ld: move interpreter string into first block of ELF file

necessary on freebsd.

R=r, dho
CC=golang-dev
https://golang.org/cl/157069

src/cmd/6l/asm.c

@@ -315,19 +315,6 @@ doelf(void)
 		elfstr[ElfStrDynstr] = addstring(shstrtab, ".dynstr");
 		elfstr[ElfStrRela] = addstring(shstrtab, ".rela");
 
-		/* interpreter string */
-		s = lookup(".interp", 0);
-		s->reachable = 1;
-		s->type = SDATA;	// TODO: rodata
-		switch(HEADTYPE) {
-		case 7:
-			addstring(lookup(".interp", 0), linuxdynld);
-			break;
-		case 9:
-			addstring(lookup(".interp", 0), freebsddynld);
-			break;
-		}
-
 		/*
 		 * hash table.
 		 * only entries that other objects need to find when
@@ -688,7 +675,14 @@ asmb(void)
 			sh->type = SHT_PROGBITS;
 			sh->flags = SHF_ALLOC;
 			sh->addralign = 1;
-			shsym(sh, lookup(".interp", 0));
+			switch(HEADTYPE) {
+			case 7:
+				elfinterp(sh, startva, linuxdynld);
+				break;
+			case 9:
+				elfinterp(sh, startva, freebsddynld);
+				break;
+			}
 
 			ph = newElfPhdr();
 			ph->type = PT_INTERP;
@@ -903,7 +897,8 @@ asmb(void)
 		a += elfwritehdr();
 		a += elfwritephdrs();
 		a += elfwriteshdrs();
-		if (a > ELFRESERVE)
+		cflush();
+		if(a+elfwriteinterp() > ELFRESERVE)
 			diag("ELFRESERVE too small: %d > %d", a, ELFRESERVE);
 		break;
 	}

src/cmd/8l/asm.c

@@ -304,14 +304,6 @@ doelf(void)
 		s = lookup(".interp", 0);
 		s->reachable = 1;
 		s->type = SDATA;	// TODO: rodata
-		switch(HEADTYPE) {
-		case 7:
-			addstring(lookup(".interp", 0), linuxdynld);
-			break;
-		case 9:
-			addstring(lookup(".interp", 0), freebsddynld);
-			break;
-		}
 
 		/*
 		 * hash table - empty for now.
@@ -794,7 +786,14 @@ asmb(void)
 			sh->type = SHT_PROGBITS;
 			sh->flags = SHF_ALLOC;
 			sh->addralign = 1;
-			shsym(sh, lookup(".interp", 0));
+			switch(HEADTYPE) {
+			case 7:
+				elfinterp(sh, startva, linuxdynld);
+				break;
+			case 9:
+				elfinterp(sh, startva, freebsddynld);
+				break;
+			}
 
 			ph = newElfPhdr();
 			ph->type = PT_INTERP;
@@ -1012,10 +1011,9 @@ asmb(void)
 		a += elfwritehdr();
 		a += elfwritephdrs();
 		a += elfwriteshdrs();
-		if (a > ELFRESERVE) {
-			diag("ELFRESERVE too small: %d > %d", a, ELFRESERVE);
-		}
 		cflush();
+		if(a+elfwriteinterp() > ELFRESERVE)
+			diag("ELFRESERVE too small: %d > %d", a, ELFRESERVE);
 		break;
 	}
 	cflush();

src/cmd/ld/elf.c

@@ -17,6 +17,7 @@ static	int	elf64;
 static	ElfEhdr	hdr;
 static	ElfPhdr	*phdr[NSECT];
 static	ElfShdr	*shdr[NSECT];
+static	char	*interp;
 
 /*
  Initialize the global variable that describes the ELF header. It will be updated as
@@ -278,3 +279,29 @@ elfwritedynentsymsize(Sym *s, int tag, Sym *t)
 		adduint32(s, tag);
 	addsize(s, t);
 }
+
+int
+elfwriteinterp(void)
+{
+	int n;
+	
+	if(interp == nil)
+		return 0;
+
+	n = strlen(interp)+1;
+	seek(cout, ELFRESERVE-n, 0);
+	write(cout, interp, n);
+	return n;
+}
+
+void
+elfinterp(ElfShdr *sh, uint64 startva, char *p)
+{
+	int n;
+	
+	interp = p;
+	n = strlen(interp)+1;
+	sh->addr = startva + ELFRESERVE - n;
+	sh->off = ELFRESERVE - n;
+	sh->size = n;
+}

src/cmd/ld/elf.h

@@ -961,11 +961,14 @@ uint64	startelf(void);
 uint64	endelf(void);
 extern	int	numelfphdr;
 extern	int	numelfshdr;
+int	elfwriteinterp(void);
+void	elfinterp(ElfShdr*, uint64, char*);
 
 /*
  * Total amount of space to reserve at the start of the file
- * for Header, PHeaders, and SHeaders.
+ * for Header, PHeaders, SHeaders, and interp.
  * May waste some.
+ * On FreeBSD, cannot be larger than a page.
  */
 #define	ELFRESERVE	2048