[llvm-dev] PDB questions

[Zachary Turner via llvm-dev]

For the first and third questions, the easiest thing to do would be run
llvm-pdbutil under a debugger and step through the code. Code that looks
simple and innocuous can often have a lot of stuff hidden behind it. For
example you could step through that loop that iterates the debug
subsections and look at the value of Reader.getOffset() every time, and see
if it matches with your own code (probably it doesn’t). Or you could dump
the entire contents of the C13Substrem and see if the bytes match up
between your own implementation. It looks like you’re reading all 0s, so
maybe you’re just not even reading the right data.

For the second question, unfortunately I don’t know of a better way. If
the/names stream starts with a magic header, maybe you could walk each
stream looking for that. But it maybe possible to have a rare false
positive that way.

BTW, have you considered just using llvm’s library rather than porting it?
It certainly seems like less work
On Thu, Aug 30, 2018 at 11:49 PM Andrew Kelley <superjoe30 at gmail.com> wrote:

> One more:
>
> 3. In the purpose of mapping source file index to string, I found this
> code:
>
> Expected<codeview::DebugChecksumsSubsectionRef>
> ModuleDebugStreamRef::findChecksumsSubsection() const {
>   codeview::DebugChecksumsSubsectionRef Result;
>   for (const auto &SS : subsections()) {
>     if (SS.kind() != DebugSubsectionKind::FileChecksums)
>       continue;
>
>     if (auto EC = Result.initialize(SS.getRecordData()))
>       return std::move(EC);
>     return Result;
>   }
>   return Result;
> }
>
> Subsections() is populated here:
>
>   if (auto EC = Reader.readSubstream(C13LinesSubstream, C13Size))
>     return EC;
>
>   BinaryStreamReader SymbolReader(SymbolsSubstream.StreamData);
>   if (auto EC =
>           SymbolReader.readArray(SymbolArray,
> SymbolReader.bytesRemaining()))
>     return EC;
>
>   BinaryStreamReader SubsectionsReader(C13LinesSubstream.StreamData);
>   if (auto EC = SubsectionsReader.readArray(Subsections,
>
> SubsectionsReader.bytesRemaining()))
>     return EC;
>
> So it looks like there should be one of these just after the C13Lines
> substream:
>
> struct DebugSubsectionHeader {
>   support::ulittle32_t Kind;   // codeview::DebugSubsectionKind enum
>   support::ulittle32_t Length; // number of bytes occupied by this record.
> };
>
> But when I look there with my own code I only see zeroes:
>
> read C13 line info 142964 bytes
> DebugSubsectionHeader{ .Kind = DebugSubsectionKind.None, .Length = 0 }
> DebugSubsectionHeader{ .Kind = DebugSubsectionKind.None, .Length = 0 }
> DebugSubsectionHeader{ .Kind = DebugSubsectionKind.None, .Length = 0 }
> DebugSubsectionHeader{ .Kind = DebugSubsectionKind.None, .Length = 0 }
> DebugSubsectionHeader{ .Kind = DebugSubsectionKind.None, .Length = 0 }
> DebugSubsectionHeader{ .Kind = DebugSubsectionKind.None, .Length = 0 }
> DebugSubsectionHeader{ .Kind = DebugSubsectionKind.None, .Length = 0 }
> <repeats until end of stream>
>
> Any clues?
>
>
> On Fri, Aug 31, 2018 at 2:17 AM Andrew Kelley <superjoe30 at gmail.com>
> wrote:
>
>> Zachary,
>>
>> Thanks for the help on IRC earlier. I've got code that can capture a
>> stack trace and then discover for each address, its module, function,
>> source index, line, and column.
>>
>> I still have a couple of loose ends though. Do you know what's going on
>> here?
>>
>> 1. There appears to be 8 bytes before every LineFragmentHeader. Here's
>> some of my own debug output, which matches llvm-pdbutil's output. You can
>> see it says "unknown bytes: ...".
>>
>> read C13 line info 136720 bytes
>> unknown bytes: f2 00 00 00  60 00 00 00
>> LineFragmentHeader{ .RelocOffset = 0, .RelocSegment = 5, .Flags =
>> LineFlags{ .LF_HaveColumns = true, .unused = 0 }, .CodeSize = 52 }
>> has column: true
>> LineBlockFragmentHeader{ .NameIndex = 0, .NumLines = 6, .BlockSize = 84 }
>> LineNumberEntry{ .Offset = 0, .Flags = 101 } Flags{ .Start = 101, .End =
>> 17, .IsStatement = false }
>> <snip some LineNumberEntry's>
>> ColumnNumberEntry{ .StartColumn = 5, .EndColumn = 0 }
>> ColumnNumberEntry{ .StartColumn = 30, .EndColumn = 0 }
>> unknown bytes: f2 00 00 00  f0 00 00 00
>> LineFragmentHeader{ .RelocOffset = 64, .RelocSegment = 5, .Flags =
>> LineFlags{ .LF_HaveColumns = true, .unused = 0 }, .CodeSize = 366 }
>> has column: true
>> LineBlockFragmentHeader{ .NameIndex = 8, .NumLines = 18, .BlockSize = 228
>> }
>> LineNumberEntry{ .Offset = 0, .Flags = 53 } Flags{ .Start = 53, .End =
>> 20, .IsStatement = false }
>> LineNumberEntry{ .Offset = 20, .Flags = 54 } Flags{ .Start = 54, .End =
>> 24, .IsStatement = false }
>> <etc>
>>
>> Do you know what's going on with these 8 bytes? I have scoured
>> llvm-pdbutil's source but I cannot find where these bytes are coming from.
>>
>> 2. Is there a simpler way to find out which is the /names (string table)
>> stream index without porting the entire hash table implementation?
>>
>>
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