include/asm-generic/bug.h: clarify valid uses of WARN()
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Message ID 20180620103716.61636-1-dvyukov@gmail.com
State New, archived
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  • include/asm-generic/bug.h: clarify valid uses of WARN()
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Commit Message

Dmitry Vyukov June 20, 2018, 10:37 a.m. UTC
From: Dmitry Vyukov <dvyukov@google.com>

Explicitly state that WARN*() should be used only for recoverable
kernel issues/bugs and that it should not be used for any kind of
invalid external inputs or transient conditions.

Motivation: it's a very useful capability to be able to understand
if a particular kernel splat means a kernel bug or simply an invalid
user-space program. For the former one wants to notify kernel developers,
while notifying kernel developers for the latter is annoying.
Even a kernel developer may not know what to do with a WARNING
in an unfamiliar subsystem. This is especially critical for any automated
testing systems that may use panic_on_warn and mail kernel developers.

The clear separation also serves as an additional documentation:
is it a condition that must never occur because of additional
checks/logic elsewhere? or is it simply a check for invalid inputs
or unfortunate conditions?

Use of pr_err() for user messages also leads to better error messages.
"Something is wrong in file foo on line X" is not particularly useful
message for end user. pr_err() forces developers to write more meaningful
error messages for user.

As of now we are almost there. We are doing systematic kernel testing
with panic_on_warn and are not seeing massive amounts of false positives.
But every now and then another WARN on ENOMEM or invalid inputs pops up
and leads to a lengthy argument each time. The goal of this change
is to officially document the rules.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
---
 include/asm-generic/bug.h | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

Comments

Greg Kroah-Hartman June 21, 2018, 12:02 a.m. UTC | #1
On Wed, Jun 20, 2018 at 12:37:16PM +0200, Dmitry Vyukov wrote:
> From: Dmitry Vyukov <dvyukov@google.com>
> 
> Explicitly state that WARN*() should be used only for recoverable
> kernel issues/bugs and that it should not be used for any kind of
> invalid external inputs or transient conditions.
> 
> Motivation: it's a very useful capability to be able to understand
> if a particular kernel splat means a kernel bug or simply an invalid
> user-space program. For the former one wants to notify kernel developers,
> while notifying kernel developers for the latter is annoying.
> Even a kernel developer may not know what to do with a WARNING
> in an unfamiliar subsystem. This is especially critical for any automated
> testing systems that may use panic_on_warn and mail kernel developers.
> 
> The clear separation also serves as an additional documentation:
> is it a condition that must never occur because of additional
> checks/logic elsewhere? or is it simply a check for invalid inputs
> or unfortunate conditions?
> 
> Use of pr_err() for user messages also leads to better error messages.
> "Something is wrong in file foo on line X" is not particularly useful
> message for end user. pr_err() forces developers to write more meaningful
> error messages for user.
> 
> As of now we are almost there. We are doing systematic kernel testing
> with panic_on_warn and are not seeing massive amounts of false positives.
> But every now and then another WARN on ENOMEM or invalid inputs pops up
> and leads to a lengthy argument each time. The goal of this change
> is to officially document the rules.
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> ---
>  include/asm-generic/bug.h | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)

Nice!

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Patch
diff mbox series

diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index a7613e1b0c87..20561a60db9c 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -75,9 +75,19 @@  struct bug_entry {
 
 /*
  * WARN(), WARN_ON(), WARN_ON_ONCE, and so on can be used to report
- * significant issues that need prompt attention if they should ever
- * appear at runtime.  Use the versions with printk format strings
- * to provide better diagnostics.
+ * significant kernel issues that need prompt attention if they should ever
+ * appear at runtime.
+ *
+ * Do not use these macros when checking for invalid external inputs
+ * (e.g. invalid system call arguments, or invalid data coming from
+ * network/devices), and on transient conditions like ENOMEM or EAGAIN.
+ * These macros should be used for recoverable kernel issues only.
+ * For invalid external inputs, transient conditions, etc use
+ * pr_err[_once/_ratelimited]() followed by dump_stack(), if necessary.
+ * Do not include "BUG"/"WARNING" in format strings manually to make these
+ * conditions distinguishable from kernel issues.
+ *
+ * Use the versions with printk format strings to provide better diagnostics.
  */
 #ifndef __WARN_TAINT
 extern __printf(3, 4)