[PATCH v2] checkpatch: add warning for avoiding .L prefix symbols in assembly files

[Aditya Srivastava <yashsri421@gmail.com>]

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 in arch/x86/entry/* and arch/x86/lib/*, if it denotes
range of code via SYM_*_START/END annotation pair.

Suggested-by: Mark Brown <broonie@kernel.org>
Link: https://groups.google.com/g/clang-built-linux/c/-drkmLgu-cU/m/4staOlf-CgAJ
Signed-off-by: Aditya Srivastava <yashsri421@gmail.com>
---
* Applies perfectly on next-20210122

Changes in v2:
- Reduce the check to only SYM_*_START/END lines
- Reduce the check for only .S files in arch/x86/entry/* and arch/x86/lib/* as suggested by Josh and Nick
- Modify commit message

 scripts/checkpatch.pl | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 7030c4d6d126..e36cdf96dfe3 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -3590,6 +3590,13 @@ sub process {
 			}
 		}
 
+# check for .L prefix local symbols in .S files
+		if ($realfile =~ m@^arch/x86/(?:entry|lib)/.*\.S$@ &&
+		    $line =~ /^\+\s*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)$/);
 
-- 
2.17.1