Merge tag 'nolibc.2022.07.27a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull nolibc updates from Paul McKenney:
 "This provides nolibc updates, perhaps most notably improved testing
  via the 'cd tools/include/nolibc; make headers' command. This should
  be considered a smoke test. More thorough testing is in the works"

* tag 'nolibc.2022.07.27a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
  tools/nolibc: add a help target to list supported targets
  tools/nolibc: make the default target build the headers
  tools/nolibc: fix the makefile to also work as "make -C tools ..."
  tools/nolibc/stdio: Add format attribute to enable printf warnings
  tools/nolibc/stdlib: Support overflow checking for older compiler versions

tools/Makefile

@@ -78,6 +78,9 @@ bpf/%: FORCE
 libapi: FORCE
 	$(call descend,lib/api)
 
+nolibc: FORCE
+	$(call descend,include/nolibc)
+
 nolibc_%: FORCE
 	$(call descend,include/nolibc,$(patsubst nolibc_%,%,$@))
 

tools/include/nolibc/Makefile

@@ -7,13 +7,46 @@ ifeq ($(srctree),)
 srctree := $(patsubst %/tools/include/,%,$(dir $(CURDIR)))
 endif
 
+# when run as make -C tools/ nolibc_<foo> the arch is not set
+ifeq ($(ARCH),)
+include $(srctree)/scripts/subarch.include
+ARCH = $(SUBARCH)
+endif
+
+# OUTPUT is only set when run from the main makefile, otherwise
+# it defaults to this nolibc directory.
+OUTPUT ?= $(CURDIR)/
+
+ifeq ($(V),1)
+Q=
+else
+Q=@
+endif
+
 nolibc_arch := $(patsubst arm64,aarch64,$(ARCH))
 arch_file := arch-$(nolibc_arch).h
 all_files := ctype.h errno.h nolibc.h signal.h std.h stdio.h stdlib.h string.h \
              sys.h time.h types.h unistd.h
 
 # install all headers needed to support a bare-metal compiler
-all:
+all: headers
+
+install: help
+
+help:
+	@echo "Supported targets under nolibc:"
+	@echo "  all                 call \"headers\""
+	@echo "  clean               clean the sysroot"
+	@echo "  headers             prepare a sysroot in tools/include/nolibc/sysroot"
+	@echo "  headers_standalone  like \"headers\", and also install kernel headers"
+	@echo "  help                this help"
+	@echo ""
+	@echo "These targets may also be called from tools as \"make nolibc_<target>\"."
+	@echo ""
+	@echo "Currently using the following variables:"
+	@echo "  ARCH    = $(ARCH)"
+	@echo "  OUTPUT  = $(OUTPUT)"
+	@echo ""
 
 # Note: when ARCH is "x86" we concatenate both x86_64 and i386
 headers:
@@ -36,7 +69,7 @@ headers:
 
 headers_standalone: headers
 	$(Q)$(MAKE) -C $(srctree) headers
-	$(Q)$(MAKE) -C $(srctree) headers_install INSTALL_HDR_PATH=$(OUTPUT)/sysroot
+	$(Q)$(MAKE) -C $(srctree) headers_install INSTALL_HDR_PATH=$(OUTPUT)sysroot
 
 clean:
 	$(call QUIET_CLEAN, nolibc) rm -rf "$(OUTPUT)sysroot"

tools/include/nolibc/stdio.h

@@ -273,7 +273,7 @@ int vfprintf(FILE *stream, const char *fmt, va_list args)
 	return written;
 }
 
-static __attribute__((unused))
+static __attribute__((unused, format(printf, 2, 3)))
 int fprintf(FILE *stream, const char *fmt, ...)
 {
 	va_list args;
@@ -285,7 +285,7 @@ int fprintf(FILE *stream, const char *fmt, ...)
 	return ret;
 }
 
-static __attribute__((unused))
+static __attribute__((unused, format(printf, 1, 2)))
 int printf(const char *fmt, ...)
 {
 	va_list args;

tools/include/nolibc/stdlib.h

@@ -128,10 +128,9 @@ void *malloc(size_t len)
 static __attribute__((unused))
 void *calloc(size_t size, size_t nmemb)
 {
-	void *orig;
-	size_t res = 0;
+	size_t x = size * nmemb;
 
-	if (__builtin_expect(__builtin_mul_overflow(nmemb, size, &res), 0)) {
+	if (__builtin_expect(size && ((x / size) != nmemb), 0)) {
 		SET_ERRNO(ENOMEM);
 		return NULL;
 	}
@@ -140,7 +139,7 @@ void *calloc(size_t size, size_t nmemb)
 	 * No need to zero the heap, the MAP_ANONYMOUS in malloc()
 	 * already does it.
 	 */
-	return malloc(res);
+	return malloc(x);
 }
 
 static __attribute__((unused))