runtime: expose auxv for use by x/sys/cpu

Updates #57336

Change-Id: I181885f59bac59360b855d3990326ea2b268bd28
Reviewed-on: https://go-review.googlesource.com/c/go/+/458256
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os3_solaris.go

@@ -601,8 +601,9 @@ func sysargs(argc int32, argv **byte) {
 	n++
 
 	// now argv+n is auxv
-	auxv := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
-	sysauxv(auxv[:])
+	auxvp := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
+	pairs := sysauxv(auxvp[:])
+	auxv = auxvp[: pairs*2 : pairs*2]
 }
 
 const (
@@ -611,8 +612,9 @@ const (
 	_AT_SUN_EXECNAME = 2014 // exec() path name
 )
 
-func sysauxv(auxv []uintptr) {
-	for i := 0; auxv[i] != _AT_NULL; i += 2 {
+func sysauxv(auxv []uintptr) (pairs int) {
+	var i int
+	for i = 0; auxv[i] != _AT_NULL; i += 2 {
 		tag, val := auxv[i], auxv[i+1]
 		switch tag {
 		case _AT_PAGESZ:
@@ -621,6 +623,7 @@ func sysauxv(auxv []uintptr) {
 			executablePath = gostringnocopy((*byte)(unsafe.Pointer(val)))
 		}
 	}
+	return i / 2
 }
 
 // sigPerThreadSyscall is only used on linux, so we assign a bogus signal

src/runtime/os_dragonfly.go

@@ -296,8 +296,9 @@ func sysargs(argc int32, argv **byte) {
 	// skip NULL separator
 	n++
 
-	auxv := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
-	sysauxv(auxv[:])
+	auxvp := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
+	pairs := sysauxv(auxvp[:])
+	auxv = auxvp[: pairs*2 : pairs*2]
 }
 
 const (
@@ -305,14 +306,16 @@ const (
 	_AT_PAGESZ = 6
 )
 
-func sysauxv(auxv []uintptr) {
-	for i := 0; auxv[i] != _AT_NULL; i += 2 {
+func sysauxv(auxv []uintptr) (pairs int) {
+	var i int
+	for i = 0; auxv[i] != _AT_NULL; i += 2 {
 		tag, val := auxv[i], auxv[i+1]
 		switch tag {
 		case _AT_PAGESZ:
 			physPageSize = val
 		}
 	}
+	return i / 2
 }
 
 // raise sends a signal to the calling thread.

src/runtime/os_freebsd.go

@@ -409,8 +409,9 @@ func sysargs(argc int32, argv **byte) {
 	n++
 
 	// now argv+n is auxv
-	auxv := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
-	sysauxv(auxv[:])
+	auxvp := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
+	pairs := sysauxv(auxvp[:])
+	auxv = auxvp[: pairs*2 : pairs*2]
 }
 
 const (
@@ -421,8 +422,9 @@ const (
 	_AT_HWCAP2   = 26 // CPU feature flags 2
 )
 
-func sysauxv(auxv []uintptr) {
-	for i := 0; auxv[i] != _AT_NULL; i += 2 {
+func sysauxv(auxv []uintptr) (pairs int) {
+	var i int
+	for i = 0; auxv[i] != _AT_NULL; i += 2 {
 		tag, val := auxv[i], auxv[i+1]
 		switch tag {
 		// _AT_NCPUS from auxv shouldn't be used due to golang.org/issue/15206
@@ -434,6 +436,7 @@ func sysauxv(auxv []uintptr) {
 
 		archauxv(tag, val)
 	}
+	return i / 2
 }
 
 // sysSigaction calls the sigaction system call.

src/runtime/os_linux.go

@@ -226,6 +226,8 @@ var addrspace_vec [1]byte
 
 func mincore(addr unsafe.Pointer, n uintptr, dst *byte) int32
 
+var auxvreadbuf [128]uintptr
+
 func sysargs(argc int32, argv **byte) {
 	n := argc + 1
 
@@ -238,8 +240,10 @@ func sysargs(argc int32, argv **byte) {
 	n++
 
 	// now argv+n is auxv
-	auxv := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
-	if sysauxv(auxv[:]) != 0 {
+	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
 	}
 	// In some situations we don't get a loader-provided
@@ -269,23 +273,24 @@ func sysargs(argc int32, argv **byte) {
 		munmap(p, size)
 		return
 	}
-	var buf [128]uintptr
-	n = read(fd, noescape(unsafe.Pointer(&buf[0])), int32(unsafe.Sizeof(buf)))
+
+	n = read(fd, noescape(unsafe.Pointer(&auxvreadbuf[0])), int32(unsafe.Sizeof(auxvreadbuf)))
 	closefd(fd)
 	if n < 0 {
 		return
 	}
 	// Make sure buf is terminated, even if we didn't read
 	// the whole file.
-	buf[len(buf)-2] = _AT_NULL
-	sysauxv(buf[:])
+	auxvreadbuf[len(auxvreadbuf)-2] = _AT_NULL
+	pairs := sysauxv(auxvreadbuf[:])
+	auxv = auxvreadbuf[: pairs*2 : pairs*2]
 }
 
 // startupRandomData holds random bytes initialized at startup. These come from
 // the ELF AT_RANDOM auxiliary vector.
 var startupRandomData []byte
 
-func sysauxv(auxv []uintptr) int {
+func sysauxv(auxv []uintptr) (pairs int) {
 	var i int
 	for ; auxv[i] != _AT_NULL; i += 2 {
 		tag, val := auxv[i], auxv[i+1]

src/runtime/os_netbsd.go

@@ -402,8 +402,9 @@ func sysargs(argc int32, argv **byte) {
 	n++
 
 	// now argv+n is auxv
-	auxv := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
-	sysauxv(auxv[:])
+	auxvp := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*goarch.PtrSize))
+	pairs := sysauxv(auxvp[:])
+	auxv = auxvp[: pairs*2 : pairs*2]
 }
 
 const (
@@ -411,14 +412,16 @@ const (
 	_AT_PAGESZ = 6 // Page size in bytes
 )
 
-func sysauxv(auxv []uintptr) {
-	for i := 0; auxv[i] != _AT_NULL; i += 2 {
+func sysauxv(auxv []uintptr) (pairs int) {
+	var i int
+	for i = 0; auxv[i] != _AT_NULL; i += 2 {
 		tag, val := auxv[i], auxv[i+1]
 		switch tag {
 		case _AT_PAGESZ:
 			physPageSize = val
 		}
 	}
+	return i / 2
 }
 
 // raise sends signal to the calling thread.

src/runtime/runtime.go

@@ -151,3 +151,12 @@ func syscall_runtimeUnsetenv(key string) {
 func writeErrStr(s string) {
 	write(2, unsafe.Pointer(unsafe.StringData(s)), int32(len(s)))
 }
+
+// auxv is populated on relevant platforms but defined here for all platforms
+// so x/sys/cpu can assume the getAuxv symbol exists without keeping its list
+// of auxv-using GOOS build tags in sync.
+//
+// It contains an even number of elements, (tag, value) pairs.
+var auxv []uintptr
+
+func getAuxv() []uintptr { return auxv } // accessed from x/sys/cpu; see issue 57336