cmd/internal/objfile: align the load address of ELF binary

The ELF ABI just requires that the address and the offset of a
segment are congruent modulo the alignment, but does not require
the start address to be aligned. While usually the segment's
start address is aligned, apparently the zig linker generates
binary with unaligned address.

At the run time, the memory mapping that contains the segment
starts at an aligned address (rounding down). Use the aligned
address for the load address, which matches the mapping.
Apparently this is what the pprof library expects.

Fixes #59466.

Change-Id: Ife78909b20b7bc975ac4c76f2c5f5db325ddec9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/483035
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>

Author: cherrymui

src/cmd/internal/objfile/elf.go

@@ -134,7 +134,12 @@ func (f *elfFile) goarch() string {
 func (f *elfFile) loadAddress() (uint64, error) {
 	for _, p := range f.elf.Progs {
 		if p.Type == elf.PT_LOAD && p.Flags&elf.PF_X != 0 {
-			return p.Vaddr, nil
+			// The memory mapping that contains the segment
+			// starts at an aligned address. Apparently this
+			// is what pprof expects, as it uses this and the
+			// start address of the mapping to compute PC
+			// delta.
+			return p.Vaddr - p.Vaddr%p.Align, nil
 		}
 	}
 	return 0, fmt.Errorf("unknown load address")