write_err_android.go

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package runtime
     6  
     7  import "unsafe"
     8  
     9  var (
    10  	writeHeader = []byte{6 /* ANDROID_LOG_ERROR */, 'G', 'o', 0}
    11  	writePath   = []byte("/dev/log/main\x00")
    12  	writeLogd   = []byte("/dev/socket/logdw\x00")
    13  
    14  	// guarded by printlock/printunlock.
    15  	writeFD  uintptr
    16  	writeBuf [1024]byte
    17  	writePos int
    18  )
    19  
    20  // Prior to Android-L, logging was done through writes to /dev/log files implemented
    21  // in kernel ring buffers. In Android-L, those /dev/log files are no longer
    22  // accessible and logging is done through a centralized user-mode logger, logd.
    23  //
    24  // https://android.googlesource.com/platform/system/core/+/refs/tags/android-6.0.1_r78/liblog/logd_write.c
    25  type loggerType int32
    26  
    27  const (
    28  	unknown loggerType = iota
    29  	legacy
    30  	logd
    31  	// TODO(hakim): logging for emulator?
    32  )
    33  
    34  var logger loggerType
    35  
    36  func writeErr(b []byte) {
    37  	if logger == unknown {
    38  		// Use logd if /dev/socket/logdw is available.
    39  		if v := uintptr(access(&writeLogd[0], 0x02 /* W_OK */)); v == 0 {
    40  			logger = logd
    41  			initLogd()
    42  		} else {
    43  			logger = legacy
    44  			initLegacy()
    45  		}
    46  	}
    47  
    48  	// Write to stderr for command-line programs.
    49  	write(2, unsafe.Pointer(&b[0]), int32(len(b)))
    50  
    51  	// Log format: "<header>\x00<message m bytes>\x00"
    52  	//
    53  	// <header>
    54  	//   In legacy mode: "<priority 1 byte><tag n bytes>".
    55  	//   In logd mode: "<android_log_header_t 11 bytes><priority 1 byte><tag n bytes>"
    56  	//
    57  	// The entire log needs to be delivered in a single syscall (the NDK
    58  	// does this with writev). Each log is its own line, so we need to
    59  	// buffer writes until we see a newline.
    60  	var hlen int
    61  	switch logger {
    62  	case logd:
    63  		hlen = writeLogdHeader()
    64  	case legacy:
    65  		hlen = len(writeHeader)
    66  	}
    67  
    68  	dst := writeBuf[hlen:]
    69  	for _, v := range b {
    70  		if v == 0 { // android logging won't print a zero byte
    71  			v = '0'
    72  		}
    73  		dst[writePos] = v
    74  		writePos++
    75  		if v == '\n' || writePos == len(dst)-1 {
    76  			dst[writePos] = 0
    77  			write(writeFD, unsafe.Pointer(&writeBuf[0]), int32(hlen+writePos))
    78  			for i := range dst {
    79  				dst[i] = 0
    80  			}
    81  			writePos = 0
    82  		}
    83  	}
    84  }
    85  
    86  func initLegacy() {
    87  	// In legacy mode, logs are written to /dev/log/main
    88  	writeFD = uintptr(open(&writePath[0], 0x1 /* O_WRONLY */, 0))
    89  	if writeFD == 0 {
    90  		// It is hard to do anything here. Write to stderr just
    91  		// in case user has root on device and has run
    92  		//	adb shell setprop log.redirect-stdio true
    93  		msg := []byte("runtime: cannot open /dev/log/main\x00")
    94  		write(2, unsafe.Pointer(&msg[0]), int32(len(msg)))
    95  		exit(2)
    96  	}
    97  
    98  	// Prepopulate the invariant header part.
    99  	copy(writeBuf[:len(writeHeader)], writeHeader)
   100  }
   101  
   102  // used in initLogdWrite but defined here to avoid heap allocation.
   103  var logdAddr sockaddr_un
   104  
   105  func initLogd() {
   106  	// In logd mode, logs are sent to the logd via a unix domain socket.
   107  	logdAddr.family = _AF_UNIX
   108  	copy(logdAddr.path[:], writeLogd)
   109  
   110  	// We are not using non-blocking I/O because writes taking this path
   111  	// are most likely triggered by panic, we cannot think of the advantage of
   112  	// non-blocking I/O for panic but see disadvantage (dropping panic message),
   113  	// and blocking I/O simplifies the code a lot.
   114  	fd := socket(_AF_UNIX, _SOCK_DGRAM|_O_CLOEXEC, 0)
   115  	if fd < 0 {
   116  		msg := []byte("runtime: cannot create a socket for logging\x00")
   117  		write(2, unsafe.Pointer(&msg[0]), int32(len(msg)))
   118  		exit(2)
   119  	}
   120  
   121  	errno := connect(fd, unsafe.Pointer(&logdAddr), int32(unsafe.Sizeof(logdAddr)))
   122  	if errno < 0 {
   123  		msg := []byte("runtime: cannot connect to /dev/socket/logdw\x00")
   124  		write(2, unsafe.Pointer(&msg[0]), int32(len(msg)))
   125  		// TODO(hakim): or should we just close fd and hope for better luck next time?
   126  		exit(2)
   127  	}
   128  	writeFD = uintptr(fd)
   129  
   130  	// Prepopulate invariant part of the header.
   131  	// The first 11 bytes will be populated later in writeLogdHeader.
   132  	copy(writeBuf[11:11+len(writeHeader)], writeHeader)
   133  }
   134  
   135  // writeLogdHeader populates the header and returns the length of the payload.
   136  func writeLogdHeader() int {
   137  	hdr := writeBuf[:11]
   138  
   139  	// The first 11 bytes of the header corresponds to android_log_header_t
   140  	// as defined in system/core/include/private/android_logger.h
   141  	//   hdr[0] log type id (unsigned char), defined in <log/log.h>
   142  	//   hdr[1:2] tid (uint16_t)
   143  	//   hdr[3:11] log_time defined in <log/log_read.h>
   144  	//      hdr[3:7] sec unsigned uint32, little endian.
   145  	//      hdr[7:11] nsec unsigned uint32, little endian.
   146  	hdr[0] = 0 // LOG_ID_MAIN
   147  	sec, nsec, _ := time_now()
   148  	packUint32(hdr[3:7], uint32(sec))
   149  	packUint32(hdr[7:11], uint32(nsec))
   150  
   151  	// TODO(hakim):  hdr[1:2] = gettid?
   152  
   153  	return 11 + len(writeHeader)
   154  }
   155  
   156  func packUint32(b []byte, v uint32) {
   157  	// little-endian.
   158  	b[0] = byte(v)
   159  	b[1] = byte(v >> 8)
   160  	b[2] = byte(v >> 16)
   161  	b[3] = byte(v >> 24)
   162  }
   163
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