Unlike perf callchain relying on walk_stackframe(), dump_backtrace()
has its own backtrace logic. A major difference between them is the
moment a symbol is recorded. Perf writes down a symbol *before*
calling unwind_frame(), but dump_backtrace() prints it out *after*
unwind_frame(). As a result, the last valid symbol cannot be hooked
in case of dump_backtrace(). This patch addresses the issue as
synchronising dump_backtrace() with perf callchain.

A simple test and its results are as follows:

- crash trigger

 $ sudo echo c > /proc/sysrq-trigger

- current status

 Call trace:
 [<fffffe00003dc738>] sysrq_handle_crash+0x24/0x30
 [<fffffe00003dd2ac>] __handle_sysrq+0x128/0x19c
 [<fffffe00003dd730>] write_sysrq_trigger+0x60/0x74
 [<fffffe0000249fc4>] proc_reg_write+0x84/0xc0
 [<fffffe00001f2638>] __vfs_write+0x44/0x104
 [<fffffe00001f2e60>] vfs_write+0x98/0x1a8
 [<fffffe00001f3730>] SyS_write+0x50/0xb0

- with this change

 Call trace:
 [<fffffe00003dc738>] sysrq_handle_crash+0x24/0x30
 [<fffffe00003dd2ac>] __handle_sysrq+0x128/0x19c
 [<fffffe00003dd730>] write_sysrq_trigger+0x60/0x74
 [<fffffe0000249fc4>] proc_reg_write+0x84/0xc0
 [<fffffe00001f2638>] __vfs_write+0x44/0x104
 [<fffffe00001f2e60>] vfs_write+0x98/0x1a8
 [<fffffe00001f3730>] SyS_write+0x50/0xb0
 [<fffffe00000939ec>] el0_svc_naked+0x20/0x28

Note that this patch does not cover a case where MMU is disabled. The
last stack frame of swapper, for example, has PC in a form of physical
address. Unfortunately, a simple conversion using phys_to_virt() cannot
cover all scenarios since PC is retrieved from LR - 4, not LR. It is
a big tradeoff to change both head.S and unwind_frame() for only a few
of symbols in *.S. Thus, this hunk does not take care of the case.

Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: James Morse <james.morse@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jungseok Lee <jungseoklee85@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>