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      1 //===--- HeaderMap.cpp - A file that acts like dir of symlinks ------------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file implements the HeaderMap interface.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #include "clang/Lex/HeaderMap.h"
     15 #include "clang/Basic/CharInfo.h"
     16 #include "clang/Basic/FileManager.h"
     17 #include "llvm/ADT/SmallString.h"
     18 #include "llvm/Support/DataTypes.h"
     19 #include "llvm/Support/MathExtras.h"
     20 #include "llvm/Support/MemoryBuffer.h"
     21 #include <cstdio>
     22 #include <memory>
     23 using namespace clang;
     24 
     25 //===----------------------------------------------------------------------===//
     26 // Data Structures and Manifest Constants
     27 //===----------------------------------------------------------------------===//
     28 
     29 enum {
     30   HMAP_HeaderMagicNumber = ('h' << 24) | ('m' << 16) | ('a' << 8) | 'p',
     31   HMAP_HeaderVersion = 1,
     32 
     33   HMAP_EmptyBucketKey = 0
     34 };
     35 
     36 namespace clang {
     37 struct HMapBucket {
     38   uint32_t Key;          // Offset (into strings) of key.
     39 
     40   uint32_t Prefix;     // Offset (into strings) of value prefix.
     41   uint32_t Suffix;     // Offset (into strings) of value suffix.
     42 };
     43 
     44 struct HMapHeader {
     45   uint32_t Magic;           // Magic word, also indicates byte order.
     46   uint16_t Version;         // Version number -- currently 1.
     47   uint16_t Reserved;        // Reserved for future use - zero for now.
     48   uint32_t StringsOffset;   // Offset to start of string pool.
     49   uint32_t NumEntries;      // Number of entries in the string table.
     50   uint32_t NumBuckets;      // Number of buckets (always a power of 2).
     51   uint32_t MaxValueLength;  // Length of longest result path (excluding nul).
     52   // An array of 'NumBuckets' HMapBucket objects follows this header.
     53   // Strings follow the buckets, at StringsOffset.
     54 };
     55 } // end namespace clang.
     56 
     57 /// HashHMapKey - This is the 'well known' hash function required by the file
     58 /// format, used to look up keys in the hash table.  The hash table uses simple
     59 /// linear probing based on this function.
     60 static inline unsigned HashHMapKey(StringRef Str) {
     61   unsigned Result = 0;
     62   const char *S = Str.begin(), *End = Str.end();
     63 
     64   for (; S != End; S++)
     65     Result += toLowercase(*S) * 13;
     66   return Result;
     67 }
     68 
     69 
     70 
     71 //===----------------------------------------------------------------------===//
     72 // Verification and Construction
     73 //===----------------------------------------------------------------------===//
     74 
     75 /// HeaderMap::Create - This attempts to load the specified file as a header
     76 /// map.  If it doesn't look like a HeaderMap, it gives up and returns null.
     77 /// If it looks like a HeaderMap but is obviously corrupted, it puts a reason
     78 /// into the string error argument and returns null.
     79 const HeaderMap *HeaderMap::Create(const FileEntry *FE, FileManager &FM) {
     80   // If the file is too small to be a header map, ignore it.
     81   unsigned FileSize = FE->getSize();
     82   if (FileSize <= sizeof(HMapHeader)) return nullptr;
     83 
     84   auto FileBuffer = FM.getBufferForFile(FE);
     85   if (!FileBuffer) return nullptr;  // Unreadable file?
     86   const char *FileStart = (*FileBuffer)->getBufferStart();
     87 
     88   // We know the file is at least as big as the header, check it now.
     89   const HMapHeader *Header = reinterpret_cast<const HMapHeader*>(FileStart);
     90 
     91   // Sniff it to see if it's a headermap by checking the magic number and
     92   // version.
     93   bool NeedsByteSwap;
     94   if (Header->Magic == HMAP_HeaderMagicNumber &&
     95       Header->Version == HMAP_HeaderVersion)
     96     NeedsByteSwap = false;
     97   else if (Header->Magic == llvm::ByteSwap_32(HMAP_HeaderMagicNumber) &&
     98            Header->Version == llvm::ByteSwap_16(HMAP_HeaderVersion))
     99     NeedsByteSwap = true;  // Mixed endianness headermap.
    100   else
    101     return nullptr;  // Not a header map.
    102 
    103   if (Header->Reserved != 0) return nullptr;
    104 
    105   // Okay, everything looks good, create the header map.
    106   return new HeaderMap(std::move(*FileBuffer), NeedsByteSwap);
    107 }
    108 
    109 //===----------------------------------------------------------------------===//
    110 //  Utility Methods
    111 //===----------------------------------------------------------------------===//
    112 
    113 
    114 /// getFileName - Return the filename of the headermap.
    115 const char *HeaderMap::getFileName() const {
    116   return FileBuffer->getBufferIdentifier();
    117 }
    118 
    119 unsigned HeaderMap::getEndianAdjustedWord(unsigned X) const {
    120   if (!NeedsBSwap) return X;
    121   return llvm::ByteSwap_32(X);
    122 }
    123 
    124 /// getHeader - Return a reference to the file header, in unbyte-swapped form.
    125 /// This method cannot fail.
    126 const HMapHeader &HeaderMap::getHeader() const {
    127   // We know the file is at least as big as the header.  Return it.
    128   return *reinterpret_cast<const HMapHeader*>(FileBuffer->getBufferStart());
    129 }
    130 
    131 /// getBucket - Return the specified hash table bucket from the header map,
    132 /// bswap'ing its fields as appropriate.  If the bucket number is not valid,
    133 /// this return a bucket with an empty key (0).
    134 HMapBucket HeaderMap::getBucket(unsigned BucketNo) const {
    135   HMapBucket Result;
    136   Result.Key = HMAP_EmptyBucketKey;
    137 
    138   const HMapBucket *BucketArray =
    139     reinterpret_cast<const HMapBucket*>(FileBuffer->getBufferStart() +
    140                                         sizeof(HMapHeader));
    141 
    142   const HMapBucket *BucketPtr = BucketArray+BucketNo;
    143   if ((const char*)(BucketPtr+1) > FileBuffer->getBufferEnd()) {
    144     Result.Prefix = 0;
    145     Result.Suffix = 0;
    146     return Result;  // Invalid buffer, corrupt hmap.
    147   }
    148 
    149   // Otherwise, the bucket is valid.  Load the values, bswapping as needed.
    150   Result.Key    = getEndianAdjustedWord(BucketPtr->Key);
    151   Result.Prefix = getEndianAdjustedWord(BucketPtr->Prefix);
    152   Result.Suffix = getEndianAdjustedWord(BucketPtr->Suffix);
    153   return Result;
    154 }
    155 
    156 /// getString - Look up the specified string in the string table.  If the string
    157 /// index is not valid, it returns an empty string.
    158 const char *HeaderMap::getString(unsigned StrTabIdx) const {
    159   // Add the start of the string table to the idx.
    160   StrTabIdx += getEndianAdjustedWord(getHeader().StringsOffset);
    161 
    162   // Check for invalid index.
    163   if (StrTabIdx >= FileBuffer->getBufferSize())
    164     return nullptr;
    165 
    166   // Otherwise, we have a valid pointer into the file.  Just return it.  We know
    167   // that the "string" can not overrun the end of the file, because the buffer
    168   // is nul terminated by virtue of being a MemoryBuffer.
    169   return FileBuffer->getBufferStart()+StrTabIdx;
    170 }
    171 
    172 //===----------------------------------------------------------------------===//
    173 // The Main Drivers
    174 //===----------------------------------------------------------------------===//
    175 
    176 /// dump - Print the contents of this headermap to stderr.
    177 void HeaderMap::dump() const {
    178   const HMapHeader &Hdr = getHeader();
    179   unsigned NumBuckets = getEndianAdjustedWord(Hdr.NumBuckets);
    180 
    181   fprintf(stderr, "Header Map %s:\n  %d buckets, %d entries\n",
    182           getFileName(), NumBuckets,
    183           getEndianAdjustedWord(Hdr.NumEntries));
    184 
    185   for (unsigned i = 0; i != NumBuckets; ++i) {
    186     HMapBucket B = getBucket(i);
    187     if (B.Key == HMAP_EmptyBucketKey) continue;
    188 
    189     const char *Key    = getString(B.Key);
    190     const char *Prefix = getString(B.Prefix);
    191     const char *Suffix = getString(B.Suffix);
    192     fprintf(stderr, "  %d. %s -> '%s' '%s'\n", i, Key, Prefix, Suffix);
    193   }
    194 }
    195 
    196 /// LookupFile - Check to see if the specified relative filename is located in
    197 /// this HeaderMap.  If so, open it and return its FileEntry.
    198 const FileEntry *HeaderMap::LookupFile(
    199     StringRef Filename, FileManager &FM) const {
    200 
    201   SmallString<1024> Path;
    202   StringRef Dest = lookupFilename(Filename, Path);
    203   if (Dest.empty())
    204     return nullptr;
    205 
    206   return FM.getFile(Dest);
    207 }
    208 
    209 StringRef HeaderMap::lookupFilename(StringRef Filename,
    210                                     SmallVectorImpl<char> &DestPath) const {
    211   const HMapHeader &Hdr = getHeader();
    212   unsigned NumBuckets = getEndianAdjustedWord(Hdr.NumBuckets);
    213 
    214   // If the number of buckets is not a power of two, the headermap is corrupt.
    215   // Don't probe infinitely.
    216   if (NumBuckets & (NumBuckets-1))
    217     return StringRef();
    218 
    219   // Linearly probe the hash table.
    220   for (unsigned Bucket = HashHMapKey(Filename);; ++Bucket) {
    221     HMapBucket B = getBucket(Bucket & (NumBuckets-1));
    222     if (B.Key == HMAP_EmptyBucketKey) return StringRef(); // Hash miss.
    223 
    224     // See if the key matches.  If not, probe on.
    225     if (!Filename.equals_lower(getString(B.Key)))
    226       continue;
    227 
    228     // If so, we have a match in the hash table.  Construct the destination
    229     // path.
    230     StringRef Prefix = getString(B.Prefix);
    231     StringRef Suffix = getString(B.Suffix);
    232     DestPath.clear();
    233     DestPath.append(Prefix.begin(), Prefix.end());
    234     DestPath.append(Suffix.begin(), Suffix.end());
    235     return StringRef(DestPath.begin(), DestPath.size());
    236   }
    237 }
    238