string.go

     1  // Copyright 2016 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 pe
     6  
     7  import (
     8  	"bytes"
     9  	"encoding/binary"
    10  	"fmt"
    11  	"io"
    12  )
    13  
    14  // cstring converts ASCII byte sequence b to string.
    15  // It stops once it finds 0 or reaches end of b.
    16  func cstring(b []byte) string {
    17  	i := bytes.IndexByte(b, 0)
    18  	if i == -1 {
    19  		i = len(b)
    20  	}
    21  	return string(b[:i])
    22  }
    23  
    24  // StringTable is a COFF string table.
    25  type StringTable []byte
    26  
    27  func readStringTable(fh *FileHeader, r io.ReadSeeker) (StringTable, error) {
    28  	// COFF string table is located right after COFF symbol table.
    29  	if fh.PointerToSymbolTable <= 0 {
    30  		return nil, nil
    31  	}
    32  	offset := fh.PointerToSymbolTable + COFFSymbolSize*fh.NumberOfSymbols
    33  	_, err := r.Seek(int64(offset), seekStart)
    34  	if err != nil {
    35  		return nil, fmt.Errorf("fail to seek to string table: %v", err)
    36  	}
    37  	var l uint32
    38  	err = binary.Read(r, binary.LittleEndian, &l)
    39  	if err != nil {
    40  		return nil, fmt.Errorf("fail to read string table length: %v", err)
    41  	}
    42  	// string table length includes itself
    43  	if l <= 4 {
    44  		return nil, nil
    45  	}
    46  	l -= 4
    47  	buf := make([]byte, l)
    48  	_, err = io.ReadFull(r, buf)
    49  	if err != nil {
    50  		return nil, fmt.Errorf("fail to read string table: %v", err)
    51  	}
    52  	return StringTable(buf), nil
    53  }
    54  
    55  // TODO(brainman): decide if start parameter should be int instead of uint32
    56  
    57  // String extracts string from COFF string table st at offset start.
    58  func (st StringTable) String(start uint32) (string, error) {
    59  	// start includes 4 bytes of string table length
    60  	if start < 4 {
    61  		return "", fmt.Errorf("offset %d is before the start of string table", start)
    62  	}
    63  	start -= 4
    64  	if int(start) > len(st) {
    65  		return "", fmt.Errorf("offset %d is beyond the end of string table", start)
    66  	}
    67  	return cstring(st[start:]), nil
    68  }
    69
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