linux-kernel - Re: [PATCH] uprobe: Do not use UPROBE_SWBP

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Date: Thu, 20 Jun 2024 12:00:55 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Andrii Nakryiko <andrii@...nel.org>, Oleg Nesterov <oleg@...hat.com>, 
	Nathan Chancellor <nathan@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH] uprobe: Do not use UPROBE_SWBP_INSN as static initializer

On Tue, Jun 18, 2024 at 12:43 PM Jiri Olsa <jolsa@...nel.org> wrote:
>
> Nathan reported compilation fail for loongarch arch:
>
>   kernel/events/uprobes.c: In function 'arch_uprobe_trampoline':
>   arch/loongarch/include/asm/uprobes.h:12:33: error: initializer element is not constant
>      12 | #define UPROBE_SWBP_INSN        larch_insn_gen_break(BRK_UPROBE_BP)
>         |                                 ^~~~~~~~~~~~~~~~~~~~
>   kernel/events/uprobes.c:1479:39: note: in expansion of macro 'UPROBE_SWBP_INSN'
>    1479 |         static uprobe_opcode_t insn = UPROBE_SWBP_INSN;
>
> Loongarch defines UPROBE_SWBP_INSN as function call, so we can't
> use it to initialize static variable.
>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Fixes: ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/events/uprobes.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>

Can we instead ask loongarch folks to rewrite it to be a constant?
Having this as a function call is both an inconvenience and potential
performance problem (a minor one, but still). I would imagine it's not
hard to hard-code an instruction as a constant here.

> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 2816e65729ac..6986bd993702 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1476,8 +1476,9 @@ static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
>
>  void * __weak arch_uprobe_trampoline(unsigned long *psize)
>  {
> -       static uprobe_opcode_t insn = UPROBE_SWBP_INSN;
> +       static uprobe_opcode_t insn;
>
> +       insn = insn ?: UPROBE_SWBP_INSN;
>         *psize = UPROBE_SWBP_INSN_SIZE;
>         return &insn;
>  }
> --
> 2.45.1
>