checkpatch: add warning for avoiding .L prefix symbols in assembly files

objtool requires that all code must be contained in an ELF symbol.  Symbol
names that have a '.L' prefix do not emit symbol table entries, as they
have special meaning for the assembler.

'.L' prefixed symbols can be used within a code region, but should be
avoided for denoting a range of code via 'SYM_*_START/END' annotations.

Add a new check to emit a warning on finding the usage of '.L' symbols for
'.S' files, if it denotes range of code via SYM_*_START/END annotation
pair.

Link: https://lkml.kernel.org/r/20210123190459.9701-1-yashsri421@gmail.com
Link: https://lore.kernel.org/lkml/20210112210154.GI4646@sirena.org.ukSigned-off-by: Aditya Srivastava <yashsri421@gmail.com>
Suggested-by: Mark Brown <broonie@kernel.org>
Acked-by: Joe Perches <joe@perches.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Aditya Srivastava <yashsri421@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -3599,6 +3599,13 @@ sub process {
 			}
 		}
 
+# check for .L prefix local symbols in .S files
+		if ($realfile =~ /\.S$/ &&
+		    $line =~ /^\+\s*(?:[A-Z]+_)?SYM_[A-Z]+_(?:START|END)(?:_[A-Z_]+)?\s*\(\s*\.L/) {
+			WARN("AVOID_L_PREFIX",
+			     "Avoid using '.L' prefixed local symbol names for denoting a range of code via 'SYM_*_START/END' annotations; see Documentation/asm-annotations.rst\n" . $herecurr);
+		}
+
 # check we are in a valid source file C or perl if not then ignore this hunk
 		next if ($realfile !~ /\.(h|c|pl|dtsi|dts)$/);