better libc detection, use DT_TEXTREL when possible instead of making a segment RWX

manip-exe.h

@@ -6,9 +6,9 @@
 #include <elf.h>
 #include <link.h>
 
-#if (sizeof(ElfW(Ehdr)) != sizeof(Elf64_Ehdr))
+/*#if sizeof(ElfW(Ehdr)) != sizeof(Elf64_Ehdr)
 #error "Need 64-bit ELF for now, sorry"
-#endif
+#endif*/
 
 static bool manip_elf(ElfW(Ehdr)* elf, const char* symname) {
 	if (elf->e_ident[0] != ELFMAG0 || elf->e_ident[1] != ELFMAG1
@@ -33,13 +33,23 @@ static bool manip_elf(ElfW(Ehdr)* elf, const char* symname) {
 
 	ElfW(Phdr)* phdr = (ElfW(Phdr)*)(elf->e_phoff + (size_t)elf), *ph_loadexec;
 	ElfW(Dyn)* dyn = NULL;
+	bool textrel = false;
+	enum {
+		libc_unk, libc_g /* glibc */, libc_musl, libc_uc /* uclibc */
+	} libc = libc_unk;
+	// first, try to determine the libc used
 	for (size_t i = 0; i < elf->e_phnum;
 			++i, phdr = (ElfW(Phdr)*)((size_t)phdr + elf->e_phentsize)) {
-		if (phdr->p_type == PT_LOAD) {
+		if (phdr->p_type == PT_INTERP) {
-			// need this at runtime for the relocs to work (yeah, sorry)
+			const char* str = (char*)elf + phdr->p_offset;
-			if (phdr->p_flags & PF_X) {
+			if (!strcmp(str, "/lib64/ld-linux-x86-64.so.2")) {
-				ph_loadexec = phdr;
+				libc = libc_g;
-				phdr->p_flags |= PF_W;
+				printf("\tglibc\n");
+			} else if (!strcmp(str, "/lib/ld-musl-x86_64.so.1")) {
+				libc = libc_musl;
+				printf("\tmusl\n");
+			} else {
+				printf("\tunknown libc: '%s'\n", str);
 			}
 		}
 
@@ -49,6 +59,55 @@ static bool manip_elf(ElfW(Ehdr)* elf, const char* symname) {
 		}
 	}
 
+	if (libc == libc_unk) {
+		ElfW(Dyn)* dd = dyn;
+		const char* ddstr = NULL;
+		for (size_t i = 0; dd->d_tag != DT_NULL; ++i, ++dd) {
+			if (dd->d_tag == DT_STRTAB)
+				ddstr = (const char*)((size_t)elf + dd->d_un.d_ptr);
+		}
+		dd = dyn;
+		for (size_t i = 0; dd->d_tag != DT_NULL; ++i, ++dd) {
+			if (dd->d_tag == DT_NEEDED) {
+				const char* s = ddstr + dd->d_un.d_val;
+
+				if (strstr(s, "libc.so") != s) continue;
+
+				if (!strcmp(s, "libc.so.6")) {
+					libc = libc_g;
+					printf("\tglibc\n");
+					break;
+				} else if (!strcmp(s, "libc.so.0")) {
+					libc = libc_uc;
+					printf("\tuclibc\n");
+					break;
+				} else if (!strcmp(s, "libc.so")) {
+					libc = libc_musl;
+					printf("\tmusl\n");
+					break;
+				} else {
+					printf("\tunknown libc: '%s'\n", s);
+				}
+			}
+		}
+	}
+
+	phdr = (ElfW(Phdr)*)(elf->e_phoff + (size_t)elf);
+	for (size_t i = 0; i < elf->e_phnum;
+			++i, phdr = (ElfW(Phdr)*)((size_t)phdr + elf->e_phentsize)) {
+		if (phdr->p_type == PT_LOAD) {
+			// need this at runtime for the relocs to work (yeah, sorry)
+			if (phdr->p_flags & PF_X) {
+				ph_loadexec = phdr;
+				if (libc == libc_g || libc == libc_uc) {
+					textrel = true;
+				} else {
+					phdr->p_flags |= PF_W;
+				}
+			}
+		}
+	}
+
 	if (dyn == NULL) {
 		fprintf(stderr, "No DYNAMIC table! bailing out...\n");
 		return true;
@@ -63,6 +122,7 @@ static bool manip_elf(ElfW(Ehdr)* elf, const char* symname) {
 	ElfW(Addr) relaoff = 0;
 
 	for (size_t i = 0; dyn->d_tag != DT_NULL; ++i, ++dyn) {
+		if (dyn->d_tag == DT_TEXTREL) textrel = false;
 		if (dyn->d_tag == DT_RELA) {
 			relatab = (ElfW(Rela)*)((size_t)elf + dyn->d_un.d_ptr);
 			relaoff = dyn->d_un.d_ptr;
@@ -80,6 +140,10 @@ static bool manip_elf(ElfW(Ehdr)* elf, const char* symname) {
 		if (dyn->d_tag == DT_SYMENT) syment = dyn->d_un.d_val;
 		if (dyn->d_tag == DT_STRSZ ) strsz  = dyn->d_un.d_val;
 	}
+	if (textrel) {
+		dyn->d_tag = DT_TEXTREL; // need this one
+		dyn[1].d_tag = DT_NULL;
+	}
 
 	if (!relasz || !relaent || !relatab) {
 		fprintf(stderr, "No RELA/RELASZ/RELAENT in DYN!\n");