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  //go:build linux && (386 || amd64 || arm || arm64 || mips64 || mips64le || ppc64 || ppc64le)
     6  // +build linux
     7  // +build 386 amd64 arm arm64 mips64 mips64le ppc64 ppc64le
     8  
     9  package runtime
    10  
    11  import "unsafe"
    12  
    13  // Look up symbols in the Linux vDSO.
    14  
    15  // This code was originally based on the sample Linux vDSO parser at
    16  // https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/vDSO/parse_vdso.c
    17  
    18  // This implements the ELF dynamic linking spec at
    19  // http://sco.com/developers/gabi/latest/ch5.dynamic.html
    20  
    21  // The version section is documented at
    22  // https://refspecs.linuxfoundation.org/LSB_3.2.0/LSB-Core-generic/LSB-Core-generic/symversion.html
    23  
    24  const (
    25  	_AT_SYSINFO_EHDR = 33
    26  
    27  	_PT_LOAD    = 1 /* Loadable program segment */
    28  	_PT_DYNAMIC = 2 /* Dynamic linking information */
    29  
    30  	_DT_NULL     = 0          /* Marks end of dynamic section */
    31  	_DT_HASH     = 4          /* Dynamic symbol hash table */
    32  	_DT_STRTAB   = 5          /* Address of string table */
    33  	_DT_SYMTAB   = 6          /* Address of symbol table */
    34  	_DT_GNU_HASH = 0x6ffffef5 /* GNU-style dynamic symbol hash table */
    35  	_DT_VERSYM   = 0x6ffffff0
    36  	_DT_VERDEF   = 0x6ffffffc
    37  
    38  	_VER_FLG_BASE = 0x1 /* Version definition of file itself */
    39  
    40  	_SHN_UNDEF = 0 /* Undefined section */
    41  
    42  	_SHT_DYNSYM = 11 /* Dynamic linker symbol table */
    43  
    44  	_STT_FUNC = 2 /* Symbol is a code object */
    45  
    46  	_STT_NOTYPE = 0 /* Symbol type is not specified */
    47  
    48  	_STB_GLOBAL = 1 /* Global symbol */
    49  	_STB_WEAK   = 2 /* Weak symbol */
    50  
    51  	_EI_NIDENT = 16
    52  
    53  	// Maximum indices for the array types used when traversing the vDSO ELF structures.
    54  	// Computed from architecture-specific max provided by vdso_linux_*.go
    55  	vdsoSymTabSize     = vdsoArrayMax / unsafe.Sizeof(elfSym{})
    56  	vdsoDynSize        = vdsoArrayMax / unsafe.Sizeof(elfDyn{})
    57  	vdsoSymStringsSize = vdsoArrayMax     // byte
    58  	vdsoVerSymSize     = vdsoArrayMax / 2 // uint16
    59  	vdsoHashSize       = vdsoArrayMax / 4 // uint32
    60  
    61  	// vdsoBloomSizeScale is a scaling factor for gnuhash tables which are uint32 indexed,
    62  	// but contain uintptrs
    63  	vdsoBloomSizeScale = unsafe.Sizeof(uintptr(0)) / 4 // uint32
    64  )
    65  
    66  /* How to extract and insert information held in the st_info field.  */
    67  func _ELF_ST_BIND(val byte) byte { return val >> 4 }
    68  func _ELF_ST_TYPE(val byte) byte { return val & 0xf }
    69  
    70  type vdsoSymbolKey struct {
    71  	name    string
    72  	symHash uint32
    73  	gnuHash uint32
    74  	ptr     *uintptr
    75  }
    76  
    77  type vdsoVersionKey struct {
    78  	version string
    79  	verHash uint32
    80  }
    81  
    82  type vdsoInfo struct {
    83  	valid bool
    84  
    85  	/* Load information */
    86  	loadAddr   uintptr
    87  	loadOffset uintptr /* loadAddr - recorded vaddr */
    88  
    89  	/* Symbol table */
    90  	symtab     *[vdsoSymTabSize]elfSym
    91  	symstrings *[vdsoSymStringsSize]byte
    92  	chain      []uint32
    93  	bucket     []uint32
    94  	symOff     uint32
    95  	isGNUHash  bool
    96  
    97  	/* Version table */
    98  	versym *[vdsoVerSymSize]uint16
    99  	verdef *elfVerdef
   100  }
   101  
   102  // see vdso_linux_*.go for vdsoSymbolKeys[] and vdso*Sym vars
   103  
   104  func vdsoInitFromSysinfoEhdr(info *vdsoInfo, hdr *elfEhdr) {
   105  	info.valid = false
   106  	info.loadAddr = uintptr(unsafe.Pointer(hdr))
   107  
   108  	pt := unsafe.Pointer(info.loadAddr + uintptr(hdr.e_phoff))
   109  
   110  	// We need two things from the segment table: the load offset
   111  	// and the dynamic table.
   112  	var foundVaddr bool
   113  	var dyn *[vdsoDynSize]elfDyn
   114  	for i := uint16(0); i < hdr.e_phnum; i++ {
   115  		pt := (*elfPhdr)(add(pt, uintptr(i)*unsafe.Sizeof(elfPhdr{})))
   116  		switch pt.p_type {
   117  		case _PT_LOAD:
   118  			if !foundVaddr {
   119  				foundVaddr = true
   120  				info.loadOffset = info.loadAddr + uintptr(pt.p_offset-pt.p_vaddr)
   121  			}
   122  
   123  		case _PT_DYNAMIC:
   124  			dyn = (*[vdsoDynSize]elfDyn)(unsafe.Pointer(info.loadAddr + uintptr(pt.p_offset)))
   125  		}
   126  	}
   127  
   128  	if !foundVaddr || dyn == nil {
   129  		return // Failed
   130  	}
   131  
   132  	// Fish out the useful bits of the dynamic table.
   133  
   134  	var hash, gnuhash *[vdsoHashSize]uint32
   135  	info.symstrings = nil
   136  	info.symtab = nil
   137  	info.versym = nil
   138  	info.verdef = nil
   139  	for i := 0; dyn[i].d_tag != _DT_NULL; i++ {
   140  		dt := &dyn[i]
   141  		p := info.loadOffset + uintptr(dt.d_val)
   142  		switch dt.d_tag {
   143  		case _DT_STRTAB:
   144  			info.symstrings = (*[vdsoSymStringsSize]byte)(unsafe.Pointer(p))
   145  		case _DT_SYMTAB:
   146  			info.symtab = (*[vdsoSymTabSize]elfSym)(unsafe.Pointer(p))
   147  		case _DT_HASH:
   148  			hash = (*[vdsoHashSize]uint32)(unsafe.Pointer(p))
   149  		case _DT_GNU_HASH:
   150  			gnuhash = (*[vdsoHashSize]uint32)(unsafe.Pointer(p))
   151  		case _DT_VERSYM:
   152  			info.versym = (*[vdsoVerSymSize]uint16)(unsafe.Pointer(p))
   153  		case _DT_VERDEF:
   154  			info.verdef = (*elfVerdef)(unsafe.Pointer(p))
   155  		}
   156  	}
   157  
   158  	if info.symstrings == nil || info.symtab == nil || (hash == nil && gnuhash == nil) {
   159  		return // Failed
   160  	}
   161  
   162  	if info.verdef == nil {
   163  		info.versym = nil
   164  	}
   165  
   166  	if gnuhash != nil {
   167  		// Parse the GNU hash table header.
   168  		nbucket := gnuhash[0]
   169  		info.symOff = gnuhash[1]
   170  		bloomSize := gnuhash[2]
   171  		info.bucket = gnuhash[4+bloomSize*uint32(vdsoBloomSizeScale):][:nbucket]
   172  		info.chain = gnuhash[4+bloomSize*uint32(vdsoBloomSizeScale)+nbucket:]
   173  		info.isGNUHash = true
   174  	} else {
   175  		// Parse the hash table header.
   176  		nbucket := hash[0]
   177  		nchain := hash[1]
   178  		info.bucket = hash[2 : 2+nbucket]
   179  		info.chain = hash[2+nbucket : 2+nbucket+nchain]
   180  	}
   181  
   182  	// That's all we need.
   183  	info.valid = true
   184  }
   185  
   186  func vdsoFindVersion(info *vdsoInfo, ver *vdsoVersionKey) int32 {
   187  	if !info.valid {
   188  		return 0
   189  	}
   190  
   191  	def := info.verdef
   192  	for {
   193  		if def.vd_flags&_VER_FLG_BASE == 0 {
   194  			aux := (*elfVerdaux)(add(unsafe.Pointer(def), uintptr(def.vd_aux)))
   195  			if def.vd_hash == ver.verHash && ver.version == gostringnocopy(&info.symstrings[aux.vda_name]) {
   196  				return int32(def.vd_ndx & 0x7fff)
   197  			}
   198  		}
   199  
   200  		if def.vd_next == 0 {
   201  			break
   202  		}
   203  		def = (*elfVerdef)(add(unsafe.Pointer(def), uintptr(def.vd_next)))
   204  	}
   205  
   206  	return -1 // cannot match any version
   207  }
   208  
   209  func vdsoParseSymbols(info *vdsoInfo, version int32) {
   210  	if !info.valid {
   211  		return
   212  	}
   213  
   214  	apply := func(symIndex uint32, k vdsoSymbolKey) bool {
   215  		sym := &info.symtab[symIndex]
   216  		typ := _ELF_ST_TYPE(sym.st_info)
   217  		bind := _ELF_ST_BIND(sym.st_info)
   218  		// On ppc64x, VDSO functions are of type _STT_NOTYPE.
   219  		if typ != _STT_FUNC && typ != _STT_NOTYPE || bind != _STB_GLOBAL && bind != _STB_WEAK || sym.st_shndx == _SHN_UNDEF {
   220  			return false
   221  		}
   222  		if k.name != gostringnocopy(&info.symstrings[sym.st_name]) {
   223  			return false
   224  		}
   225  		// Check symbol version.
   226  		if info.versym != nil && version != 0 && int32(info.versym[symIndex]&0x7fff) != version {
   227  			return false
   228  		}
   229  
   230  		*k.ptr = info.loadOffset + uintptr(sym.st_value)
   231  		return true
   232  	}
   233  
   234  	if !info.isGNUHash {
   235  		// Old-style DT_HASH table.
   236  		for _, k := range vdsoSymbolKeys {
   237  			for chain := info.bucket[k.symHash%uint32(len(info.bucket))]; chain != 0; chain = info.chain[chain] {
   238  				if apply(chain, k) {
   239  					break
   240  				}
   241  			}
   242  		}
   243  		return
   244  	}
   245  
   246  	// New-style DT_GNU_HASH table.
   247  	for _, k := range vdsoSymbolKeys {
   248  		symIndex := info.bucket[k.gnuHash%uint32(len(info.bucket))]
   249  		if symIndex < info.symOff {
   250  			continue
   251  		}
   252  		for ; ; symIndex++ {
   253  			hash := info.chain[symIndex-info.symOff]
   254  			if hash|1 == k.gnuHash|1 {
   255  				// Found a hash match.
   256  				if apply(symIndex, k) {
   257  					break
   258  				}
   259  			}
   260  			if hash&1 != 0 {
   261  				// End of chain.
   262  				break
   263  			}
   264  		}
   265  	}
   266  }
   267  
   268  func vdsoauxv(tag, val uintptr) {
   269  	switch tag {
   270  	case _AT_SYSINFO_EHDR:
   271  		if val == 0 {
   272  			// Something went wrong
   273  			return
   274  		}
   275  		var info vdsoInfo
   276  		// TODO(rsc): I don't understand why the compiler thinks info escapes
   277  		// when passed to the three functions below.
   278  		info1 := (*vdsoInfo)(noescape(unsafe.Pointer(&info)))
   279  		vdsoInitFromSysinfoEhdr(info1, (*elfEhdr)(unsafe.Pointer(val)))
   280  		vdsoParseSymbols(info1, vdsoFindVersion(info1, &vdsoLinuxVersion))
   281  	}
   282  }
   283  
   284  // vdsoMarker reports whether PC is on the VDSO page.
   285  //go:nosplit
   286  func inVDSOPage(pc uintptr) bool {
   287  	for _, k := range vdsoSymbolKeys {
   288  		if *k.ptr != 0 {
   289  			page := *k.ptr &^ (physPageSize - 1)
   290  			return pc >= page && pc < page+physPageSize
   291  		}
   292  	}
   293  	return false
   294  }
   295
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