On Tue, Nov 06, 2018 at 11:32:50AM +0000, Dave P Martin wrote: > On Tue, Nov 06, 2018 at 11:00:19AM +0000, Mark Rutland wrote: > > 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. > > Which is a bit odd, since every call to a noreturn function is a tail- > call by definition, and other tail-calls are typically optimised to a B > (thus interfering with backtracing in all except the noreturn case). > > Avoiding this would require a change to the compiler, and since there's > no obvious correct answer anyway, I guess we shouldn't hold our breath. > > > 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. > > Agreed. > > > 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. > > -fpatchable-function-entry=1,1 does almost the right thing, by > inserting 1 NOP at the start of each function, and putting the function > entry point after that (1) NOP.
Neat hack, but unfortunately insufficient in general since: * The next function may be notrace or asm, and hence will not have a preceding NOP. * There may not be a next function (e.g. for the final instruction in .text), and the LR value may point at some data symbol. * This relies on the preceding NOP being accounted as part of the previous function, which feels like a bug given we should have the function size somewhere. Generally, I think that trying to bodge around the exiting behaviour is going to cause just as many problems as it solves, and worse, makes it harder to consistently analyse a backtrace. IMO, we shouldn't change the kernel here. Thanks, Mark.
