[RESEND v3] arm64: Add USER_STACKTRACE support

[Will Deacon]

On Tue, Dec 19, 2023 at 10:22:29AM +0800, chenqiwu wrote:
> Currently, userstacktrace is unsupported for ftrace and uprobe
> tracers on arm64. This patch uses the perf_callchain_user() code
> as blueprint to implement the arch_stack_walk_user() which add
> userstacktrace support on arm64.
> Meanwhile, we can use arch_stack_walk_user() to simplify the
> implementation of perf_callchain_user().
> This patch is tested pass with ftrace, uprobe and perf tracers
> profiling userstacktrace cases.
> 
> changes in v3:
>   - update perf_callchain_user() to use arch_stack_walk_user()
>     and delete the redundant code as Mark's suggestion in v2.
>   - update the commit message.
> 
> Tested-by: chenqiwu <qiwu.chen at transsion.com>
> Signed-off-by: chenqiwu <qiwu.chen at transsion.com>
> ---
>  arch/arm64/Kconfig                 |   1 +
>  arch/arm64/kernel/perf_callchain.c | 118 +---------------------------
>  arch/arm64/kernel/stacktrace.c     | 120 +++++++++++++++++++++++++++++
>  3 files changed, 125 insertions(+), 114 deletions(-)

This mostly looks good to me, with one potential issue:

> @@ -107,35 +25,7 @@ void perf_callchain_user(struct perf_callchain_entry_ctx *entry,
>  		return;
>  	}
>  
> -	perf_callchain_store(entry, regs->pc);
> -
> -	if (!compat_user_mode(regs)) {
> -		/* AARCH64 mode */
> -		struct frame_tail __user *tail;
> -
> -		tail = (struct frame_tail __user *)regs->regs[29];
> -
> -		while (entry->nr < entry->max_stack &&

The old code is checking entry->nr against entry->max_stack here...

> +void arch_stack_walk_user(stack_trace_consume_fn consume_entry, void *cookie,
> +					const struct pt_regs *regs)
> +{
> +	if (!consume_entry(cookie, regs->pc))
> +		return;
> +
> +	if (!compat_user_mode(regs)) {
> +		/* AARCH64 mode */
> +		struct frame_tail __user *tail;
> +
> +		tail = (struct frame_tail __user *)regs->regs[29];
> +		while (tail && !((unsigned long)tail & 0x7))
> +			tail = unwind_user_frame(tail, cookie, consume_entry);

... but it looks like you've dropped that with the rework. Why is that ok?

Will