[PATCH 1/2] checkpatch: special case extern struct in .c

[Jim Cromie]

The warning "externs should be avoided in .c files" wants an exception
for linker symbols (named in vmlinux.lds.h etc), like those that mark
the __start, __stop/__end symbols delimiting many kernel sections.

Since checkpatch already checks REALNAME to avoid looking at patch
chunks changing vmlinux.lds.h, add a new else-if block to look at them
instead.  As a simple heuristic, treat all words (in the + patch-lines)
as candidate symbols, to screen later warnings about the same symbols
being found in following chunks that change *.c files.

Where the "# check for new externs in .c files." is done, precede it
with a new else-if block to isolate one common extern-in-c use case:
"extern struct foo bar[]".  For this case, we can issue a more
informative warning:

  WARN("AVOID_EXTERNS",
     "found a file-scoped extern type:$st_type name:$st_name in .c file\n"
     . "is this a linker symbol ?\n" . $herecurr);

NOTE: The "screening" is a regex match, not an exact match.  This
accepts __start_foo and __stop_foo symbols found in a *.c file, if
"foo" was found previously in a vmlinux.lds.h chunk.

It does require that the patch adding "externs in .c's" also have the
additions to vmlinux.lds.h.  And it requires vmlinux.lds.h chunks
before .c chunks.

Cc: apw@xxxxxxxxxxxxx
Cc: joe@xxxxxxxxxxx
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Jim Cromie <jim.cromie@xxxxxxxxx>
---
 scripts/checkpatch.pl | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 880fde13d9b8..6aabcc1f66c1 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -74,6 +74,8 @@ my $git_command ='export LANGUAGE=en_US.UTF-8; git';
 my $tabsize = 8;
 my ${CONFIG_} = "CONFIG_";
 
+my %maybe_linker_symbol; # for externs in c exceptions, when seen in *vmlinux.lds.h
+
 sub help {
 	my ($exitcode) = @_;
 
@@ -6051,6 +6053,9 @@ sub process {
 
 # check for line continuations outside of #defines, preprocessor #, and asm
 
+		} elsif ($realfile =~ m@/vmlinux.lds.h$@) {
+		    $line =~ s/(\w+)/$maybe_linker_symbol{$1}++/ge;
+		    #print "REAL: $realfile\nln: $line\nkeys:", sort keys %maybe_linker_symbol;
 		} else {
 			if ($prevline !~ /^..*\\$/ &&
 			    $line !~ /^\+\s*\#.*\\$/ &&		# preprocessor
@@ -7119,6 +7124,21 @@ sub process {
 				     "arguments for function declarations should follow identifier\n" . $herecurr);
 			}
 
+		} elsif ($realfile =~ /\.c$/ && defined $stat &&
+		    $stat =~ /^\+extern struct\s+(\w+)\s+(\w+)\[\];/)
+		{
+			my ($st_type, $st_name) = ($1, $2);
+
+			for my $s (keys %maybe_linker_symbol) {
+			    #print "Linker symbol? $st_name : $s\n";
+			    goto LIKELY_LINKER_SYMBOL
+				if $st_name =~ /$s/;
+			}
+			WARN("AVOID_EXTERNS",
+			     "found a file-scoped extern type:$st_type name:$st_name in .c file\n"
+			     . "is this a linker symbol ?\n" . $herecurr);
+		  LIKELY_LINKER_SYMBOL:
+
 		} elsif ($realfile =~ /\.c$/ && defined $stat &&
 		    $stat =~ /^.\s*extern\s+/)
 		{
-- 
2.41.0