[perf tool] record failure with 3.14-rc4

[Will Deacon]

Hi guys,

If I try to run perf record as a non-root user, I end up with the following
(unhelpful) error:

  $ perf record -e cycles ls
  [...]
  Not enough memory for reading perf file header

This is because the addresses in /proc/kallsyms always read as 0x0 when
viewed by a non-privileged user, causing kallsyms__get_function_start to
return 0x0 in args.start. machine__create_kernel_maps then treats this as
an error an returns -1 to perf_session__create_kernel_maps, causing
perf_session__new to fail and perf to exit.

The perf tool code in 3.13 is perfectly happy creating maps at 0x0, but I
can see this has changed quite substantially in the recent merge window.

Any ideas? We could fix kallsyms__get_function_start to return < 0 on
failure then fix the checks in the callers. Adrian?

Will
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