False positive for 'Possible unnecessary KERN_ERR' warning in checkpatch

From: Niklas Söderlund
Date: Tue Sep 14 2021 - 12:05:25 EST


Hi Joe,

Maybe you are already aware of this, but in case you are not.

The issue is the checkpatch check for unnecessary KERN_<LEVEL> for log
functions. If a single line contains a statement that match a log
function name from $logFunctions, such as foo_err() and the same line
contains a KERN_<LEVEL> statement the 'WARNING: Possible unnecessary
KERN_ERR' is triggered. This is true even if the KERN_<LEVEL> statement
is not part of the arguments to the foo_err() definition that triggers
the first part of the check.

This can be demonstrated by,

./scripts/checkpatch.pl --mailback --git c821e617896e99b8

Where we get (among others) the warning,

WARNING: Possible unnecessary KERN_ERR
#38: FILE: drivers/net/ethernet/netronome/nfp/nfp_net.h:63:
+#define nn_err(nn, fmt, args...) nn_pr(nn, KERN_ERR, fmt, ## args)

Looking at the code in checkpatch.pl we have,

our $logFunctions = qr{(?x:
printk(?:_ratelimited|_once|_deferred_once|_deferred|)|
(?:[a-z0-9]+_){1,2}(?:printk|emerg|alert|crit|err|warning|warn|notice|info|debug|dbg|vdbg|devel|cont|WARN)(?:_ratelimited|_once|)|
TP_printk|
WARN(?:_RATELIMIT|_ONCE|)|
panic|
MODULE_[A-Z_]+|
seq_vprintf|seq_printf|seq_puts
)};

...

# check for logging functions with KERN_<LEVEL>
if ($line !~ /printk(?:_ratelimited|_once)?\s*\(/ &&
$line =~ /\b$logFunctions\s*\(.*\b(KERN_[A-Z]+)\b/) {
my $level = $1;
if (WARN("UNNECESSARY_KERN_LEVEL",
"Possible unnecessary $level\n" . $herecurr) &&
$fix) {
$fixed[$fixlinenr] =~ s/\s*$level\s*//;
}
}

Looking at the line from above that triggers the warning,

#define nn_err(nn, fmt, args...) nn_pr(nn, KERN_ERR, fmt, ## args)

We see that the warning is triggers by the regexp but that it matches
the first part on nn_err( and then the second part of on the second
argument to nn_pr, KERN_ERR. I believe this to be a false positive.

Unfortunately my Perl skills are not good enough to fix the check to only
look for KERN_[A-Z]+ inside the argument list to the log function name
that matches the first part of the regexp.

--
Regards,
Niklas Söderlund