From 9a41e57c2edeb32ae3cd530e761133883d491075 Mon Sep 17 00:00:00 2001
From: dzwdz
Date: Mon, 18 Jul 2022 18:06:20 +0200
Subject: user: basic elf relocations, PIE support

---
 src/usertestelf.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 63 insertions(+), 1 deletion(-)

(limited to 'src/usertestelf.c')

diff --git a/src/usertestelf.c b/src/usertestelf.c
index 19f0096..c3ab289 100644
--- a/src/usertestelf.c
+++ b/src/usertestelf.c
@@ -1,6 +1,68 @@
+#include <shared/printf.h>
+#include <user/lib/elf.h>
 #include <user/lib/syscall.c>
 
+const char *str = "Hello!\n", *str2 = "World.\n";
+
+__attribute__((visibility("hidden")))
+extern struct Elf64_Dyn _DYNAMIC[];
+
+__attribute__((visibility("hidden")))
+extern char _image_base;
+
+static void printf_backend(void *arg, const char *buf, size_t len) {
+	(void)arg;
+	_syscall_write(1, buf, len, -1);
+}
+
+int printf(const char *fmt, ...) {
+	int ret = 0;
+	va_list argp;
+	va_start(argp, fmt);
+	ret = __printf_internal(fmt, argp, printf_backend, 0);
+	va_end(argp);
+	return ret;
+}
+
+
+static struct Elf64_Dyn *dyn_gettag(Elf64_Xword tag) {
+	for (size_t i = 0;; i++) {
+		if (_DYNAMIC[i].d_tag == tag) return &_DYNAMIC[i];
+		if (_DYNAMIC[i].d_tag == DT_NULL) return NULL;
+	}
+}
+
+static void reloc(void) {
+	printf("hi?\n");
+	// TODO DT_REL
+	if (dyn_gettag(DT_PLTGOT) || dyn_gettag(DT_JMPREL)) {
+		printf("elf: unimplemented tag in _DYNAMIC\n");
+	}
+
+	struct Elf64_Dyn *rela_tag = dyn_gettag(DT_RELA);
+	if (rela_tag) {
+		/* not checking pointer validity,
+		 * crashing on an invalid elf is fine */
+		size_t relasz = dyn_gettag(DT_RELASZ)->d_val;
+		size_t relaent = dyn_gettag(DT_RELAENT)->d_val;
+		for (size_t o = 0; o < relasz; o += relaent) {
+			struct Elf64_Rela *r = &_image_base + rela_tag->d_ptr + o;
+			uintptr_t *target = &_image_base + r->r_offset;
+
+			switch (ELF64_R_TYPE(r->r_info)) {
+				case R_X86_64_RELATIVE:
+					*target = &_image_base + r->r_addend;
+					break;
+				default:
+					printf("elf: unsupported relocation type\n");
+			}
+		}
+	}
+}
+
 int main(void) {
-	_syscall_write(1, "Hello!", 6, 0);
+	reloc();
+	printf(str);
+	printf(str2);
 	_syscall_exit(0);
 }
-- 
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