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SubjectRe: [PATCH] dynamic_debug: allow to work if debugfs is disabled
On Wed, Jan 22, 2020 at 08:43:43AM +0100, Greg Kroah-Hartman wrote:
> With the realization that having debugfs enabled on "production" systems is
> generally not a good idea, debugfs is being disabled from more and more
> platforms over time. However, the functionality of dynamic debugging still is
> needed at times, and since it relies on debugfs for its user api, having
> debugfs disabled also forces dynamic debug to be disabled.

Why is the dyndbg= command-line option not sufficient for these use-cases?

> To get around this, move the "control" file for dynamic_debug to procfs IFF
> debugfs is disabled. This lets people turn on debugging as needed at runtime
> for individual driverfs and subsystems.

Hmm. If something called "dynamic_debug" is getting moved out of debugfs,
this does raise the question as to what (if anything) should be left behind.
I worry this is a bit of a slippery slope...

Anywho, comments below.

> Reported-by: many different companies
> Cc: Jason Baron <jbaron@akamai.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> .../admin-guide/dynamic-debug-howto.rst | 3 +++
> lib/Kconfig.debug | 2 +-
> lib/dynamic_debug.c | 17 ++++++++++++++---
> 3 files changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/admin-guide/dynamic-debug-howto.rst b/Documentation/admin-guide/dynamic-debug-howto.rst
> index 252e5ef324e5..41f43a373a6a 100644
> --- a/Documentation/admin-guide/dynamic-debug-howto.rst
> +++ b/Documentation/admin-guide/dynamic-debug-howto.rst
> @@ -54,6 +54,9 @@ If you make a mistake with the syntax, the write will fail thus::
> <debugfs>/dynamic_debug/control
> -bash: echo: write error: Invalid argument
>
> +Note, for systems without 'debugfs' enabled, the control file can be
> +also found in ``/proc/dynamic_debug/control``.
> +
> Viewing Dynamic Debug Behaviour
> ===============================
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 5ffe144c9794..01d4add8b963 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -98,7 +98,7 @@ config DYNAMIC_DEBUG
> bool "Enable dynamic printk() support"
> default n
> depends on PRINTK
> - depends on DEBUG_FS
> + depends on (DEBUG_FS || PROC_FS)
> help
>
> Compiles debug level messages into the kernel, which would noti

The help text here also needs updating, since it refers to debugfs.

> diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
> index c60409138e13..077b2d6623ac 100644
> --- a/lib/dynamic_debug.c
> +++ b/lib/dynamic_debug.c
> @@ -993,13 +993,24 @@ static __initdata int ddebug_init_success;
>
> static int __init dynamic_debug_init_debugfs(void)
> {
> - struct dentry *dir;
> + struct dentry *debugfs_dir;
> + struct proc_dir_entry *procfs_dir;
>
> if (!ddebug_init_success)
> return -ENODEV;
>
> - dir = debugfs_create_dir("dynamic_debug", NULL);
> - debugfs_create_file("control", 0644, dir, NULL, &ddebug_proc_fops);
> + /* Create the control file in debugfs if it is enabled */
> + if (debugfs_initialized) {
> + debugfs_dir = debugfs_create_dir("dynamic_debug", NULL);
> + debugfs_create_file("control", 0644, debugfs_dir, NULL,
> + &ddebug_proc_fops);
> + return 0;
> + }
> +
> + /* No debugfs so put it in procfs instead */
> + procfs_dir = proc_mkdir("dynamic_debug", NULL);
> + if (procfs_dir)
> + proc_create("control", 0x644, procfs_dir, &ddebug_proc_fops);

Shouldn't this be octal rather than hex? Even then, I don't understand what
use it is being able to read but not write to this file. Perhaps make it
0600 for /proc ?

Will

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