Re: [PATCH] Perf: support to unwind userspace application stacks generated with thumb.

[Jiping Ma]

On 04/16/2020 03:10 PM, Will Deacon wrote:
> On Thu, Apr 16, 2020 at 01:38:29PM +0800, Jiping Ma wrote:
> > Only push sp and lr in the stack for thumb mode. it will go
> > through the stack find sp and lr.
> >
> > Change this to the more detailed description of the patch
> >
> > Signed-off-by: Jiping Ma <jiping.ma2@xxxxxxxxxxxxx>
> > ---
> >   arch/arm64/kernel/perf_callchain.c | 36 +++++++++++++++++++++++++++---
> >   1 file changed, 33 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c
> > index bcafd7dcfe8b..97dde271c121 100644
> > --- a/arch/arm64/kernel/perf_callchain.c
> > +++ b/arch/arm64/kernel/perf_callchain.c
> > @@ -104,6 +104,30 @@ compat_user_backtrace(struct compat_frame_tail __user *tail,
> >   
> >   	return (struct compat_frame_tail __user *)compat_ptr(buftail.fp) - 1;
> >   }
> > +
> > +void
> > +user_backtrace_thumb(struct perf_callchain_entry_ctx *entry,
> > +		     struct pt_regs *regs)
> > +{
> > +	u32 sp;
> > +	u32 *sp_t;
> > +	/*
> > +	 * Only push sp, lr to stack.
> > +	 */
> > +	for (sp = regs->compat_sp; (sp < current->mm->start_stack) &&
> > +		(entry->nr < entry->max_stack); sp += 4) {
> > +		sp_t = (u32 *)(unsigned long)sp;
> > +		if ((*sp_t > regs->compat_sp) &&
> > +			(*sp_t < current->mm->start_stack)) {
> > +			if (*(sp_t + 1) < current->mm->end_code &&
> > +				*(sp_t + 1) > current->mm->start_code) {
> > +				perf_callchain_store(entry,  *(sp_t + 1)-1);
> > +				sp += 4;
> > +			}
> > +		}
> > +	}
> > +}
> This looks like a pile of fragile heuristics to me. Why don't you just use
> libunwind in userspace, the same way you'd have to if you compiled without
> framepointers?
Hi, Will

Could you share more details for libunwind in userspace? Following is 
our build command for test app.
bt_perf.thumb: arm-wrs-linux-gnueabi-gcc -march=armv7ve -mthumb 
-mfpu=neon -mfloat-abi=softfp -mcpu=cortex-a15 
--sysroot=sysroots/cortexa15t2-neon-wrs-linux-gnueabi -O0 -g -ggdb -Wall 
-funwind-tables -fno-omit-frame-pointer -frecord-gcc-switches 
-mapcs-frame bt_perf.c -o bt_perf.thumb

Thanks,
Jiping

> Will