Handle PowerPC64 kernel mapping in getBase().

The kernel mapping record has the offset of 0xc000000000000000 on
PowerPC64 along with the same value for the mapping start.  Both values
come from arch/powerpc/Kconfig.  This case is not handled by any of the
conditions in the current getBase() code, so update the existing code
handling the kernel case to handle the PowerPC64 case.

internal/elfexec/elfexec.go

 // use the address of the _stext symbol as the mmap start.  _stext
 // offset can be obtained with `nm vmlinux | grep _stext`
 func GetBase(fh *elf.FileHeader, loadSegment *elf.ProgHeader, stextOffset *uint64, start, limit, offset uint64) (uint64, error) {
-	const pageSize = 4096
+	const (
+		pageSize = 4096
+		// PAGE_OFFSET for PowerPC64, see arch/powerpc/Kconfig in the kernel sources.
+		pageOffsetPpc64 = 0xc000000000000000
+	)
 
 	if start == 0 && offset == 0 &&
 		(limit == ^uint64(0) || limit == 0) {
 		if loadSegment.Vaddr == start-offset {
 			return offset, nil
 		}
-		if start > loadSegment.Vaddr && limit > start && offset == 0 {
+		if start >= loadSegment.Vaddr && limit > start && (offset == 0 || offset == pageOffsetPpc64) {
 			// Some kernels look like:
 			//       VADDR=0xffffffff80200000
 			// stextOffset=0xffffffff80200198
 			//       Start=0xffffffff83200000
 			//       Limit=0xffffffff84200000
-			//      Offset=0
+			//      Offset=0 (0xc000000000000000 for PowerPC64)
 			// So the base should be:
 			if stextOffset != nil && (start%pageSize) == (*stextOffset%pageSize) {
 				// perf uses the address of _stext as start.  Some tools may

internal/elfexec/elfexec_test.go

 	kernelHeader := &elf.ProgHeader{
 		Vaddr: 0xffffffff81000000,
 	}
+	ppc64KernelHeader := &elf.ProgHeader{
+		Vaddr: 0xc000000000000000,
+	}
 
 	testcases := []struct {
 		label                string
 		{"exec offset 2", fhExec, lsOffset, nil, 0x200000, 0x600000, 0, 0, false},
 		{"exec nomap", fhExec, nil, nil, 0, 0, 0, 0, false},
 		{"exec kernel", fhExec, kernelHeader, uint64p(0xffffffff81000198), 0xffffffff82000198, 0xffffffff83000198, 0, 0x1000000, false},
+		{"exec PPC64 kernel", fhExec, ppc64KernelHeader, uint64p(0xc000000000000000), 0xc000000000000000, 0xd00000001a730000, 0xc000000000000000, 0x0, false},
 		{"exec chromeos kernel", fhExec, kernelHeader, uint64p(0xffffffff81000198), 0, 0x10197, 0, 0x7efffe68, false},
 		{"exec chromeos kernel 2", fhExec, kernelHeader, uint64p(0xffffffff81000198), 0, 0x10198, 0, 0x7efffe68, false},
 		{"exec chromeos kernel 3", fhExec, kernelHeader, uint64p(0xffffffff81000198), 0x198, 0x100000, 0, 0x7f000000, false},