[PATCH v3 02/11] tools/nolibc: add new crt.h with _start_c

[Zhangjin Wu]

As the environ and _auxv support added for nolibc, the assembly _start
function becomes more and more complex and therefore makes the porting
of nolibc to new architectures harder and harder.

To simplify portability, this C version of _start_c() is added to do
most of the assembly start operations in C, which reduces the complexity
a lot and will eventually simplify the porting of nolibc to the new
architectures.

The new _start_c() only requires a stack pointer argument, it will find
argv, envp and _auxv for us, and then call main(), finally, it exit()
with main's return status. With this new _start_c(), the future new
architectures only require to add very few assembly instructions.

As suggested by Thomas, users may use a different signature of main
(e.g. void main(void)), a _nolibc_main alias is added for main to
silence the warning about potential conflicting types.

Suggested-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/90fdd255-32f4-4caf-90ff-06456b53dac3@xxxxxxxx/
Signed-off-by: Zhangjin Wu <falcon@xxxxxxxxxxx>
---
 tools/include/nolibc/Makefile |  1 +
 tools/include/nolibc/crt.h    | 59 +++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 tools/include/nolibc/crt.h

diff --git a/tools/include/nolibc/Makefile b/tools/include/nolibc/Makefile
index 64d67b080744..909b6eb500fe 100644
--- a/tools/include/nolibc/Makefile
+++ b/tools/include/nolibc/Makefile
@@ -27,6 +27,7 @@ nolibc_arch := $(patsubst arm64,aarch64,$(ARCH))
 arch_file := arch-$(nolibc_arch).h
 all_files := \
 		compiler.h \
+		crt.h \
 		ctype.h \
 		errno.h \
 		nolibc.h \
diff --git a/tools/include/nolibc/crt.h b/tools/include/nolibc/crt.h
new file mode 100644
index 000000000000..f9db2389acd2
--- /dev/null
+++ b/tools/include/nolibc/crt.h
@@ -0,0 +1,59 @@
+/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
+/*
+ * C Run Time support for NOLIBC
+ * Copyright (C) 2023 Zhangjin Wu <falcon@xxxxxxxxxxx>
+ */
+
+#ifndef _NOLIBC_CRT_H
+#define _NOLIBC_CRT_H
+
+char **environ __attribute__((weak));
+const unsigned long *_auxv __attribute__((weak));
+
+typedef int (_nolibc_main_fn)(int, char **, char **);
+static void exit(int);
+
+void _start_c(long *sp)
+{
+	int argc, i;
+	char **argv;
+	char **envp;
+	/* silence potential warning: conflicting types for 'main' */
+	_nolibc_main_fn _nolibc_main __asm__ ("main");
+
+	/*
+	 * sp  :  argc          <-- argument count, required by main()
+	 * argv:  argv[0]       <-- argument vector, required by main()
+	 *        argv[1]
+	 *        ...
+	 *        argv[argc-1]
+	 *        null
+	 * envp:  envp[0]       <-- environment variables, required by main() and getenv()
+	 *        envp[1]
+	 *        ...
+	 *        null
+	 * _auxv: auxv[0]       <-- auxiliary vector, required by getauxval()
+	 *        auxv[1]
+	 *        ...
+	 *        null
+	 */
+
+	/* assign argc and argv */
+	argc = sp[0];
+	argv = (void *)(sp + 1);
+
+	/* find envp */
+	envp = argv + argc + 1;
+	environ = envp;
+
+	/* find auxv */
+	i = 0;
+	while (envp[i])
+		i++;
+	_auxv = (void *)(envp + i + 1);
+
+	/* go to application */
+	exit(_nolibc_main(argc, argv, envp));
+}
+
+#endif /* _NOLIBC_CRT_H */
-- 
2.25.1