linux-kernel - Re: [PATCH] selftests/livepatch: adopt to newer sysctl error format

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Date:   Fri, 10 Jul 2020 14:31:08 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>, Shuah Khan <shuah@...nel.org>,
        live-patching@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests/livepatch: adopt to newer sysctl error format

On Fri, Jul 10, 2020 at 05:27:35PM +0200, Petr Mladek wrote:
> On Fri 2020-07-10 10:40:43, Kamalesh Babulal wrote:
> > With procfs v3.3.16, the sysctl command doesn't prints the set key and
> > value on error.  This change breaks livepatch selftest test-ftrace.sh,
> > that tests the interaction of sysctl ftrace_enabled:
> > 

Good catch, it looks like it was this procps commit that modified that
behavior:

  commit da82fe49b1476d227874905068adb69577e11d96
  Author: Patrick Steinhardt <ps@....im>
  Date:   Tue May 29 13:29:03 2018 +0200
  
      sysctl: do not report set key in case `close_stream` fails
      
      As we're using buffered I/O when writing kernel parameters, write errors
      may get delayed until we close the `FILE` stream. As we are currently
      outputting the key that is to be set disregarding the return value of
      `close_stream`, we may end up in a situation where we report error and
      success:
      
          $ sysctl kernel.printk_ratelimit=100000000000000
          sysctl: setting key "kernel.printk_ratelimit": error code 22
          kernel.printk_ratelimit = 100000000000000
      
      Fix the issue by only outputting the updated value in case
      `close_stream` does not report an error.
      
      Signed-off-by: Patrick Steinhardt <ps@....im>

And I'd agree that echoing the failed new value was confusing to see
from a user's perspective.

> >  # selftests: livepatch: test-ftrace.sh
> >  # TEST: livepatch interaction with ftrace_enabled sysctl ... not ok
> >  #
> >  # --- expected
> >  # +++ result
> >  # @@ -16,7 +16,7 @@ livepatch: 'test_klp_livepatch': initial
> >  #  livepatch: 'test_klp_livepatch': starting patching transition
> >  #  livepatch: 'test_klp_livepatch': completing patching transition
> >  #  livepatch: 'test_klp_livepatch': patching complete
> >  # -livepatch: sysctl: setting key "kernel.ftrace_enabled": Device or
> >     resource busy kernel.ftrace_enabled = 0
> >  # +livepatch: sysctl: setting key "kernel.ftrace_enabled": Device or
> >     resource busy
> >  #  % echo 0 > /sys/kernel/livepatch/test_klp_livepatch/enabled
> >  #  livepatch: 'test_klp_livepatch': initializing unpatching transition
> >  #  livepatch: 'test_klp_livepatch': starting unpatching transition
> >  #
> >  # ERROR: livepatch kselftest(s) failed
> > 
> > on setting sysctl kernel.ftrace_enabled={0,1} value successfully, the
> > set key and value is displayed.
> > 
> > This patch fixes it by limiting the output from both the cases to eight
> > words, that includes the error message or set key and value on failure
> > and success. The upper bound of eight words is enough to display the
> > only tracked error message. Also, adjust the check_result string in
> > test-ftrace.sh to match the expected output.
> 
> This looks really tricky.
> 
> I wonder if we could use "sysctl -q" to refuse printing the value
> even with older versions. The following patch works here with
> sysctl 3.3.15:
> 

FWIW, --quiet was added to procps way back in 2004, so it should be safe
to use... and there's already a bunch of net selftests using it.

> diff --git a/tools/testing/selftests/livepatch/functions.sh b/tools/testing/selftests/livepatch/functions.sh
> index 2aab9791791d..47aa4c762bb4 100644
> --- a/tools/testing/selftests/livepatch/functions.sh
> +++ b/tools/testing/selftests/livepatch/functions.sh
> @@ -64,7 +64,8 @@ function set_dynamic_debug() {
>  }
>  
>  function set_ftrace_enabled() {
> -	result=$(sysctl kernel.ftrace_enabled="$1" 2>&1 | paste --serial --delimiters=' ')
> +	result=$(sysctl -q kernel.ftrace_enabled="$1" 2>&1 && \
> +		 sysctl kernel.ftrace_enabled 2>&1)
>  	echo "livepatch: $result" > /dev/kmsg
>  }
>  
> diff --git a/tools/testing/selftests/livepatch/test-ftrace.sh b/tools/testing/selftests/livepatch/test-ftrace.sh
> index e2a76887f40a..aa967c5d0558 100755
> --- a/tools/testing/selftests/livepatch/test-ftrace.sh
> +++ b/tools/testing/selftests/livepatch/test-ftrace.sh
> @@ -53,7 +53,7 @@ livepatch: '$MOD_LIVEPATCH': initializing patching transition
>  livepatch: '$MOD_LIVEPATCH': starting patching transition
>  livepatch: '$MOD_LIVEPATCH': completing patching transition
>  livepatch: '$MOD_LIVEPATCH': patching complete
> -livepatch: sysctl: setting key \"kernel.ftrace_enabled\": Device or resource busy kernel.ftrace_enabled = 0
> +livepatch: sysctl: setting key \"kernel.ftrace_enabled\": Device or resource busy
>  % echo 0 > /sys/kernel/livepatch/$MOD_LIVEPATCH/enabled
>  livepatch: '$MOD_LIVEPATCH': initializing unpatching transition
>  livepatch: '$MOD_LIVEPATCH': starting unpatching transition
> 
> 

I think this method is less fragile than word count / cutting and we get
to drop that strange 'paste' invocation (I had to look that up in the
mapages to remember why we used it).

Regards,

-- Joe