[PATCH v3] um: drivers: call kernel_strrchr() explicitly in cow_user.c

[Michael Bommarito <michael.bommarito@gmail.com>]

Building ARCH=um on glibc >= 2.43 fails:

  arch/um/drivers/cow_user.c: error: implicit declaration of
  function 'strrchr' [-Wimplicit-function-declaration]

glibc 2.43's C23 const-preserving strrchr() macro does not survive
UML's global -Dstrrchr=kernel_strrchr remap from arch/um/Makefile.
Call kernel_strrchr() directly in cow_user.c so the source no longer
depends on the -D rewrite.

Fixes: 2c51a4bc0233 ("um: fix strrchr() problems")
Suggested-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Assisted-by: Codex:gpt-5-4
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
v2: https://lore.kernel.org/all/20260407181528.879358-1-michael.bommarito@gmail.com/

Changes since v2: trim commit message per review.

 arch/um/drivers/cow_user.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/um/drivers/cow_user.c b/arch/um/drivers/cow_user.c
index 29b46581ddd1..ec8e6121b402 100644
--- a/arch/um/drivers/cow_user.c
+++ b/arch/um/drivers/cow_user.c
@@ -15,6 +15,12 @@
 #include "cow.h"
 #include "cow_sys.h"

+/*
+ * arch/um/Makefile remaps strrchr to kernel_strrchr; call the kernel
+ * name directly to avoid glibc >= 2.43's C23 strrchr macro.
+ */
+extern char *kernel_strrchr(const char *, int);
+
 #define PATH_LEN_V1 256

 /* unsigned time_t works until year 2106 */
@@ -153,7 +159,7 @@ static int absolutize(char *to, int size, char *from)
 			   errno);
 		return -1;
 	}
-	slash = strrchr(from, '/');
+	slash = kernel_strrchr(from, '/');
 	if (slash != NULL) {
 		*slash = '\0';
 		if (chdir(from)) {
--
2.49.0