[PATCH v2] Documentation/checkpatch: Prefer str_has_prefix over strncmp

[Chuhong Yuan]

Add "strncmp() on string prefix" to
Documentation/process/deprecated.rst since using strncmp()
to check whether a string starts with a prefix is error-prone.
The safe replacement is str_has_prefix().

Also add check to the newly introduced deprecated_string_apis
in checkpatch.pl.

This patch depends on patch:
"Documentation/checkpatch: Prefer stracpy/strscpy over
strcpy/strlcpy/strncpy."

Signed-off-by: Chuhong Yuan <hslester96@xxxxxxxxx>
---
Changes in v2:
  - Use "strncmp() on string prefix" instead of
    "strncmp()" to make it more precise.
  - Remove "c:func" and use "strncmp" directly
    in description.

 Documentation/process/deprecated.rst | 8 ++++++++
 scripts/checkpatch.pl                | 1 +
 2 files changed, 9 insertions(+)

diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index 56280e108d5a..96fa32aba189 100644
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -109,6 +109,14 @@ the given limit of bytes to copy. This is inefficient and can lead to
 linear read overflows if a source string is not NUL-terminated. The
 safe replacement is stracpy() or strscpy().
 
+strncmp() on string prefix
+--------------------------
+strncmp() is often used to test if a string starts with a prefix by
+strncmp(str, prefix, length of prefix). This is error-prone because length
+of prefix can have counting error if using a constant length, or use
+sizeof(prefix) without - 1. Also, if the prefix is a pointer, sizeof(prefix)
+leads to a wrong size. The safe replacement is str_has_prefix().
+
 Variable Length Arrays (VLAs)
 -----------------------------
 Using stack VLAs produces much worse machine code than statically
diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 0ae9ae01d855..38e82d2ac286 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -609,6 +609,7 @@ our %deprecated_string_apis = (
 	"strcpy"		=> "stracpy or strscpy",
 	"strlcpy"		=> "stracpy or strscpy",
 	"strncpy"		=> "stracpy or strscpy - for non-NUL-terminated uses, strncpy dest should be __nonstring",
+	"strncmp"		=> "str_has_prefix",
 );
 
 #Create a search pattern for all these strings apis to speed up a loop below
-- 
2.20.1