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.hu...@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.
>
> 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
