Re: [PATCH] arch/arm64 : fix error in dump_backtrace

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Nov 06, 2018 at 03:19:35PM +0800, Zhaoyang Huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> In some cases, the instruction of "bl foo1" will be the last one of the
> foo2[1], which will cause the lr be the first instruction of the adjacent
> foo3[2]. Hence, the backtrace will show the weird result as bellow[3].
> The patch will fix it by miner 4 of the lr when dump_backtrace

This has come up in the past (and a similar patch has been applied, then
reverted).

In general, we don't know that a function call was made via BL, and therefore
cannot know that LR - 4 is the address of the caller. The caller could set up
the LR as it likes, then B or BR to the callee, and depending on how the basic
blocks get laid out in memory, LR - 4 might point at something completely
different.

More ideally, the compiler wouldn't end a function with a BL. When does that
happen, and is there some way we could arrange for that to not happen? e.g.
somehow pad a NOP after the BL.

Thanks,
Mark.

> 
> [1]
> 0xffffff80081e6b04 <handle_mm_fault+3996>:      adrp    x0, 0xffffff8008ca8000
> 0xffffff80081e6b08 <handle_mm_fault+4000>:      add     x0, x0, #0x5a8
> 0xffffff80081e6b0c <handle_mm_fault+4004>:      bl      0xffffff80081b0ca0 <panic>
> 0xffffff80081e6b10 <access_remote_vm>:  stp     x29, x30, [sp,#-64]!
> 0xffffff80081e6b14 <access_remote_vm+4>:        mov     x29, sp
> 
> [2]
> crash_arm64> rd ffffffc02eec3bd0 2
> ffffffc02eec3bd0:  ffffffc02eec3cb0 ffffff80081e6b10
> 
> [3]
> wrong:
> [<ffffff80081b0d90>] panic+0xf0/0x24c
> [<ffffff80081e6b10>] access_remote_vm+0x0/0x5c
> [<ffffff800809d7b0>] do_page_fault+0x290/0x3b8
> [<ffffff8008081570>] do_mem_abort+0x64/0xdc
> 
> correct:
> [ffffffc02eec3bd0] panic at ffffff80081b0da4
> [ffffffc02eec3cb0] handle_mm_fault at ffffff80081e6b0c
> [ffffffc02eec3d80] do_page_fault at ffffff800809d7ac
> [ffffffc02eec3df0] do_mem_abort at ffffff800808156c
> 
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> ---
>  arch/arm64/kernel/traps.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index d399d45..7a097cc 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -113,7 +113,7 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
>  
>  	if (tsk == current) {
>  		frame.fp = (unsigned long)__builtin_frame_address(0);
> -		frame.pc = (unsigned long)dump_backtrace;
> +		frame.pc = (unsigned long)dump_backtrace + 4;
>  	} else {
>  		/*
>  		 * task blocked in __switch_to
> @@ -130,7 +130,7 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
>  	do {
>  		/* skip until specified stack frame */
>  		if (!skip) {
> -			dump_backtrace_entry(frame.pc);
> +			dump_backtrace_entry(frame.pc - 4);
>  		} else if (frame.fp == regs->regs[29]) {
>  			skip = 0;
>  			/*
> -- 
> 1.9.1
>