I was looking at a report where pprof wouldn't symbolize the data
collected for a Chrome binary using Linux perf (b/112303003). The mmap
information is:

start: 0000000002, limit: 0000000006, offset: 0000000002

The ELF file header:

elf.FileHeader{Class:elf.ELFCLASS64, Data:elf.ELFDATA2LSB, Version:elf.EV_CURRENT, OSABI:elf.ELFOSABI_NONE, ABIVersion:0x0, ByteOrder:binary.LittleEndian, Type:elf.ET_EXEC, Machine:elf.EM_X86_64, Entry:0x272e000}

The code segment:

elf.ProgHeader{Type:elf.PT_LOAD, Flags:elf.PF_X+elf.PF_R, Off:0x252f000,
Vaddr:0x272e000, Paddr:0x272e000, Filesz:0x43da610, Memsz:0x43da610,
Align:0x1000}

The dynamic loader here mapped 0x6b09000-0x272e000 = 0x43db000 bytes
starting 0x252f000 file offset into 0x272e000 virtual address, exactly
as instructed by the program header (so, no ASLR). Thus, the base
adjustment should be zero. Yet, the current GetBase produced the base of
0x252f000 which is wrong. The reason for that is that the ET_EXEC branch
of GetBase doesn't handle the general case of non-zero mmap file offset,
but rather only supports a couple of special cases. This change makes
handling the case of user-mode ET_EXEC more generic.