elfexec.go

     1  // Copyright 2014 Google Inc. All Rights Reserved.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package elfexec provides utility routines to examine ELF binaries.
    16  package elfexec
    17  
    18  import (
    19  	"bufio"
    20  	"debug/elf"
    21  	"encoding/binary"
    22  	"fmt"
    23  	"io"
    24  )
    25  
    26  const (
    27  	maxNoteSize        = 1 << 20 // in bytes
    28  	noteTypeGNUBuildID = 3
    29  )
    30  
    31  // elfNote is the payload of a Note Section in an ELF file.
    32  type elfNote struct {
    33  	Name string // Contents of the "name" field, omitting the trailing zero byte.
    34  	Desc []byte // Contents of the "desc" field.
    35  	Type uint32 // Contents of the "type" field.
    36  }
    37  
    38  // parseNotes returns the notes from a SHT_NOTE section or PT_NOTE segment.
    39  func parseNotes(reader io.Reader, alignment int, order binary.ByteOrder) ([]elfNote, error) {
    40  	r := bufio.NewReader(reader)
    41  
    42  	// padding returns the number of bytes required to pad the given size to an
    43  	// alignment boundary.
    44  	padding := func(size int) int {
    45  		return ((size + (alignment - 1)) &^ (alignment - 1)) - size
    46  	}
    47  
    48  	var notes []elfNote
    49  	for {
    50  		noteHeader := make([]byte, 12) // 3 4-byte words
    51  		if _, err := io.ReadFull(r, noteHeader); err == io.EOF {
    52  			break
    53  		} else if err != nil {
    54  			return nil, err
    55  		}
    56  		namesz := order.Uint32(noteHeader[0:4])
    57  		descsz := order.Uint32(noteHeader[4:8])
    58  		typ := order.Uint32(noteHeader[8:12])
    59  
    60  		if uint64(namesz) > uint64(maxNoteSize) {
    61  			return nil, fmt.Errorf("note name too long (%d bytes)", namesz)
    62  		}
    63  		var name string
    64  		if namesz > 0 {
    65  			// Documentation differs as to whether namesz is meant to include the
    66  			// trailing zero, but everyone agrees that name is null-terminated.
    67  			// So we'll just determine the actual length after the fact.
    68  			var err error
    69  			name, err = r.ReadString('\x00')
    70  			if err == io.EOF {
    71  				return nil, fmt.Errorf("missing note name (want %d bytes)", namesz)
    72  			} else if err != nil {
    73  				return nil, err
    74  			}
    75  			namesz = uint32(len(name))
    76  			name = name[:len(name)-1]
    77  		}
    78  
    79  		// Drop padding bytes until the desc field.
    80  		for n := padding(len(noteHeader) + int(namesz)); n > 0; n-- {
    81  			if _, err := r.ReadByte(); err == io.EOF {
    82  				return nil, fmt.Errorf(
    83  					"missing %d bytes of padding after note name", n)
    84  			} else if err != nil {
    85  				return nil, err
    86  			}
    87  		}
    88  
    89  		if uint64(descsz) > uint64(maxNoteSize) {
    90  			return nil, fmt.Errorf("note desc too long (%d bytes)", descsz)
    91  		}
    92  		desc := make([]byte, int(descsz))
    93  		if _, err := io.ReadFull(r, desc); err == io.EOF {
    94  			return nil, fmt.Errorf("missing desc (want %d bytes)", len(desc))
    95  		} else if err != nil {
    96  			return nil, err
    97  		}
    98  
    99  		notes = append(notes, elfNote{Name: name, Desc: desc, Type: typ})
   100  
   101  		// Drop padding bytes until the next note or the end of the section,
   102  		// whichever comes first.
   103  		for n := padding(len(desc)); n > 0; n-- {
   104  			if _, err := r.ReadByte(); err == io.EOF {
   105  				// We hit the end of the section before an alignment boundary.
   106  				// This can happen if this section is at the end of the file or the next
   107  				// section has a smaller alignment requirement.
   108  				break
   109  			} else if err != nil {
   110  				return nil, err
   111  			}
   112  		}
   113  	}
   114  	return notes, nil
   115  }
   116  
   117  // GetBuildID returns the GNU build-ID for an ELF binary.
   118  //
   119  // If no build-ID was found but the binary was read without error, it returns
   120  // (nil, nil).
   121  func GetBuildID(f *elf.File) ([]byte, error) {
   122  	findBuildID := func(notes []elfNote) ([]byte, error) {
   123  		var buildID []byte
   124  		for _, note := range notes {
   125  			if note.Name == "GNU" && note.Type == noteTypeGNUBuildID {
   126  				if buildID == nil {
   127  					buildID = note.Desc
   128  				} else {
   129  					return nil, fmt.Errorf("multiple build ids found, don't know which to use")
   130  				}
   131  			}
   132  		}
   133  		return buildID, nil
   134  	}
   135  
   136  	for _, p := range f.Progs {
   137  		if p.Type != elf.PT_NOTE {
   138  			continue
   139  		}
   140  		notes, err := parseNotes(p.Open(), int(p.Align), f.ByteOrder)
   141  		if err != nil {
   142  			return nil, err
   143  		}
   144  		if b, err := findBuildID(notes); b != nil || err != nil {
   145  			return b, err
   146  		}
   147  	}
   148  	for _, s := range f.Sections {
   149  		if s.Type != elf.SHT_NOTE {
   150  			continue
   151  		}
   152  		notes, err := parseNotes(s.Open(), int(s.Addralign), f.ByteOrder)
   153  		if err != nil {
   154  			return nil, err
   155  		}
   156  		if b, err := findBuildID(notes); b != nil || err != nil {
   157  			return b, err
   158  		}
   159  	}
   160  	return nil, nil
   161  }
   162  
   163  // kernelBase calculates the base for kernel mappings, which usually require
   164  // special handling. For kernel mappings, tools (like perf) use the address of
   165  // the kernel relocation symbol (_text or _stext) as the mmap start. Additionally,
   166  // for obfuscation, ChromeOS profiles have the kernel image remapped to the 0-th page.
   167  func kernelBase(loadSegment *elf.ProgHeader, stextOffset *uint64, start, limit, offset uint64) (uint64, bool) {
   168  	const (
   169  		// PAGE_OFFSET for PowerPC64, see arch/powerpc/Kconfig in the kernel sources.
   170  		pageOffsetPpc64 = 0xc000000000000000
   171  		pageSize        = 4096
   172  	)
   173  
   174  	if loadSegment.Vaddr == start-offset {
   175  		return offset, true
   176  	}
   177  	if start == 0 && limit != 0 && stextOffset != nil {
   178  		// ChromeOS remaps its kernel to 0. Nothing else should come
   179  		// down this path. Empirical values:
   180  		//       VADDR=0xffffffff80200000
   181  		// stextOffset=0xffffffff80200198
   182  		return start - *stextOffset, true
   183  	}
   184  	if start >= loadSegment.Vaddr && limit > start && (offset == 0 || offset == pageOffsetPpc64 || offset == start) {
   185  		// Some kernels look like:
   186  		//       VADDR=0xffffffff80200000
   187  		// stextOffset=0xffffffff80200198
   188  		//       Start=0xffffffff83200000
   189  		//       Limit=0xffffffff84200000
   190  		//      Offset=0 (0xc000000000000000 for PowerPC64) (== Start for ASLR kernel)
   191  		// So the base should be:
   192  		if stextOffset != nil && (start%pageSize) == (*stextOffset%pageSize) {
   193  			// perf uses the address of _stext as start. Some tools may
   194  			// adjust for this before calling GetBase, in which case the page
   195  			// alignment should be different from that of stextOffset.
   196  			return start - *stextOffset, true
   197  		}
   198  
   199  		return start - loadSegment.Vaddr, true
   200  	}
   201  	if start%pageSize != 0 && stextOffset != nil && *stextOffset%pageSize == start%pageSize {
   202  		// ChromeOS remaps its kernel to 0 + start%pageSize. Nothing
   203  		// else should come down this path. Empirical values:
   204  		//       start=0x198 limit=0x2f9fffff offset=0
   205  		//       VADDR=0xffffffff81000000
   206  		// stextOffset=0xffffffff81000198
   207  		return start - *stextOffset, true
   208  	}
   209  	return 0, false
   210  }
   211  
   212  // GetBase determines the base address to subtract from virtual
   213  // address to get symbol table address. For an executable, the base
   214  // is 0. Otherwise, it's a shared library, and the base is the
   215  // address where the mapping starts. The kernel needs special handling.
   216  func GetBase(fh *elf.FileHeader, loadSegment *elf.ProgHeader, stextOffset *uint64, start, limit, offset uint64) (uint64, error) {
   217  
   218  	if start == 0 && offset == 0 && (limit == ^uint64(0) || limit == 0) {
   219  		// Some tools may introduce a fake mapping that spans the entire
   220  		// address space. Assume that the address has already been
   221  		// adjusted, so no additional base adjustment is necessary.
   222  		return 0, nil
   223  	}
   224  
   225  	switch fh.Type {
   226  	case elf.ET_EXEC:
   227  		if loadSegment == nil {
   228  			// Assume fixed-address executable and so no adjustment.
   229  			return 0, nil
   230  		}
   231  		if stextOffset == nil && start > 0 && start < 0x8000000000000000 {
   232  			// A regular user-mode executable. Compute the base offset using same
   233  			// arithmetics as in ET_DYN case below, see the explanation there.
   234  			// Ideally, the condition would just be "stextOffset == nil" as that
   235  			// represents the address of _stext symbol in the vmlinux image. Alas,
   236  			// the caller may skip reading it from the binary (it's expensive to scan
   237  			// all the symbols) and so it may be nil even for the kernel executable.
   238  			// So additionally check that the start is within the user-mode half of
   239  			// the 64-bit address space.
   240  			return start - offset + loadSegment.Off - loadSegment.Vaddr, nil
   241  		}
   242  		// Various kernel heuristics and cases are handled separately.
   243  		if base, match := kernelBase(loadSegment, stextOffset, start, limit, offset); match {
   244  			return base, nil
   245  		}
   246  		// ChromeOS can remap its kernel to 0, and the caller might have not found
   247  		// the _stext symbol. Split this case from kernelBase() above, since we don't
   248  		// want to apply it to an ET_DYN user-mode executable.
   249  		if start == 0 && limit != 0 && stextOffset == nil {
   250  			return start - loadSegment.Vaddr, nil
   251  		}
   252  
   253  		return 0, fmt.Errorf("don't know how to handle EXEC segment: %v start=0x%x limit=0x%x offset=0x%x", *loadSegment, start, limit, offset)
   254  	case elf.ET_REL:
   255  		if offset != 0 {
   256  			return 0, fmt.Errorf("don't know how to handle mapping.Offset")
   257  		}
   258  		return start, nil
   259  	case elf.ET_DYN:
   260  		// The process mapping information, start = start of virtual address range,
   261  		// and offset = offset in the executable file of the start address, tells us
   262  		// that a runtime virtual address x maps to a file offset
   263  		// fx = x - start + offset.
   264  		if loadSegment == nil {
   265  			return start - offset, nil
   266  		}
   267  		// Kernels compiled as PIE can be ET_DYN as well. Use heuristic, similar to
   268  		// the ET_EXEC case above.
   269  		if base, match := kernelBase(loadSegment, stextOffset, start, limit, offset); match {
   270  			return base, nil
   271  		}
   272  		// The program header, if not nil, indicates the offset in the file where
   273  		// the executable segment is located (loadSegment.Off), and the base virtual
   274  		// address where the first byte of the segment is loaded
   275  		// (loadSegment.Vaddr). A file offset fx maps to a virtual (symbol) address
   276  		// sx = fx - loadSegment.Off + loadSegment.Vaddr.
   277  		//
   278  		// Thus, a runtime virtual address x maps to a symbol address
   279  		// sx = x - start + offset - loadSegment.Off + loadSegment.Vaddr.
   280  		return start - offset + loadSegment.Off - loadSegment.Vaddr, nil
   281  	}
   282  	return 0, fmt.Errorf("don't know how to handle FileHeader.Type %v", fh.Type)
   283  }
   284  
   285  // FindTextProgHeader finds the program segment header containing the .text
   286  // section or nil if the segment cannot be found.
   287  func FindTextProgHeader(f *elf.File) *elf.ProgHeader {
   288  	for _, s := range f.Sections {
   289  		if s.Name == ".text" {
   290  			// Find the LOAD segment containing the .text section.
   291  			for _, p := range f.Progs {
   292  				if p.Type == elf.PT_LOAD && p.Flags&elf.PF_X != 0 && s.Addr >= p.Vaddr && s.Addr < p.Vaddr+p.Memsz {
   293  					return &p.ProgHeader
   294  				}
   295  			}
   296  		}
   297  	}
   298  	return nil
   299  }
   300  
   301  // ProgramHeadersForMapping returns the program segment headers that overlap
   302  // the runtime mapping with file offset mapOff and memory size mapSz. We skip
   303  // over segments zero file size because their file offset values are unreliable.
   304  // Even if overlapping, a segment is not selected if its aligned file offset is
   305  // greater than the mapping file offset, or if the mapping includes the last
   306  // page of the segment, but not the full segment and the mapping includes
   307  // additional pages after the segment end.
   308  // The function returns a slice of pointers to the headers in the input
   309  // slice, which are valid only while phdrs is not modified or discarded.
   310  func ProgramHeadersForMapping(phdrs []elf.ProgHeader, mapOff, mapSz uint64) []*elf.ProgHeader {
   311  	const (
   312  		// pageSize defines the virtual memory page size used by the loader. This
   313  		// value is dependent on the memory management unit of the CPU. The page
   314  		// size is 4KB virtually on all the architectures that we care about, so we
   315  		// define this metric as a constant. If we encounter architectures where
   316  		// page sie is not 4KB, we must try to guess the page size on the system
   317  		// where the profile was collected, possibly using the architecture
   318  		// specified in the ELF file header.
   319  		pageSize       = 4096
   320  		pageOffsetMask = pageSize - 1
   321  	)
   322  	mapLimit := mapOff + mapSz
   323  	var headers []*elf.ProgHeader
   324  	for i := range phdrs {
   325  		p := &phdrs[i]
   326  		// Skip over segments with zero file size. Their file offsets can have
   327  		// arbitrary values, see b/195427553.
   328  		if p.Filesz == 0 {
   329  			continue
   330  		}
   331  		segLimit := p.Off + p.Memsz
   332  		// The segment must overlap the mapping.
   333  		if p.Type == elf.PT_LOAD && mapOff < segLimit && p.Off < mapLimit {
   334  			// If the mapping offset is strictly less than the page aligned segment
   335  			// offset, then this mapping comes from a different segment, fixes
   336  			// b/179920361.
   337  			alignedSegOffset := uint64(0)
   338  			if p.Off > (p.Vaddr & pageOffsetMask) {
   339  				alignedSegOffset = p.Off - (p.Vaddr & pageOffsetMask)
   340  			}
   341  			if mapOff < alignedSegOffset {
   342  				continue
   343  			}
   344  			// If the mapping starts in the middle of the segment, it covers less than
   345  			// one page of the segment, and it extends at least one page past the
   346  			// segment, then this mapping comes from a different segment.
   347  			if mapOff > p.Off && (segLimit < mapOff+pageSize) && (mapLimit >= segLimit+pageSize) {
   348  				continue
   349  			}
   350  			headers = append(headers, p)
   351  		}
   352  	}
   353  	return headers
   354  }
   355  
   356  // HeaderForFileOffset attempts to identify a unique program header that
   357  // includes the given file offset. It returns an error if it cannot identify a
   358  // unique header.
   359  func HeaderForFileOffset(headers []*elf.ProgHeader, fileOffset uint64) (*elf.ProgHeader, error) {
   360  	var ph *elf.ProgHeader
   361  	for _, h := range headers {
   362  		if fileOffset >= h.Off && fileOffset < h.Off+h.Memsz {
   363  			if ph != nil {
   364  				// Assuming no other bugs, this can only happen if we have two or
   365  				// more small program segments that fit on the same page, and a
   366  				// segment other than the last one includes uninitialized data, or
   367  				// if the debug binary used for symbolization is stripped of some
   368  				// sections, so segment file sizes are smaller than memory sizes.
   369  				return nil, fmt.Errorf("found second program header (%#v) that matches file offset %x, first program header is %#v. Is this a stripped binary, or does the first program segment contain uninitialized data?", *h, fileOffset, *ph)
   370  			}
   371  			ph = h
   372  		}
   373  	}
   374  	if ph == nil {
   375  		return nil, fmt.Errorf("no program header matches file offset %x", fileOffset)
   376  	}
   377  	return ph, nil
   378  }
   379
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