Re: [PATCH v15 01/10] unwind_user: Add user space unwinding API with frame pointer support

[Jens Remus <jremus@linux.ibm.com>]

On 25.07.2025 20:55, Steven Rostedt wrote:
> From: Josh Poimboeuf <jpoimboe@kernel.org>
> 
> Introduce a generic API for unwinding user stacks.
> 
> In order to expand user space unwinding to be able to handle more complex
> scenarios, such as deferred unwinding and reading user space information,
> create a generic interface that all architectures can use that support the
> various unwinding methods.
> 
> This is an alternative method for handling user space stack traces from
> the simple stack_trace_save_user() API. This does not replace that
> interface, but this interface will be used to expand the functionality of
> user space stack walking.
> 
> None of the structures introduced will be exposed to user space tooling.
> 
> Support for frame pointer unwinding is added. For an architecture to
> support frame pointer unwinding it needs to enable
> CONFIG_HAVE_UNWIND_USER_FP and define ARCH_INIT_USER_FP_FRAME.
> 
> By encoding the frame offsets in struct unwind_user_frame, much of this
> code can also be reused for future unwinder implementations like sframe.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> Co-developed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Link: https://lore.kernel.org/all/20250710164301.3094-2-mathieu.desnoyers@efficios.com/
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Co-developed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Reviewed-by: Jens Remus <jremus@linux.ibm.com>

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

> +static int unwind_user_next_fp(struct unwind_user_state *state)
> +{
> +	struct unwind_user_frame *frame = &fp_frame;

Optional: Pointer to const?

> +	unsigned long cfa, fp, ra = 0;

Nit: Is initialization of ra really required?  I don't see where ra
would be getting used without being set beforehand.

> +	unsigned int shift;
> +
> +	if (frame->use_fp) {
> +		if (state->fp < state->sp)
> +			return -EINVAL;
> +		cfa = state->fp;
> +	} else {
> +		cfa = state->sp;
> +	}
> +
> +	/* Get the Canonical Frame Address (CFA) */
> +	cfa += frame->cfa_off;
> +
> +	/* stack going in wrong direction? */
> +	if (cfa <= state->sp)
> +		return -EINVAL;
> +
> +	/* Make sure that the address is word aligned */
> +	shift = sizeof(long) == 4 ? 2 : 3;
> +	if (cfa & ((1 << shift) - 1))
> +		return -EINVAL;
> +
> +	/* Find the Return Address (RA) */
> +	if (get_user(ra, (unsigned long *)(cfa + frame->ra_off)))
> +		return -EINVAL;
> +
> +	if (frame->fp_off && get_user(fp, (unsigned long __user *)(cfa + frame->fp_off)))
> +		return -EINVAL;
> +
> +	state->ip = ra;
> +	state->sp = cfa;
> +	if (frame->fp_off)
> +		state->fp = fp;
> +	return 0;
> +}

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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