tools/nolibc: export environ as a weak symbol on arm

The environ is retrieved from the _start code and is easy to store at
this moment. Let's declare the variable weak and store the value into
it. By not being static it will be visible to all units. By being weak,
if some programs already declared it, they will continue to be able to
use it. This was tested in arm and thumb1 and thumb2 modes, and for each
mode, both with environ inherited from _start and extracted from envp.

Signed-off-by: Willy Tarreau <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: wtarreau

tools/include/nolibc/arch-arm.h

@@ -196,6 +196,8 @@ struct sys_stat_struct {
 	_arg1;                                                                \
 })
 
+char **environ __attribute__((weak));
+
 /* startup code */
 void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
 {
@@ -206,6 +208,8 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
 		"add %r2, %r0, $1\n"          // envp = (argc + 1) ...
 		"lsl %r2, %r2, $2\n"          //        * 4        ...
 		"add %r2, %r2, %r1\n"         //        + argv
+		"ldr %r3, 1f\n"               // r3 = &environ (see below)
+		"str %r2, [r3]\n"             // store envp into environ
 
 		"mov %r3, $8\n"               // AAPCS : sp must be 8-byte aligned in the
 		"neg %r3, %r3\n"              //         callee, and bl doesn't push (lr=pc)
@@ -215,7 +219,10 @@ void __attribute__((weak,noreturn,optimize("omit-frame-pointer"))) _start(void)
 		"bl main\n"                   // main() returns the status code, we'll exit with it.
 		"movs r7, $1\n"               // NR_exit == 1
 		"svc $0x00\n"
-	  );
+		".align 2\n"                  // below are the pointers to a few variables
+		"1:\n"
+		".word environ\n"
+	);
 	__builtin_unreachable();
 }