Re: [PATCH] treewide: remove current_text_addr

["H. Peter Anvin" <hpa@xxxxxxxxx>]

Here is a full-blown (user space) test program demonstrating the whole
technique and how to use it.

	-hpa

#include <stddef.h>
#include <string.h>

#define _RET_IP_ ((unsigned long)__builtin_return_address(0))
#define noinline __attribute__((noinline))
#define used __attribute__((used))
/* __always_inline is defined in glibc already */
#define ifconst(x,y)	__builtin_choose_expr(__builtin_constant_p(x),(x),(y))
static inline void notailcall(void)
{
	asm volatile("");
}

/* Change this to a null string to make all functions global */
#define STATIC static

struct myputs_string {
	unsigned short len;
	char str[0];
};

STATIC int _myputs_struct(const struct myputs_string * const strs);
STATIC int _myputs_string(const char *str);
STATIC int __myputs(unsigned long ip, const char *str, size_t len);

#if 1

#include <stdio.h>

STATIC void dump_caller(unsigned long where)
{
	const char *opname = NULL;
	const char *wheretoname = NULL;
	char ichar;
	unsigned long whereto = 0;

#if defined(__i386__) || defined(__x86_64__)
	char opname_buf[4];
	unsigned char opcode;
	
	where -= 5;
	opcode = *(unsigned char *)where;

	switch (opcode) {
	case 0xe8:
		opname = "call";
		whereto = where + 5 + *(signed int *)(where + 1);
		break;
	case 0xe9:
		opname = "jmp";
		whereto = where + 5 + *(signed int *)(where + 1);
		break;
	default:
		snprintf(opname_buf, sizeof opname_buf, "?%02x", opcode);
		opname = opname_buf;
		break;
	}

#elif defined(__sparc__)
	const char regtype[4] = "gilo";
	unsigned int opcode, op1, op3, ibit;
	signed int simm13, simm30;
	char opname_buf[32];
	char *p;

	where -= 8;
	
	opcode = *(signed int *)where;
	op1 = opcode >> 30;
	op3 = (opcode >> 19) & 0x3f;
	ibit = (opcode >> 13) & 1;
	simm13 = (opcode & 0x1fff) << 2;
	simm30 = (opcode & 0x3fffffff) << 2;

	opname = opname_buf;
	
	if (op1 == 1) {
		opname = "call";
		whereto = where + simm30;
	} else if (op1 == 2 && op3 == 0x38) {
		if (ibit) {
			snprintf(opname_buf, sizeof opname_buf,
				 "jmpl %%%c%u %c 0x%x",
				 regtype[(opcode >> 17) & 3],
				 (opcode >> 14) & 7,
				 simm13 < 0 ? '-' : '+',
				 abs(simm13));
		} else {
			snprintf(opname_buf, sizeof opname_buf,
				 "jmpl %%%c%u + %%%c%u",
				 regtype[(opcode >> 17) & 3],
				 (opcode >> 14) & 7,
				 regtype[(opcode >> 3) & 3],
				 opcode & 7);
		}
	} else {
		snprintf(opname_buf, sizeof opname_buf,
			 "?0x08x", opcode);
	}
#else
	/* Unknown architecture */
#endif
	if (whereto == (unsigned long)_myputs_struct) {
		wheretoname = "_myputs_struct";
	} else if (whereto == (unsigned long)_myputs_string) {
		wheretoname = "_myputs_string";
	} else {
		wheretoname = "?";
	}

	ichar = '[';
	
	if (opname) {
		printf("%c%p: %s",
		       ichar, (void *)where, opname);
		ichar = ' ';
	}
	if (whereto) {
		printf("%c%p <%s>", ichar, (void *)whereto, wheretoname);
		ichar = ' ';
	}
	if (ichar != '[')
		putchar(']');
}
	
STATIC int __myputs(unsigned long where, const char *str, size_t len)
{
	size_t slen = strlen(str);
	size_t rv;
	
	len--;
	rv = printf("%p: \"%.*s\"%*s", (void *)where, (int)len, str,
		    16-(int)slen, "");
	dump_caller(where);
	if (slen != len)
		printf(" <err: strlen = %zu, len = %zu>\n", slen, len);
	else
		printf(" <ok: len = %zu>\n", len);
	
	return rv;
}

STATIC int noinline _myputs_struct(const struct myputs_string * const strs)
{
	return __myputs(_RET_IP_, strs->str, strs->len);
}

STATIC int noinline _myputs_string(const char *str)
{
	return __myputs(_RET_IP_, str, strlen(str)+1);
}
#endif

#define myputs(s)							\
({									\
	int _rv;							\
	if (__builtin_constant_p(s) &&					\
	    __builtin_constant_p(strlen(s)) &&				\
	    strlen(s)+1 == sizeof(s) &&					\
	    sizeof(s) <= (size_t)65535) {				\
	static const struct {						\
		struct myputs_string _mps_hdr;				\
		char _mps_str[sizeof(s)];				\
	} _mps = {							\
		._mps_hdr.len = sizeof(s),				\
		._mps_str = ifconst(s,""),				\
	};								\
		_rv = _myputs_struct(&_mps._mps_hdr);			\
	} else {							\
		_rv = _myputs_string(s);				\
	}								\
	notailcall();							\
	_rv;								\
})

STATIC int test1(void);
STATIC int test2(const char *strx);

STATIC int test1(void)
{
	return myputs("Foobar");
}

STATIC int test2(const char *strx)
{
	return myputs(strx);
}

int main(int argc, char *argv[])
{
	(void)argc;

	test1();
	test2(argv[0]);
	return 0;
}