Re: [PATCH v3 07/19] unwind: Add user space unwinding API

[Jens Remus]

On 28.10.2024 22:47, Josh Poimboeuf wrote:
> Introduce a user space unwinder API which provides a generic way to
> unwind user stacks.

...

> diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c

...

> +int unwind_user_next(struct unwind_user_state *state)
> +{
> +	struct unwind_user_frame _frame;
> +	struct unwind_user_frame *frame = &_frame;
> +	unsigned long prev_ip, cfa, fp, ra = 0;
> +
> +	if (state->done)
> +		return -EINVAL;
> +
> +	prev_ip = state->ip;
> +
> +	switch (state->type) {
> +	case UNWIND_USER_TYPE_FP:
> +		frame = &fp_frame;
> +		break;
> +	default:
> +		BUG();
> +	}
> +
> +	cfa = (frame->use_fp ? state->fp : state->sp) + frame->cfa_off;
> +
> +	if (frame->ra_off && get_user(ra, (unsigned long __user *)(cfa + frame->ra_off)))
> +		goto the_end;
> +
> +	if (ra == prev_ip)
> +		goto the_end;

This seems too restrictive to me, as it effectively prevents
unwinding from recursive functions, e.g. Glibc internal merge sort
msort_with_tmp():

$ perf record -F 9999 --call-graph fp /usr/bin/objdump -wdWF /usr/bin/objdump
$ perf script
...
objdump    8314 236064.515562:     100010 task-clock:ppp:
                 100630a compare_symbols+0x2a (/usr/bin/objdump)
             3ffb9e58e7c msort_with_tmp.part.0+0x15c (/usr/lib64/libc.so.6)
             3ffb9e58d76 msort_with_tmp.part.0+0x56 (/usr/lib64/libc.so.6)
[unwinding unexpectedly stops]

Would it be an option to only stop unwinding if both the IP and SP do
not change?

if (sp == prev_sp && ra == prev_ra)
	gote the_end;

> +
> +	if (frame->fp_off && get_user(fp, (unsigned long __user *)(cfa + frame->fp_off)))
> +		goto the_end;
> +
> +	state->sp = cfa;
> +	state->ip = ra;
> +	if (frame->fp_off)
> +		state->fp = fp;
> +
> +	return 0;
> +
> +the_end:
> +	state->done = true;
> +	return -EINVAL;
> +}

...

Thanks and regards,
Jens
--
Jens Remus
Linux on Z Development (D3303) and z/VSE Support
+49-7031-16-1128 Office
jremus@xxxxxxxxxx

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