Fix bug in DWARF v4 location decoding

This fixes a long standing bug in the DWARF v4 location decoding logic.
Previously, the logic would not recognize "base address selection"
entries at all and assume an entry was either an end-of-list or a
location list entry thus resulting in incorrect decoding in release
builds, and an assert in debug builds.

See DWARF v4 spec section 2.6.2 for details.

2 files changed, 43 insertions, 4 deletions

diff --git a/src/readDwarf.cpp b/src/readDwarf.cpp
index 8e4ca4d..6562980 100644
--- a/src/readDwarf.cpp
+++ b/src/readDwarf.cpp
@@ -414,9 +414,12 @@ void mergeSpecification(DWARF_InfoData& id, const CV2PDB& context)
 LOCCursor::LOCCursor(const DIECursor& parent, unsigned long off)

 	: parent(parent)

 {

+	// Default the base address to the compilation unit (DWARF v4 2.6.2)

 	base = parent.cu->base_address;

 	isLocLists = (parent.cu->version >= 5);

 

+	// DWARF v4 uses .debug_loc, DWARF v5 uses .debug_loclists with a different

+	// schema.

 	const PESection& sec = isLocLists ? parent.img->debug_loclists : parent.img->debug_loc;

 	ptr = sec.byteAt(off);

 	end = sec.endByte();

@@ -426,6 +429,8 @@ bool LOCCursor::readNext(LOCEntry& entry)
 {

 	if (isLocLists)

 	{

+		// DWARF v5 location list parsing.

+

 		if (parent.debug & DbgDwarfLocLists)

 			fprintf(stderr, "%s:%d: loclists off=%x DIEoff=%x:\n", __FUNCTION__, __LINE__,

 					parent.img->debug_loclists.sectOff(ptr), parent.entryOff);

@@ -488,6 +493,8 @@ bool LOCCursor::readNext(LOCEntry& entry)
 	}

 	else

 	{

+		// The logic here is goverened by DWARF4 section 2.6.2.

+

 		if (ptr >= end)

 			return false;

 

@@ -495,10 +502,28 @@ bool LOCCursor::readNext(LOCEntry& entry)
 			fprintf(stderr, "%s:%d: loclist off=%x DIEoff=%x:\n", __FUNCTION__, __LINE__,

 					parent.img->debug_loc.sectOff(ptr), parent.entryOff);

 

+		// Extract the begin and end offset

+		// TODO: Why is this truncating to 32 bit?

 		entry.beg_offset = (unsigned long) parent.RDAddr(ptr);

 		entry.end_offset = (unsigned long) parent.RDAddr(ptr);

-		if (!entry.beg_offset && !entry.end_offset)

+

+		// Check for a base-address-selection entry.

+		if (entry.beg_offset == -1U) {

+			// This is a base address selection entry and thus has no location

+			// description.

+			// Update the base address with this entry's value.

+			base = entry.end_offset;

+

+			// Continue the scan, but don't try to decode further since there

+			// are no location description records following this type of entry.

+			return true;

+		}

+

+		// Check for end-of-list entry. (Both offsets 0)

+		if (!entry.beg_offset && !entry.end_offset) {

+			// Terminate the scan.

 			return false;

+		}

 

 		DWARF_Attribute attr;

 		attr.type = Block;

diff --git a/src/readDwarf.h b/src/readDwarf.h
index 04afa38..4b0ac98 100644
--- a/src/readDwarf.h
+++ b/src/readDwarf.h
@@ -470,9 +470,15 @@ struct Location
 class LOCEntry

 {

 public:

-	unsigned long beg_offset;

-	unsigned long end_offset;

+	// TODO: investigate making these 64bit (or vary). Also consider renaming

+	// to Value0 and Value1 since their meanings varies depending on entry type.

+	unsigned long beg_offset; // or -1U for base address selection entries

+	unsigned long end_offset; // or the base address in base address selection entries

+

+	// DWARF v5 only. See DW_LLE_default_location.

 	bool isDefault;

+

+	// Location description.

 	Location loc;

 

 	void addBase(uint32_t base)

@@ -482,16 +488,24 @@ public:
 	}

 };

 

-// Location list cursor

+// Location list cursor (see DWARF v4 and v5 Section 2.6).

 class LOCCursor

 {

 public:

 	LOCCursor(const DIECursor& parent, unsigned long off);

 

 	const DIECursor& parent;

+

+	// The base address for subsequent loc list entries read in a given list.

+	// Default to the CU base in the absense of any base address selection entries.

+	//

+	// TODO: So far we only assign to this but never actually use it.

 	uint32_t base;

+

 	byte* end;

 	byte* ptr;

+

+	// Is this image using the new debug_loclists section in DWARF v5?

 	bool isLocLists;

 

 	bool readNext(LOCEntry& entry);