linux-kernel - Re: [PATCH bpf-next 06/13] uprobes/x86: Add uprobe syscall to speed up uprobe

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Message-ID: <ec6f4159-8428-4156-9413-d5aa6b39e5eb@t-8ch.de>
Date: Sat, 14 Dec 2024 14:21:43 +0100
From: Thomas Weißschuh <thomas@...ch.de>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org, Song Liu <songliubraving@...com>, 
	Yonghong Song <yhs@...com>, John Fastabend <john.fastabend@...il.com>, 
	Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Alan Maguire <alan.maguire@...cle.com>, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 06/13] uprobes/x86: Add uprobe syscall to speed
 up uprobe

On 2024-12-13 22:52:15+0100, Jiri Olsa wrote:
> On Fri, Dec 13, 2024 at 04:12:46PM +0100, Thomas Weißschuh wrote:
> 
> SNIP
> 
> > > > > +static int __init arch_uprobes_init(void)
> > > > > +{
> > > > > +	unsigned long size = uprobe_trampoline_end - uprobe_trampoline_entry;
> > > > > +	static struct page *pages[2];
> > > > > +	struct page *page;
> > > > > +
> > > > > +	page = alloc_page(GFP_HIGHUSER);
> > > > 
> > > > That page could be in static memory, removing the need for the explicit
> > > > allocation. It could also be __ro_after_init.
> > > > Then tramp_mapping itself can be const.
> > > 
> > > hum, how would that look like? I think that to get proper page object
> > > you have to call alloc_page or some other page alloc family function..
> > > what do I miss?
> > 
> > static u8 trampoline_page[PAGE_SIZE] __ro_after_init __aligned(PAGE_SIZE);
> > static struct page *tramp_mapping_pages[2] __ro_after_init;
> > 
> > static const struct vm_special_mapping tramp_mapping = {
> > 	.name   = "[uprobes-trampoline]",
> > 	.pages  = tramp_mapping_pages,
> > 	.mremap = tramp_mremap,
> > };
> > 
> > static int __init arch_uprobes_init(void)
> > {
> > 	...
> > 	trampoline_pages[0] = virt_to_page(trampoline_page);
> > 	...
> > }
> > 
> > Untested, but it's similar to the stuff the vDSO implementations are
> > doing which I am working with at the moment.
> 
> nice idea, better than allocating the page, will do that

Or even better yet, just allocate the whole page already in the inline
asm and avoid the copying, too:

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index b2420eeee23a..c5e6ca7f998a 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -462,7 +462,7 @@ SYSCALL_DEFINE0(uprobe)

 asm (
        ".pushsection .rodata\n"
-       ".global uprobe_trampoline_entry\n"
+       ".balign " __stringify(PAGE_SIZE) "\n"
        "uprobe_trampoline_entry:\n"
        "endbr64\n"
        "push %rcx\n"
@@ -474,13 +474,11 @@ asm (
        "pop %r11\n"
        "pop %rcx\n"
        "ret\n"
-       ".global uprobe_trampoline_end\n"
-       "uprobe_trampoline_end:\n"
+       ".balign " __stringify(PAGE_SIZE) "\n"
        ".popsection\n"
 );

-extern __visible u8 uprobe_trampoline_entry[];
-extern __visible u8 uprobe_trampoline_end[];
+extern u8 uprobe_trampoline_entry[];


If you want to keep the copying for some reason, the asm code should be
in the section ".init.rodata" as its not used afterwards.

(A bit bikesheddy, I admit)


Thomas