cmd/cgo, cmd/cc, cmd/ld: detect dynamic linker automatically

Some newer Linux distributions (Ubuntu ARM at least) use a new multiarch
directory organization, where dynamic linker is no longer in the hardcoded
path in our linker.
For example, Ubuntu 12.04 ARM hardfloat places its dynamic linker at
/lib/arm-linux-gnueabihf/ld-linux.so.3

Ref: http://lackof.org/taggart/hacking/multiarch/

Also, to support Debian GNU/kFreeBSD as a FreeBSD variant, we need this capability, so it's part of issue 3533.

This CL add a new pragma (#pragma dynlinker "path") to cc.

R=iant, rsc
CC=golang-dev
https://golang.org/cl/6086043

src/cmd/5c/swt.c

@@ -405,6 +405,10 @@ outcode(void)
 		Bprint(&outbuf, "\n$$  // dynexport\n");
 		for(i=0; i<ndynexp; i++)
 			Bprint(&outbuf, "dynexport %s %s\n", dynexp[i].local, dynexp[i].remote);
+		Bprint(&outbuf, "\n$$  // dynlinker\n");
+		if(dynlinker != nil) {
+			Bprint(&outbuf, "dynlinker %s\n", dynlinker);
+		}
 		Bprint(&outbuf, "\n$$\n\n");
 	}
 	Bprint(&outbuf, "!\n");

src/cmd/5l/obj.c

@@ -106,6 +106,7 @@ main(int argc, char *argv[])
 		INITENTRY = EARGF(usage());
 		break;
 	case 'I':
+		debug['I'] = 1; // denote cmdline interpreter override
 		interpreter = EARGF(usage());
 		break;
 	case 'L':

src/cmd/6c/swt.c

@@ -259,6 +259,10 @@ outcode(void)
 		Bprint(&b, "\n$$  // dynexport\n");
 		for(i=0; i<ndynexp; i++)
 			Bprint(&b, "dynexport %s %s\n", dynexp[i].local, dynexp[i].remote);
+		Bprint(&b, "\n$$  // dynlinker\n");
+		if(dynlinker != nil) {
+			Bprint(&b, "dynlinker %s\n", dynlinker);
+		}
 		Bprint(&b, "\n$$\n\n");
 	}
 	Bprint(&b, "!\n");

src/cmd/6l/obj.c

@@ -114,6 +114,7 @@ main(int argc, char *argv[])
 		HEADTYPE = headtype(EARGF(usage()));
 		break;
 	case 'I':
+		debug['I'] = 1; // denote cmdline interpreter override
 		interpreter = EARGF(usage());
 		break;
 	case 'L':

src/cmd/8c/swt.c

@@ -263,6 +263,10 @@ outcode(void)
 		Bprint(&b, "\n$$  // dynexport\n");
 		for(i=0; i<ndynexp; i++)
 			Bprint(&b, "dynexport %s %s\n", dynexp[i].local, dynexp[i].remote);
+		Bprint(&b, "\n$$  // dynlinker\n");
+		if(dynlinker != nil) {
+			Bprint(&b, "dynlinker %s\n", dynlinker);
+		}
 		Bprint(&b, "\n$$\n\n");
 	}
 	Bprint(&b, "!\n");

src/cmd/8l/obj.c

@@ -119,6 +119,7 @@ main(int argc, char *argv[])
 		HEADTYPE = headtype(EARGF(usage()));
 		break;
 	case 'I':
+		debug['I'] = 1; // denote cmdline interpreter override
 		interpreter = EARGF(usage());
 		break;
 	case 'L':

src/cmd/cc/cc.h

@@ -774,6 +774,8 @@ void	pragtextflag(void);
 void	pragincomplete(void);
 void	pragdynimport(void);
 void	pragdynexport(void);
+void	pragdynlinker(void);
+EXTERN	char *dynlinker;
 
 /*
  * calls to machine depend part

src/cmd/cc/dpchk.c

@@ -725,3 +725,20 @@ out:
 	while(getnsc() != '\n')
 		;
 }
+
+void
+pragdynlinker(void)
+{
+	dynlinker = getquoted();
+	if(dynlinker == nil)
+		goto err;
+
+	goto out;
+
+err:
+	yyerror("usage: #pragma dynlinker \"path\"");
+
+out:
+	while(getnsc() != '\n')
+		;
+}

src/cmd/cc/lexbody

@@ -60,6 +60,13 @@ pragdynexport(void)
 		;
 }
 
+void
+pragdynlinker(void)
+{
+	while(getnsc() != '\n')
+		;
+}
+
 void
 pragfpround(void)
 {

src/cmd/cc/macbody

@@ -751,6 +751,10 @@ macprag(void)
 		pragdynexport();
 		return;
 	}
+	if(s && strcmp(s->name, "dynlinker") == 0) {
+		pragdynlinker();
+		return;
+	}
 	while(getnsc() != '\n')
 		;
 	return;

src/cmd/cgo/out.go

@@ -128,6 +128,12 @@ func dynimport(obj string) {
 	}
 
 	if f, err := elf.Open(obj); err == nil {
+		if sec := f.Section(".interp"); sec != nil {
+			if data, err := sec.Data(); err == nil && len(data) > 1 {
+				// skip trailing \0 in data
+				fmt.Fprintf(stdout, "#pragma dynlinker %q\n", string(data[:len(data)-1]))
+			}
+		}
 		sym, err := f.ImportedSymbols()
 		if err != nil {
 			fatalf("cannot load imported symbols from ELF file %s: %v", obj, err)

src/cmd/ld/go.c

@@ -69,6 +69,7 @@ ilookup(char *name)
 static void loadpkgdata(char*, char*, char*, int);
 static void loaddynimport(char*, char*, char*, int);
 static void loaddynexport(char*, char*, char*, int);
+static void loaddynlinker(char*, char*, char*, int);
 static int parsemethod(char**, char*, char**);
 static int parsepkgdata(char*, char*, char**, char*, char**, char**, char**);
 
@@ -204,7 +205,7 @@ ldpkg(Biobuf *f, char *pkg, int64 len, char *filename, int whence)
 	if(p0 != nil) {
 		p0 = strchr(p0+1, '\n');
 		if(p0 == nil) {
-			fprint(2, "%s: found $$ // dynexporg but no newline in %s\n", argv0, filename);
+			fprint(2, "%s: found $$ // dynexport but no newline in %s\n", argv0, filename);
 			if(debug['u'])
 				errorexit();
 			return;
@@ -213,13 +214,34 @@ ldpkg(Biobuf *f, char *pkg, int64 len, char *filename, int whence)
 		if(p1 == nil)
 			p1 = strstr(p0, "\n!\n");
 		if(p1 == nil) {
-			fprint(2, "%s: cannot find end of // dynexporg section in %s\n", argv0, filename);
+			fprint(2, "%s: cannot find end of // dynexport section in %s\n", argv0, filename);
 			if(debug['u'])
 				errorexit();
 			return;
 		}
 		loaddynexport(filename, pkg, p0 + 1, p1 - (p0+1));
 	}
+
+	p0 = strstr(p1, "\n$$  // dynlinker");
+	if(p0 != nil) {
+		p0 = strchr(p0+1, '\n');
+		if(p0 == nil) {
+			fprint(2, "%s: found $$ // dynlinker but no newline in %s\n", argv0, filename);
+			if(debug['u'])
+				errorexit();
+			return;
+		}
+		p1 = strstr(p0, "\n$$");
+		if(p1 == nil)
+			p1 = strstr(p0, "\n!\n");
+		if(p1 == nil) {
+			fprint(2, "%s: cannot find end of // dynlinker section in %s\n", argv0, filename);
+			if(debug['u'])
+				errorexit();
+			return;
+		}
+		loaddynlinker(filename, pkg, p0 + 1, p1 - (p0+1));
+	}
 }
 
 static void
@@ -551,6 +573,44 @@ err:
 	nerrors++;
 }
 
+static void
+loaddynlinker(char *file, char *pkg, char *p, int n)
+{
+	char *pend, *next, *dynlinker, *p0;
+
+	USED(file);
+	pend = p + n;
+	for(; p<pend; p=next) {
+		next = strchr(p, '\n');
+		if(next == nil)
+			next = "";
+		else
+			*next++ = '\0';
+		p0 = p;
+		if(strncmp(p, "dynlinker ", 10) != 0)
+			goto err;
+		p += 10;
+		dynlinker = p;
+
+		if(*dynlinker == '\0')
+			goto err;
+		if(!debug['I']) { // not overrided by cmdline
+			if(interpreter != nil && strcmp(interpreter, dynlinker) != 0) {
+				fprint(2, "%s: conflict dynlinker: %s and %s\n", argv0, interpreter, dynlinker);
+				nerrors++;
+				return;
+			}
+			free(interpreter);
+			interpreter = strdup(dynlinker);
+		}
+	}
+	return;
+
+err:
+	fprint(2, "%s: invalid dynlinker line: %s\n", argv0, p0);
+	nerrors++;
+}
+
 static int markdepth;
 
 static void