runtime: fix segmentation fault due to missing argv on musl-linux c-a…

…rchive

This change fixes a segmentation fault that occurs on musl-based systems
when using c-archive (or c-shared with LD_PRELOAD). The issue
was caused by uninitialized argv and envp, which led to crashes at runtime.

This fix ensures that both argv and envp are correctly initialized,
preventing the segmentation fault.

While this fix addresses crashes due to missing argv and envp, it does
not address the error described in issue golang#54805, where a c-shared library
loaded with dlopen triggers an error related initial-exec access to
TLS objects [1] in a dynamically loaded library.

[1]: http://git.musl-libc.org/cgit/musl/commit/?id=5c2f46a214fceeee3c3e41700c51415e0a4f1acd

Fixes golang#13492

src/runtime/cgo.go

@@ -20,6 +20,7 @@ import "unsafe"
 //go:linkname _cgo_pthread_key_created _cgo_pthread_key_created
 //go:linkname _cgo_bindm _cgo_bindm
 //go:linkname _cgo_getstackbound _cgo_getstackbound
+//go:linkname _cgo_is_musl _cgo_is_musl
 
 var (
 	_cgo_init                     unsafe.Pointer
@@ -32,6 +33,7 @@ var (
 	_cgo_pthread_key_created      unsafe.Pointer
 	_cgo_bindm                    unsafe.Pointer
 	_cgo_getstackbound            unsafe.Pointer
+	_cgo_is_musl                  unsafe.Pointer
 )
 
 // iscgo is set to true by the runtime/cgo package

src/runtime/cgo/callbacks.go

@@ -150,3 +150,11 @@ var _cgo_yield unsafe.Pointer
 //go:linkname _cgo_getstackbound _cgo_getstackbound
 var x_cgo_getstackbound byte
 var _cgo_getstackbound = &x_cgo_getstackbound
+
+// x_cgo_is_musl is set to 1 if the C library is musl.
+
+//go:cgo_import_static x_cgo_is_musl
+//go:linkname x_cgo_is_musl x_cgo_is_musl
+//go:linkname _cgo_is_musl _cgo_is_musl
+var x_cgo_is_musl byte
+var _cgo_is_musl = &x_cgo_is_musl

src/runtime/cgo/gcc_libinit.c

@@ -17,6 +17,7 @@
 #include <stdlib.h>
 #include <string.h> // strerror
 #include <time.h>
+#include <limits.h>
 #include "libcgo.h"
 #include "libcgo_unix.h"
 
@@ -179,3 +180,13 @@ pthread_key_destructor(void* g) {
 		x_crosscall2_ptr(NULL, g, 0, 0);
 	}
 }
+
+// x_cgo_is_musl reports whether the C library is musl.
+int
+x_cgo_is_musl() {
+	#if defined(__GLIBC__) || defined(__UCLIBC__)
+		return 0;
+	#else
+		return 1;
+	#endif
+}

src/runtime/os_linux.go

@@ -233,21 +233,25 @@ var auxvreadbuf [128]uintptr
 func sysargs(argc int32, argv **byte) {
 	n := argc + 1
 
-	// skip over argv, envp to get to auxv
-	for argv_index(argv, n) != nil {
-		n++
-	}
+	// auxv on argv is not available on musl library/archive. avoid for musl.
+	if !((islibrary || isarchive) && isMusl()) {
+		// skip over argv, envp to get to auxv
+		for argv_index(argv, n) != nil {
+			n++
+		}
 
-	// skip NULL separator
-	n++
+		// skip NULL separator
+		n++
 
-	// now argv+n is auxv
-	auxvp := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
+		// now argv+n is auxv
+		auxvp := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
 
-	if pairs := sysauxv(auxvp[:]); pairs != 0 {
-		auxv = auxvp[: pairs*2 : pairs*2]
-		return
+		if pairs := sysauxv(auxvp[:]); pairs != 0 {
+			auxv = auxvp[: pairs*2 : pairs*2]
+			return
+		}
 	}
+
 	// In some situations we don't get a loader-provided
 	// auxv, such as when loaded as a library on Android.
 	// Fall back to /proc/self/auxv.

src/runtime/proc.go

@@ -764,6 +764,14 @@ func getGodebugEarly() string {
 		// Similar to goenv_unix but extracts the environment value for
 		// GODEBUG directly.
 		// TODO(moehrmann): remove when general goenvs() can be called before cpuinit()
+		if (isarchive || islibrary) && isMusl() {
+			for _, value := range fetch_from_fd(procEnviron) {
+				if hasPrefix(value, prefix) {
+					return value
+				}
+			}
+		}
+
 		n := int32(0)
 		for argv_index(argv, argc+1+n) != nil {
 			n++

src/runtime/runtime1.go

@@ -25,6 +25,9 @@ const (
 var traceback_cache uint32 = 2 << tracebackShift
 var traceback_env uint32
 
+var procCmdline = []byte("/proc/self/cmdline\x00")
+var procEnviron = []byte("/proc/self/environ\x00")
+
 // gotraceback returns the current traceback settings.
 //
 // If level is 0, suppress all tracebacks.
@@ -56,6 +59,11 @@ var (
 	argv **byte
 )
 
+// isMusl reports whether the Go program is linked with musl libc.
+func isMusl() bool {
+	return asmcgocall(_cgo_is_musl, nil) == 1
+}
+
 // nosplit for use in linux startup sysargs.
 //
 //go:nosplit
@@ -73,13 +81,24 @@ func goargs() {
 	if GOOS == "windows" {
 		return
 	}
+
+	if (isarchive || islibrary) && isMusl() {
+		argslice = fetch_from_fd(procCmdline)
+		return
+	}
+
 	argslice = make([]string, argc)
 	for i := int32(0); i < argc; i++ {
 		argslice[i] = gostringnocopy(argv_index(argv, i))
 	}
 }
 
 func goenvs_unix() {
+	if (isarchive || islibrary) && isMusl() {
+		envs = fetch_from_fd(procEnviron)
+		return
+	}
+
 	// TODO(austin): ppc64 in dynamic linking mode doesn't
 	// guarantee env[] will immediately follow argv. Might cause
 	// problems.
@@ -94,6 +113,37 @@ func goenvs_unix() {
 	}
 }
 
+func fetch_from_fd(path []byte) []string {
+	fd := open(&path[0], 0 /* O_RDONLY */, 0)
+	if fd <= 0 {
+		return nil
+	}
+
+	// Read the file in 16-byte chunks.
+	var data []byte
+	var buf [16]byte
+	for {
+		n := read(fd, noescape(unsafe.Pointer(&buf[0])), int32(unsafe.Sizeof(buf)))
+		if n <= 0 { // EOF
+			break
+		}
+		data = append(data, buf[:n]...)
+	}
+
+	// Parse the data into a slice of strings.
+	var start int
+	var result = make([]string, 0, 8)
+	for i := 0; i < len(data); i++ {
+		if data[i] == 0 { // null-termination
+			result = append(result, gostring(&data[start:i][0]))
+			start = i + 1
+		}
+	}
+
+	closefd(fd)
+	return result
+}
+
 func environ() []string {
 	return envs
 }