  - Added new "bootconfig".
> >    Looks for a file appended to initrd to add boot config options.
> >    This has been discussed thoroughly at Linux Plumbers.
> >    Very useful for adding kprobes at bootup.  
> 
> Is there a way to disable this from the "real" kernel command line?

Currently no.

> 
> If I have problems, I want to boot with a known kernel command line
> (forcing text mode etc). If the bootconfig file always gets parsed,
> there's no way to do that from the bootloader (grub2, whatever)..
> 

I totally agree as I had that exact same issue while testing this. I
wanted to not have it and found it a bit annoying I needed to modify
the initrd to remove it for just a single boot up.

Would this work?

-- Steve

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ade4e6ec23e0..c4f1417f1934 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -857,6 +857,10 @@
                 on      Perform hardened usercopy checks (default).
                 off     Disable hardened usercopy checks.
 
+	disable_bootconfig
+			Disable reading the bootconfig that may be attached
+			to the initrd.
+
 	disable_radix	[PPC]
 			Disable RADIX MMU mode on POWER9
 
diff --git a/init/main.c b/init/main.c
index dd7da62d99a5..b52636cd9c1d 100644
--- a/init/main.c
+++ b/init/main.c
@@ -336,15 +336,23 @@ u32 boot_config_checksum(unsigned char *p, u32 size)
 	return ret;
 }
 
-static void __init setup_boot_config(void)
+static void __init setup_boot_config(const char *cmdline)
 {
 	u32 size, csum;
 	char *data, *copy;
+	const char *p;
 	u32 *hdr;
 
 	if (!initrd_end)
 		return;
 
+	p = strstr(cmdline, "disable_bootconfig");
+	if (p && (p == cmdline || isspace(*(p-1))) &&
+	    (!p[18] || isspace(p[18]))) {
+		pr_info("Disabling bootconfig because 'disable_bootconfig' found on the command line\n");
+		return;
+	}
+
 	hdr = (u32 *)(initrd_end - 8);
 	size = hdr[0];
 	csum = hdr[1];
@@ -379,7 +387,7 @@ static void __init setup_boot_config(void)
 	}
 }
 #else
-#define setup_boot_config()	do { } while (0)
+#define setup_boot_config(cmdline)	do { } while (0)
 #endif
 
 /* Change NUL term back to "=", to make "param" the whole string. */
@@ -760,7 +768,7 @@ asmlinkage __visible void __init start_kernel(void)
 	pr_notice("%s", linux_banner);
 	early_security_init();
 	setup_arch(&command_line);
-	setup_boot_config();
+	setup_boot_config(command_line);
 	setup_command_line(command_line);
 	setup_nr_cpu_ids();
 	setup_per_cpu_areas();