[BUG] gcov causes vread_tsc to increment kernel memory

[Steven Rostedt]

On bootup of the latest kernel my init segfaults. Debugging it, I found 
that vread_tsc (a vsyscall) increments some strange kernel memory:

0000000000000000 <vread_tsc>:
   0:   55                      push   %rbp
   1:   48 ff 05 00 00 00 00    incq   0(%rip)        # 8 <vread_tsc+0x8>
                        4: R_X86_64_PC32        .bss+0x3c
   8:   48 89 e5                mov    %rsp,%rbp
   b:   66 66 90                xchg   %ax,%ax
   e:   48 ff 05 00 00 00 00    incq   0(%rip)        # 15 <vread_tsc+0x15>
                        11: R_X86_64_PC32       .bss+0x44
  15:   66 66 90                xchg   %ax,%ax
  18:   48 ff 05 00 00 00 00    incq   0(%rip)        # 1f <vread_tsc+0x1f>
                        1b: R_X86_64_PC32       .bss+0x4c
  1f:   0f 31                   rdtsc  
	

Those "incq" is very bad to happen in vsyscall memory, since userspace can 
not modify it. You need to make something prevent profiling of vsyscall 
memory (like I do with ftrace).

-- Steve

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