Commit 9f93f3e9 authored Oct 17, 2015 by Jungseok Lee Committed by Catalin Marinas Oct 19, 2015

arm64: Synchronise dump_backtrace() with perf callchain



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>

parent 096b3224

arch/arm64/kernel/traps.c

+10 −5

Original line number	Diff line number	Diff line
		@@ -103,12 +103,12 @@ static void dump_mem(const char lvl, const char str, unsigned long bottom,
		set_fs(fs);
		}

		static void dump_backtrace_entry(unsigned long where, unsigned long stack)
		static void dump_backtrace_entry(unsigned long where)
		{
		/*
		* Note that 'where' can have a physical address, but it's not handled.
		*/
		print_ip_sym(where);
		if (in_exception_text(where))
		dump_mem("", "Exception stack", stack,
		stack + sizeof(struct pt_regs), false);
		}

		static void dump_instr(const char lvl, struct pt_regs regs)
		@@ -172,12 +172,17 @@ static void dump_backtrace(struct pt_regs regs, struct task_struct tsk)
		pr_emerg("Call trace:\n");
		while (1) {
		unsigned long where = frame.pc;
		unsigned long stack;
		int ret;

		dump_backtrace_entry(where);
		ret = unwind_frame(&frame);
		if (ret < 0)
		break;
		dump_backtrace_entry(where, frame.sp);
		stack = frame.sp;
		if (in_exception_text(where))
		dump_mem("", "Exception stack", stack,
		stack + sizeof(struct pt_regs), false);
		}
		}

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