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

[Mark Rutland]

On Tue, Nov 06, 2018 at 08:57:51AM +0000, Daniel Thompson wrote:
> On Tue, Nov 06, 2018 at 08:39:01AM +0000, Mark Rutland wrote:
> > On Tue, Nov 06, 2018 at 03:19:35PM +0800, Zhaoyang Huang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > > 
> > > 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.
> 
> It's a consequence of having __noreturn isn't it? __noreturn frees the
> compiler from the burden of having to produce a valid return stack... so
> it doesn't and unwinding becomes hard.

In that case, the compiler could equally just use B rather than BL,
which this patch doesn't solve.

The documentation for the GCC noreturn attribute [1] says:

| In order to preserve backtraces, GCC will never turn calls to noreturn
| functions into tail calls. 

... so clearly it's not intended to mess up backtracing.

IIUC we mostly use noreturn to prevent warings about uninitialised
variables and such after a call to a noreturn function. I think
optimization is a secondary concern.

We could ask the GCC folk if they can ensure that a noreturn function
call leave thes LR pointing into the caller, e.g. by padding with a NOP:

	BL	<noreturn function>
	NOP

That seems cheap enough, and would keep backtraces reliable.

Thanks,
Mark.

[1] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noreturn-function-attribute