Handle shared libraries with nonzero program headers. (#119)

Generalize GetBase for shared libraries to handle cases when both process
mapping offset and program header addresses are non-zero.

Based on process mapping information, a sample at a virtual address x maps to
a file offset fx = x - map_start + map_offset.

The program header for the segment with the .text section may have non-zero
mapping information. Thus, a file offset fx maps to a symbol address
sx = fx - ph_offset + ph_virt_addr.

Thus, sx = x - map_start + map_offset - ph_offset + ph_virt_addr, and the base
address for symbolization is map_start - map_offset + ph_offset - ph_virt_addr.

internal/elfexec/elfexec.go

 		}
 		return start, nil
 	case elf.ET_DYN:
-		if offset != 0 {
-			if loadSegment == nil || loadSegment.Vaddr == 0 {
-				return start - offset, nil
-			}
-			return 0, fmt.Errorf("Don't know how to handle mapping. Offset=%x, vaddr=%x",
-				offset, loadSegment.Vaddr)
-		}
+		// The process mapping information, start = start of virtual address range,
+		// and offset = offset in the executable file of the start address, tells us
+		// that a runtime virtual address x maps to a file offset
+		// fx = x - start + offset.
 		if loadSegment == nil {
-			return start, nil
+			return start - offset, nil
 		}
-		return start - loadSegment.Vaddr, nil
+		// The program header, if not nil, indicates the offset in the file where
+		// the executable segment is located (loadSegment.Off), and the base virtual
+		// address where the first byte of the segment is loaded
+		// (loadSegment.Vaddr). A file offset fx maps to a virtual (symbol) address
+		// sx = fx - loadSegment.Off + loadSegment.Vaddr.
+		//
+		// Thus, a runtime virtual address x maps to a symbol address
+		// sx = x - start + offset - loadSegment.Off + loadSegment.Vaddr.
+		return start - offset + loadSegment.Off - loadSegment.Vaddr, nil
 	}
 	return 0, fmt.Errorf("Don't know how to handle FileHeader.Type %v", fh.Type)
 }

internal/elfexec/elfexec_test.go

 		{"exec chromeos kernel 4", fhExec, kernelHeader, uint64p(0xffffffff81200198), 0x198, 0x100000, 0, 0x7ee00000, false},
 		{"exec chromeos kernel unremapped", fhExec, kernelHeader, uint64p(0xffffffff810001c8), 0xffffffff834001c8, 0xffffffffc0000000, 0xffffffff834001c8, 0x2400000, false},
 		{"dyn", fhDyn, nil, nil, 0x200000, 0x300000, 0, 0x200000, false},
-		{"dyn offset", fhDyn, lsOffset, nil, 0x0, 0x300000, 0, 0xFFFFFFFFFFC00000, false},
+		{"dyn map", fhDyn, lsOffset, nil, 0x0, 0x300000, 0, 0xFFFFFFFFFFE00000, false},
 		{"dyn nomap", fhDyn, nil, nil, 0x0, 0x0, 0, 0, false},
+		{"dyn map+offset", fhDyn, lsOffset, nil, 0x900000, 0xa00000, 0x200000, 0x500000, false},
 		{"rel", fhRel, nil, nil, 0x2000000, 0x3000000, 0, 0x2000000, false},
 		{"rel nomap", fhRel, nil, nil, 0x0, ^uint64(0), 0, 0, false},
 		{"rel offset", fhRel, nil, nil, 0x100000, 0x200000, 0x1, 0, true},