vdso_linux.go

Documentation: runtime

     1  // Copyright 2012 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  // +build linux
     6  // +build 386 amd64
     7  
     8  package runtime
     9  
    10  import "unsafe"
    11  
    12  // Look up symbols in the Linux vDSO.
    13  
    14  // This code was originally based on the sample Linux vDSO parser at
    15  // https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/vDSO/parse_vdso.c
    16  
    17  // This implements the ELF dynamic linking spec at
    18  // http://sco.com/developers/gabi/latest/ch5.dynamic.html
    19  
    20  // The version section is documented at
    21  // http://refspecs.linuxfoundation.org/LSB_3.2.0/LSB-Core-generic/LSB-Core-generic/symversion.html
    22  
    23  const (
    24  	_AT_SYSINFO_EHDR = 33
    25  
    26  	_PT_LOAD    = 1 /* Loadable program segment */
    27  	_PT_DYNAMIC = 2 /* Dynamic linking information */
    28  
    29  	_DT_NULL     = 0          /* Marks end of dynamic section */
    30  	_DT_HASH     = 4          /* Dynamic symbol hash table */
    31  	_DT_STRTAB   = 5          /* Address of string table */
    32  	_DT_SYMTAB   = 6          /* Address of symbol table */
    33  	_DT_GNU_HASH = 0x6ffffef5 /* GNU-style dynamic symbol hash table */
    34  	_DT_VERSYM   = 0x6ffffff0
    35  	_DT_VERDEF   = 0x6ffffffc
    36  
    37  	_VER_FLG_BASE = 0x1 /* Version definition of file itself */
    38  
    39  	_SHN_UNDEF = 0 /* Undefined section */
    40  
    41  	_SHT_DYNSYM = 11 /* Dynamic linker symbol table */
    42  
    43  	_STT_FUNC = 2 /* Symbol is a code object */
    44  
    45  	_STB_GLOBAL = 1 /* Global symbol */
    46  	_STB_WEAK   = 2 /* Weak symbol */
    47  
    48  	_EI_NIDENT = 16
    49  
    50  	// Maximum indices for the array types used when traversing the vDSO ELF structures.
    51  	// Computed from architecture-specific max provided by vdso_linux_*.go
    52  	vdsoSymTabSize     = vdsoArrayMax / unsafe.Sizeof(elfSym{})
    53  	vdsoDynSize        = vdsoArrayMax / unsafe.Sizeof(elfDyn{})
    54  	vdsoSymStringsSize = vdsoArrayMax     // byte
    55  	vdsoVerSymSize     = vdsoArrayMax / 2 // uint16
    56  	vdsoHashSize       = vdsoArrayMax / 4 // uint32
    57  
    58  	// vdsoBloomSizeScale is a scaling factor for gnuhash tables which are uint32 indexed,
    59  	// but contain uintptrs
    60  	vdsoBloomSizeScale = unsafe.Sizeof(uintptr(0)) / 4 // uint32
    61  )
    62  
    63  /* How to extract and insert information held in the st_info field.  */
    64  func _ELF_ST_BIND(val byte) byte { return val >> 4 }
    65  func _ELF_ST_TYPE(val byte) byte { return val & 0xf }
    66  
    67  type symbol_key struct {
    68  	name     string
    69  	sym_hash uint32
    70  	gnu_hash uint32
    71  	ptr      *uintptr
    72  }
    73  
    74  type version_key struct {
    75  	version  string
    76  	ver_hash uint32
    77  }
    78  
    79  type vdso_info struct {
    80  	valid bool
    81  
    82  	/* Load information */
    83  	load_addr   uintptr
    84  	load_offset uintptr /* load_addr - recorded vaddr */
    85  
    86  	/* Symbol table */
    87  	symtab     *[vdsoSymTabSize]elfSym
    88  	symstrings *[vdsoSymStringsSize]byte
    89  	chain      []uint32
    90  	bucket     []uint32
    91  	symOff     uint32
    92  	isGNUHash  bool
    93  
    94  	/* Version table */
    95  	versym *[vdsoVerSymSize]uint16
    96  	verdef *elfVerdef
    97  }
    98  
    99  var linux26 = version_key{"LINUX_2.6", 0x3ae75f6}
   100  
   101  // see vdso_linux_*.go for sym_keys[] and __vdso_* vars
   102  
   103  func vdso_init_from_sysinfo_ehdr(info *vdso_info, hdr *elfEhdr) {
   104  	info.valid = false
   105  	info.load_addr = uintptr(unsafe.Pointer(hdr))
   106  
   107  	pt := unsafe.Pointer(info.load_addr + uintptr(hdr.e_phoff))
   108  
   109  	// We need two things from the segment table: the load offset
   110  	// and the dynamic table.
   111  	var found_vaddr bool
   112  	var dyn *[vdsoDynSize]elfDyn
   113  	for i := uint16(0); i < hdr.e_phnum; i++ {
   114  		pt := (*elfPhdr)(add(pt, uintptr(i)*unsafe.Sizeof(elfPhdr{})))
   115  		switch pt.p_type {
   116  		case _PT_LOAD:
   117  			if !found_vaddr {
   118  				found_vaddr = true
   119  				info.load_offset = info.load_addr + uintptr(pt.p_offset-pt.p_vaddr)
   120  			}
   121  
   122  		case _PT_DYNAMIC:
   123  			dyn = (*[vdsoDynSize]elfDyn)(unsafe.Pointer(info.load_addr + uintptr(pt.p_offset)))
   124  		}
   125  	}
   126  
   127  	if !found_vaddr || dyn == nil {
   128  		return // Failed
   129  	}
   130  
   131  	// Fish out the useful bits of the dynamic table.
   132  
   133  	var hash, gnuhash *[vdsoHashSize]uint32
   134  	info.symstrings = nil
   135  	info.symtab = nil
   136  	info.versym = nil
   137  	info.verdef = nil
   138  	for i := 0; dyn[i].d_tag != _DT_NULL; i++ {
   139  		dt := &dyn[i]
   140  		p := info.load_offset + uintptr(dt.d_val)
   141  		switch dt.d_tag {
   142  		case _DT_STRTAB:
   143  			info.symstrings = (*[vdsoSymStringsSize]byte)(unsafe.Pointer(p))
   144  		case _DT_SYMTAB:
   145  			info.symtab = (*[vdsoSymTabSize]elfSym)(unsafe.Pointer(p))
   146  		case _DT_HASH:
   147  			hash = (*[vdsoHashSize]uint32)(unsafe.Pointer(p))
   148  		case _DT_GNU_HASH:
   149  			gnuhash = (*[vdsoHashSize]uint32)(unsafe.Pointer(p))
   150  		case _DT_VERSYM:
   151  			info.versym = (*[vdsoVerSymSize]uint16)(unsafe.Pointer(p))
   152  		case _DT_VERDEF:
   153  			info.verdef = (*elfVerdef)(unsafe.Pointer(p))
   154  		}
   155  	}
   156  
   157  	if info.symstrings == nil || info.symtab == nil || (hash == nil && gnuhash == nil) {
   158  		return // Failed
   159  	}
   160  
   161  	if info.verdef == nil {
   162  		info.versym = nil
   163  	}
   164  
   165  	if gnuhash != nil {
   166  		// Parse the GNU hash table header.
   167  		nbucket := gnuhash[0]
   168  		info.symOff = gnuhash[1]
   169  		bloomSize := gnuhash[2]
   170  		info.bucket = gnuhash[4+bloomSize*uint32(vdsoBloomSizeScale):][:nbucket]
   171  		info.chain = gnuhash[4+bloomSize*uint32(vdsoBloomSizeScale)+nbucket:]
   172  		info.isGNUHash = true
   173  	} else {
   174  		// Parse the hash table header.
   175  		nbucket := hash[0]
   176  		nchain := hash[1]
   177  		info.bucket = hash[2 : 2+nbucket]
   178  		info.chain = hash[2+nbucket : 2+nbucket+nchain]
   179  	}
   180  
   181  	// That's all we need.
   182  	info.valid = true
   183  }
   184  
   185  func vdso_find_version(info *vdso_info, ver *version_key) int32 {
   186  	if !info.valid {
   187  		return 0
   188  	}
   189  
   190  	def := info.verdef
   191  	for {
   192  		if def.vd_flags&_VER_FLG_BASE == 0 {
   193  			aux := (*elfVerdaux)(add(unsafe.Pointer(def), uintptr(def.vd_aux)))
   194  			if def.vd_hash == ver.ver_hash && ver.version == gostringnocopy(&info.symstrings[aux.vda_name]) {
   195  				return int32(def.vd_ndx & 0x7fff)
   196  			}
   197  		}
   198  
   199  		if def.vd_next == 0 {
   200  			break
   201  		}
   202  		def = (*elfVerdef)(add(unsafe.Pointer(def), uintptr(def.vd_next)))
   203  	}
   204  
   205  	return -1 // cannot match any version
   206  }
   207  
   208  func vdso_parse_symbols(info *vdso_info, version int32) {
   209  	if !info.valid {
   210  		return
   211  	}
   212  
   213  	apply := func(symIndex uint32, k symbol_key) bool {
   214  		sym := &info.symtab[symIndex]
   215  		typ := _ELF_ST_TYPE(sym.st_info)
   216  		bind := _ELF_ST_BIND(sym.st_info)
   217  		if typ != _STT_FUNC || bind != _STB_GLOBAL && bind != _STB_WEAK || sym.st_shndx == _SHN_UNDEF {
   218  			return false
   219  		}
   220  		if k.name != gostringnocopy(&info.symstrings[sym.st_name]) {
   221  			return false
   222  		}
   223  
   224  		// Check symbol version.
   225  		if info.versym != nil && version != 0 && int32(info.versym[symIndex]&0x7fff) != version {
   226  			return false
   227  		}
   228  
   229  		*k.ptr = info.load_offset + uintptr(sym.st_value)
   230  		return true
   231  	}
   232  
   233  	if !info.isGNUHash {
   234  		// Old-style DT_HASH table.
   235  		for _, k := range sym_keys {
   236  			for chain := info.bucket[k.sym_hash%uint32(len(info.bucket))]; chain != 0; chain = info.chain[chain] {
   237  				if apply(chain, k) {
   238  					break
   239  				}
   240  			}
   241  		}
   242  		return
   243  	}
   244  
   245  	// New-style DT_GNU_HASH table.
   246  	for _, k := range sym_keys {
   247  		symIndex := info.bucket[k.gnu_hash%uint32(len(info.bucket))]
   248  		if symIndex < info.symOff {
   249  			continue
   250  		}
   251  		for ; ; symIndex++ {
   252  			hash := info.chain[symIndex-info.symOff]
   253  			if hash|1 == k.gnu_hash|1 {
   254  				// Found a hash match.
   255  				if apply(symIndex, k) {
   256  					break
   257  				}
   258  			}
   259  			if hash&1 != 0 {
   260  				// End of chain.
   261  				break
   262  			}
   263  		}
   264  	}
   265  }
   266  
   267  func archauxv(tag, val uintptr) {
   268  	switch tag {
   269  	case _AT_SYSINFO_EHDR:
   270  		if val == 0 {
   271  			// Something went wrong
   272  			return
   273  		}
   274  		var info vdso_info
   275  		// TODO(rsc): I don't understand why the compiler thinks info escapes
   276  		// when passed to the three functions below.
   277  		info1 := (*vdso_info)(noescape(unsafe.Pointer(&info)))
   278  		vdso_init_from_sysinfo_ehdr(info1, (*elfEhdr)(unsafe.Pointer(val)))
   279  		vdso_parse_symbols(info1, vdso_find_version(info1, &linux26))
   280  	}
   281  }
   282
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